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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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One channel on stereo amp blows filter cap on negative rail
For the record, the amp is in an Advent 300, which I refuse to let die.
Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two |
#2
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One channel on stereo amp blows filter cap on negative rail
Soundhaspriority wrote:
"Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? Take a look at the points where the + and - caps are grounded. If that part of the ground bus is intact, but the bus is broken on the way to the supply ground, so that it is isolated, that point could float up towards +22 volts, making the total voltage over the cap that blows 44 volts. Which cap blows is whichever leaks less, just a matter of chance. So check that ground bus very carefully for integrity. Bob Morein (310) 237-6511 Hi,Bob and thanks The grounds are intact. By that I mean to say that point in the CB where electrolytic caps C312 and C313 are grounded is continuous with the ground at the power supply filter caps. Chassis is the same. All points which are supposed to be grounded are continuous as read by an ohm meter. -- Einstein forgot to carry the two |
#3
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One channel on stereo amp blows filter cap on negative rail
Tiger Luck wrote: snip The grounds are intact. By that I mean to say that point in the CB where electrolytic caps C312 and C313 are grounded is continuous with the ground at the power supply filter caps. Chassis is the same. All points which are supposed to be grounded are continuous as read by an ohm meter. Those little 100uF caps are _not_ the main 'lytics. The mains would likely be 1000uF to 10000uF (or possibly even more) and be located near the main bridge rectifier - which is on the _rest_ of the schematic along with the power transformer.. I think you'll find the main negative bus 'lytic to be open or dried out so that you're putting LOTS of ripple current on those littlle 100uF caps. Those 100uF caps are just local 'clean up' caps and can't handle the load. G² |
#4
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One channel on stereo amp blows filter cap on negative rail
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#5
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One channel on stereo amp blows filter cap on negative rail
In article ,
Tiger Luck wrote: Every piece of gear I have that has + and - power buss has the positive rail cap with the + to the rail and the - lead to ground. HOWEVER the negative rail has the - to the rail and the + to ground. Things are not always as they seem dnw |
#6
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One channel on stereo amp blows filter cap on negative rail
Chris Hornbeck wrote:
On Wed, 10 Sep 2008 21:23:45 -0700, Tiger Luck wrote: The best odds-on bet is that despite everything posted the cap is reversed. Sorry, but that's the main line. Not a chance... cap installed correctly and worked for years. Are you saying that *ONE* capacitor has failed? Not a series of replacements? The thing's 30 years old - *all* of the electrolytic cap's are bad now. That's life. Replace *all* of the electrolytics and move on. They're *all* bad. Parts are cheap. I replaced all the electro caps less than two years ago. Amp worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. BTW, if your question had been about repeated failures, the diagnosis would be: the cap got too hot. It got too hot because it was defective, or it was exposed to too high a voltage, I checked voltage across cap terminals with cap out. It read correctly. or it was exposed to reverse voltage, Trust me. Polarity was correct. or it was cooked by ESR. Equivalent series resistance? Is that a measure of quality? Caps are no-names, both from the same parts house. hmm... So, for future troubleshooting, eliminate these four and you'll have only a mystery remaining! Good fortune, Chris Hornbeck -- Einstein forgot to carry the two |
#7
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
In article ,
Tiger Luck wrote: For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? Part of the circuit might have failed so that it oscillates or pulses. For example - R313, C305, D301, C309, R335, or any of that bias circuit. Drag the leads of a big foil capacitor across the leads of C313. It should spark once to charge it. If it keeps sparking (or smokes), there's AC current there. The great thing about stereo amps is that you can put a voltmeter between the matching parts of each side to find faults. -- Google is a pro-spamming service. I will not see your reply if you use Google. |
#8
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One channel on stereo amp blows filter cap on negative rail
Tiger Luck wrote:
Chris Hornbeck wrote: On Wed, 10 Sep 2008 21:23:45 -0700, Tiger Luck wrote: The best odds-on bet is that despite everything posted the cap is reversed. Sorry, but that's the main line. Not a chance... cap installed correctly and worked for years. Are you saying that *ONE* capacitor has failed? Not a series of replacements? The thing's 30 years old - *all* of the electrolytic cap's are bad now. That's life. At work we buy 100uF 25 volt caps in bags of 1000 and have so far installed 2500 in 20 year old gear, mostly Sony. But we also have some Ampex 2" quad machines built in 1978 with _all_ the orignal electrolytics and the machines are in cherry condition. 30 years is not necessarily the end point for caps. Replace *all* of the electrolytics and move on. They're *all* bad. Parts are cheap. I replaced all the electro caps less than two years ago. Amp worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. What do you see with a scope? Something definitely funny going on. Is the transformer heating up too? BTW, if your question had been about repeated failures, the diagnosis would be: the cap got too hot. It got too hot because it was defective, or it was exposed to too high a voltage, I checked voltage across cap terminals with cap out. It read correctly. or it was exposed to reverse voltage, Trust me. Polarity was correct. or it was cooked by ESR. Equivalent series resistance? Is that a measure of quality? Oh yeah, ESR is a big deal. Caps are no-names, both from the same parts house. hmm... For little ( 1-4700 uF) caps we were using Panasonic FC series but after reading some Hydrogenaudio forum posts, I found that the FM series caps are even lower in ESR, longer llife (2000 vs 1000 hrs at 105 C) and they cost a little less. ($57.54 vs $61 / 1000 for 100uF 25V) So, for future troubleshooting, eliminate these four and you'll have only a mystery remaining! Good fortune, Chris Hornbeck Definitely want to hear how this shakes out. G² |
#9
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One channel on stereo amp blows filter cap on negative rail
"Tiger Luck" wrote in message ... snip Caps are no-names, both from the same parts house. hmm... snip Well, this may not be your problem, but things have deteriorated in the electronic component area over the last few years. Many manufacturers have out sourced production to China. Most if not all capacitor failures I have seen in recent years have made in China - or no brand. Try to find something that says made in Japan!! Cheers Dave |
#10
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One channel on stereo amp blows filter cap on negative rail
On Wed, 10 Sep 2008 18:29:26 -0700, Tiger Luck
put finger to keyboard and composed: For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? Maybe the trace from that channel to the -22V rail is partially open (ie resistive), allowing C313 to be reverse charged from the +22V rail via Q317 ??? Just for testing purposes, try running two hookup wires between the caps in both channels. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#11
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One channel on stereo amp blows filter cap on negative rail
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#12
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
"Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes. I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ... I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~} Arfa |
#13
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One channel on stereo amp blows filter cap on negative rail
I replaced all the electro caps less than two years ago. Amp
worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. It's remotely possible the replacement was defective. If the voltage across the cap's location is correct in level and polarity, then that can't be the problem. Question... How safe is it -- other than the cap blowing -- to operate the amp? I think you need to put a 'scope on a number of points and see what's happening. I don't mean to hassle you on the question of polarity, but... Japanese caps generally have a black line on one side pointing toward the negative lead. |
#14
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One channel on stereo amp blows filter cap on negative rail
Transformer has stayed cool. The 3 amp fuses on the positive and
negative rails are not blowing at this point. I don't have a 'scope, but have a line on a Philips PM3050 for $85. There is more going on here than I can read at the moment. I have the output pair on the left channel yanked at this point because they shorted out. Now the output pair on the right channel are heating up when the power is on. The electro cap that blew is on the right channel. Part of the 15 volt regulator circuitry failed and I replaced some parts there. But that's it for tonight. I think you have problems that go fary beyond a bad cap. I suspect the bad cap is a effect more than a cause. I would start by pulling the output pair on the right channel and trying to get everything else working correctly. |
#15
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One channel on stereo amp blows filter cap on negative rail
"Arfa Daily" wrote in message ... "Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes. I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ... I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~} Arfa The matching cap on the other channel may now be open, thus not exploding from a shorted rectifier in the power supply putting AC across the cap, which would explain the original cap failure also, and would also go a long way toward explaining the heating on the other channel. The multiple failure does suggest to me that you are getting AC to that negative rail. This would explain the voltage regulator failure, and I'll bet the main transformer is getting hot as well. More than once I've been misled by DC voltages only to find that it was an AC problem, either line-related or HF oscillation. Wouldn't be surprised if the main filter on that negative rail were bad also (or maybe it just hasn't exploded yet from the AC !). Too bad the schematic is not complete - I would have liked to have a full PDF on this model. Mark Z. |
#16
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One channel on stereo amp blows filter cap on negative rail
Tiger Luck wrote: Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, Which is WRONG ! Read the diagram ! The negative of C311 should be grounded together with the POSITIVE of C313 Possibly a pcb legend error. I've known it happen. Graham |
#17
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One channel on stereo amp blows filter cap on negative rail
DC wrote: "Tiger Luck" wrote Well, this may not be your problem, but things have deteriorated in the electronic component area over the last few years. Many manufacturers have out sourced production to China. Most if not all capacitor failures I have seen in recent years have made in China - or no brand. Try to find something that says made in Japan!! Panasonic are reliable. But the op says C311 and C313 have their negatives connected together AIUI. That's plain WRONG. Graham |
#18
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One channel on stereo amp blows filter cap on negative rail
Arfa Daily wrote: In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. An HF fault causing high local ripple ? Graham |
#19
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One channel on stereo amp blows filter cap on negative rail
William Sommerwerck wrote: I replaced all the electro caps less than two years ago. Amp worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. It's remotely possible the replacement was defective. If the voltage across the cap's location is correct in level and polarity, then that can't be the problem. Question... How safe is it -- other than the cap blowing -- to operate the amp? I think you need to put a 'scope on a number of points and see what's happening. I don't mean to hassle you on the question of polarity, but... Japanese caps generally have a black line on one side pointing toward the negative lead. Or a gold one etc etc. Graham |
#20
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One channel on stereo amp blows filter cap on negative rail
William Sommerwerck wrote:
I replaced all the electro caps less than two years ago. Amp worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. It's remotely possible the replacement was defective. If the voltage across the cap's location is correct in level and polarity, then that can't be the problem. Question... How safe is it -- other than the cap blowing -- to operate the amp? I think you need to put a 'scope on a number of points and see what's happening. I don't mean to hassle you on the question of polarity, but... Japanese caps generally have a black line on one side pointing toward the negative lead. These caps have a line of pluses down one side pointing to the plus lead. The CB has a plus printed next to one hole for the ECs. I installed several of these caps in this amp. They are foolproof. The amp worked perfectly for a couple of years with these caps installed. I don't have a scope, but I may buy a used Philips PM3050 for $85 just to see what I'm missing. -- Einstein forgot to carry the two |
#21
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One channel on stereo amp blows filter cap on negative rail
Tiger Luck wrote:
William Sommerwerck wrote: I replaced all the electro caps less than two years ago. Amp worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. It's remotely possible the replacement was defective. If the voltage across the cap's location is correct in level and polarity, then that can't be the problem. Question... How safe is it -- other than the cap blowing -- to operate the amp? I think you need to put a 'scope on a number of points and see what's happening. I don't mean to hassle you on the question of polarity, but... Japanese caps generally have a black line on one side pointing toward the negative lead. These caps have a line of pluses down one side pointing to the plus lead. The CB has a plus printed next to one hole for the ECs. I installed several of these caps in this amp. They are foolproof. The amp worked perfectly for a couple of years with these caps installed. I don't have a scope, but I may buy a used Philips PM3050 for $85 just to see what I'm missing. Couldn`t you just put a meter across the cap to see what polarity it`s getting? Ron -- Lune Valley Audio Public Address Systems Hire Sales Maintenance www.lunevalleyaudio.com |
#22
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One channel on stereo amp blows filter cap on negative rail
"don ward" wrote in message ... In article , Tiger Luck wrote: Every piece of gear I have that has + and - power buss has the positive rail cap with the + to the rail and the - lead to ground. HOWEVER the negative rail has the - to the rail and the + to ground. Things are not always as they seem dnw Then you must not have any pieces of gear that are in working order. On the negative rail, the - lead of the cap is on the rail and the + lead is to ground. Can't be any other way. |
#23
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One channel on stereo amp blows filter cap on negative rail
On Sep 11, 10:33*am, Tiger Luck
wrote: William Sommerwerck wrote: I replaced all the electro caps less than two years ago. Amp worked fine up until now. Polarity was and is correct. After cap blew today, new replacement cap over heated. I shut off power before it blew out. It's remotely possible the replacement was defective. If the voltage across the cap's location is correct in level and polarity, then that can't be the problem. Question... How safe is it -- other than the cap blowing -- to operate the amp? I think you need to put a 'scope on a number of points and see what's happening. I don't mean to hassle you on the question of polarity, but... Japanese caps generally have a black line on one side pointing toward the negative lead. These caps have a line of pluses down one side pointing to the plus lead. The CB has a plus printed next to one hole for the ECs. I installed several of these caps in this amp. They are foolproof. The amp worked perfectly for a couple of years with these caps installed. I don't have a scope, but I may buy a used Philips PM3050 for $85 just to see what I'm missing. -- Einstein forgot to carry the two- Hide quoted text - - Show quoted text - You need to put a scope on it. I think you're going to find ac or a lot of ripples. If you isolate the power supply don't forget to put a load on it. |
#24
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One channel on stereo amp blows filter cap on negative rail
On Thu, 11 Sep 2008 13:51:27 +0100, Eeyore
put finger to keyboard and composed: Tiger Luck wrote: Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, Which is WRONG ! Read the diagram ! The negative of C311 should be grounded together with the POSITIVE of C313 Possibly a pcb legend error. I've known it happen. Graham The OP wrote that "the corresponding cap in the other *channel* [C312] does not blow", not the other *rail*. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#25
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One channel on stereo amp blows filter cap on negative rail
William Sommerwerck wrote:
Transformer has stayed cool. The 3 amp fuses on the positive and negative rails are not blowing at this point. I don't have a 'scope, but have a line on a Philips PM3050 for $85. There is more going on here than I can read at the moment. I have the output pair on the left channel yanked at this point because they shorted out. Now the output pair on the right channel are heating up when the power is on. The electro cap that blew is on the right channel. Part of the 15 volt regulator circuitry failed and I replaced some parts there. But that's it for tonight. I think you have problems that go fary beyond a bad cap. I suspect the bad cap is a effect more than a cause. I would start by pulling the output pair on the right channel and trying to get everything else working correctly. Both output pairs are pulled. I am reading 20 volts AC to ground on each of the stepped down leads from the transformer and 40 volts between them. The positive rail is reading 26.3 volts DC to ground and the negative rail is reading 26.7 volts DC to ground after going through the bridge rectifier and the filter caps. They are reading 53 volts between them. The replacement for the electro cap that blew is reading 26.7 volts across its terminals. It connect the negative rail to ground. Positive side is grounded. It is no longer heating up when power is on. With power on and about to take some voltage readings across the empty output tranny terminals on the CB, I noticed some tiny sparking going on at the edges of the trace leading to the collector terminal of Q317, with occasional riny puffs of smoke (see schematics): http://www.davidreaton.com/PDFs/Adve..._schematic.pdf I removed as much solder as possible from the traces around all output terminals with a solder wick and cleaned up the board with switch cleaner. Sparking appears to have stopped. Began taking various voltage measurements at same points in right and left channels. Discovered huge variation on voltage across C-E terminals with output transistors removed. Right side had about 26 to 27 volts across the C-E terminals of each output transistor. The left side had no voltage across C-E terminals of one output transistor and about 52 to 54 volts across the C-E terminals of the other output transistor. Also had different readings across C-E terminals of the two pairs of differential transistors at the input. Right side had about 26-27 volts across C-E terminals of each transistor and the left side had about 7 volts across the same terminals. Disconnecting the base of Q302 resulted in both differential transistors on the left reading identical to the right side across the C-E terminals. I traced it down to where D304 connects to the base of Q302 thru a 20K resistor, R336. I reconnected the base of Q302 and disconnecting the cathode side of D304 and got the same results as disconnecting the base of Q302. Now it's just a matter of going through the same procedure until I find the faulty component. Disconnecting the base of Q302 had no effect on the voltage measurements of the output transistors in the left channel, but disconnecting D304 did. Reading across the C-E terminals of one output tranny on the right channel with D304 disconnected, the digital meter read about 27 volts and then began drifting steadily lower, similar to reading a large cap with an ohmmeter. When I switched the meter to the C-E terminals of the other output tranny on the right side, it read higher than 27 volts by as much as the first tranny's voltage diminished when it was read. And that's where it stands. Sorry for not reading and replying to every post, but when I got enough insight from what I did read to see where I needed to go, I stopped reading and went to work. Will report all findings and methods used. Thanks all -- Einstein forgot to carry the two |
#26
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
Arfa Daily wrote:
"Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes. I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ... I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~} Arfa Arfa, an update as requested and as promised. First, a repost of my reply to William Sommerwerck posted today, 9/11/08 at 2:11pm so that everything is in one post: ---------------------------------- Both output pairs are pulled. I am reading 20 volts AC to ground on each of the stepped down leads from the transformer and 40 volts AC between them. The positive rail is reading 26.3 volts DC to ground and the negative rail is reading 26.7 volts DC to ground after going through the bridge rectifier and the filter caps. They are reading 53 volts between them. The replacement for the electro cap that blew is reading 26.7 volts across its terminals. It connects the negative rail to ground. Positive side is grounded. It is no longer heating up when power is on. With power on and about to take some voltage readings across the empty output tranny terminals on the CB, I noticed some tiny sparking going on at the edges of the trace leading to the collector terminal of Q317, with occasional tiny puffs of smoke (see schematics): http://www.davidreaton.com/PDFs/Adve..._schematic.pdf I removed as much solder as possible from the traces around all output terminals with a solder wick and cleaned up the board with switch cleaner. Sparking appears to have stopped. Began taking various voltage measurements at same points in right and left channels. Discovered huge variation on voltage across C-E terminals with output transistors removed. Right side had about 26 to 27 volts across the C-E terminals of each output transistor. The left side had no voltage across C-E terminals of one output transistor and about 52 to 54 volts across the C-E terminals of the other output transistor. Also had different readings across C-E terminals of the two pairs of differential transistors at the input. Right side had about 26-27 volts across C-E terminals of each transistor and the left side had about 7 volts across the same terminals. Disconnecting the base of Q302 resulted in both differential transistors on the left reading identical to the right side across the C-E terminals. I traced it down to where D304 connects to the base of Q302 thru a 20K resistor, R336. I reconnected the base of Q302 and disconnecting the cathode side of D304 and got the same results as disconnecting the base of Q302. Now it's just a matter of going through the same procedure until I find the faulty component. Disconnecting the base of Q302 had no effect on the voltage measurements of the output transistors in the left channel, but disconnecting D304 did. Reading across the C-E terminals of one output tranny on the right channel with D304 disconnected, the digital meter read about 27 volts and then began drifting steadily lower, similar to reading a large cap with an ohmmeter. When I switched the meter to the C-E terminals of the other output tranny on the right side, it read higher than 27 volts by as much as the first tranny's voltage diminished when it was read. And that's where it stands. Sorry for not reading and replying to every post, but when I got enough insight from what I did read to see where I needed to go, I stopped reading and went to work. Will report all findings and methods used. -------------------------------------- Update: Narrowed down the search to the upper section of the right channel as seen on the schematics at the link above. Following the logic outlined above, I reconnected everything and disconnected the base of Q308 and turned the power on to check voltages discrepancies outlined above. With the unit on its side and probing the underside of the circuit board, smoke began rising from the top side opposite me. I cut power and found the following fried components: Q308 Q310 R326 R328 R324 Pulled all the fried components and checked. Q308, Q310, R326 and R328 were toast and I replaced them. R324 was still good, but replaced it. Everything except the output trannies are in place and the voltage discrepancies between the two channels have disappeared. I will now install the output trannies and give it a go. -- Einstein forgot to carry the two |
#27
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
Mark D. Zacharias wrote:
"Arfa Daily" wrote in message ... "Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes. I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ... I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~} Arfa The matching cap on the other channel may now be open, thus not exploding from a shorted rectifier in the power supply putting AC across the cap, which would explain the original cap failure also, and would also go a long way toward explaining the heating on the other channel. The multiple failure does suggest to me that you are getting AC to that negative rail. This would explain the voltage regulator failure, and I'll bet the main transformer is getting hot as well. More than once I've been misled by DC voltages only to find that it was an AC problem, either line-related or HF oscillation. Wouldn't be surprised if the main filter on that negative rail were bad also (or maybe it just hasn't exploded yet from the AC !). Too bad the schematic is not complete - I would have liked to have a full PDF on this model. Mark Z. Mark, here is the other PDF. It includes the power supply and the tuner: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf Here are two sites devoted to this receiver. Schematics came from the latter: http://home.netcarrier.com/%7Erstevens/advent.html http://www.davidreaton.com/Model_300..._main_page.htm -- Einstein forgot to carry the two |
#28
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
Tiger Luck wrote:
Arfa Daily wrote: "Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes. I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ... I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~} Arfa Arfa, an update as requested and as promised. First, a repost of my reply to William Sommerwerck posted today, 9/11/08 at 2:11pm so that everything is in one post: ---------------------------------- Both output pairs are pulled. I am reading 20 volts AC to ground on each of the stepped down leads from the transformer and 40 volts AC between them. The positive rail is reading 26.3 volts DC to ground and the negative rail is reading 26.7 volts DC to ground after going through the bridge rectifier and the filter caps. They are reading 53 volts between them. The replacement for the electro cap that blew is reading 26.7 volts across its terminals. It connects the negative rail to ground. Positive side is grounded. It is no longer heating up when power is on. With power on and about to take some voltage readings across the empty output tranny terminals on the CB, I noticed some tiny sparking going on at the edges of the trace leading to the collector terminal of Q317, with occasional tiny puffs of smoke (see schematics): http://www.davidreaton.com/PDFs/Adve..._schematic.pdf I removed as much solder as possible from the traces around all output terminals with a solder wick and cleaned up the board with switch cleaner. Sparking appears to have stopped. Began taking various voltage measurements at same points in right and left channels. Discovered huge variation on voltage across C-E terminals with output transistors removed. Right side had about 26 to 27 volts across the C-E terminals of each output transistor. The left side had no voltage across C-E terminals of one output transistor and about 52 to 54 volts across the C-E terminals of the other output transistor. Also had different readings across C-E terminals of the two pairs of differential transistors at the input. Right side had about 26-27 volts across C-E terminals of each transistor and the left side had about 7 volts across the same terminals. Disconnecting the base of Q302 resulted in both differential transistors on the left reading identical to the right side across the C-E terminals. I traced it down to where D304 connects to the base of Q302 thru a 20K resistor, R336. I reconnected the base of Q302 and disconnecting the cathode side of D304 and got the same results as disconnecting the base of Q302. Now it's just a matter of going through the same procedure until I find the faulty component. Disconnecting the base of Q302 had no effect on the voltage measurements of the output transistors in the left channel, but disconnecting D304 did. Reading across the C-E terminals of one output tranny on the right channel with D304 disconnected, the digital meter read about 27 volts and then began drifting steadily lower, similar to reading a large cap with an ohmmeter. When I switched the meter to the C-E terminals of the other output tranny on the right side, it read higher than 27 volts by as much as the first tranny's voltage diminished when it was read. And that's where it stands. Sorry for not reading and replying to every post, but when I got enough insight from what I did read to see where I needed to go, I stopped reading and went to work. Will report all findings and methods used. -------------------------------------- Update: Narrowed down the search to the upper section of the right channel as seen on the schematics at the link above. Following the logic outlined above, I reconnected everything and disconnected the base of Q308 and turned the power on to check voltages discrepancies outlined above. With the unit on its side and probing the underside of the circuit board, smoke began rising from the top side opposite me. I cut power and found the following fried components: Q308 Q310 R326 R328 R324 Pulled all the fried components and checked. Q308, Q310, R326 and R328 were toast and I replaced them. R324 was still good, but replaced it. Everything except the output trannies are in place and the voltage discrepancies between the two channels have disappeared. I will now install the output trannies and give it a go. Output trannies are back in. Speakers hooked up. Tuner tuned in. Fingers crossed. And here goes... power up... power down, got smoke. Output trannies on the left side got hot, but held. The drama continues. Will pull output trannies on left side and check right side for okayness. -- Einstein forgot to carry the two |
#29
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
"Dave" wrote in message
newsbayk.2231$yS5.534@edtnps83... "don ward" wrote in message ... In article , Tiger Luck wrote: Every piece of gear I have that has + and - power buss has the positive rail cap with the + to the rail and the - lead to ground. HOWEVER the negative rail has the - to the rail and the + to ground. Things are not always as they seem dnw Then you must not have any pieces of gear that are in working order. On the negative rail, the - lead of the cap is on the rail and the + lead is to ground. Can't be any other way. As I read it, that's what he said. |
#30
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
Tiger Luck wrote:
Tiger Luck wrote: Arfa Daily wrote: "Tiger Luck" wrote in message ... For the record, the amp is in an Advent 300, which I refuse to let die. Here's a PDF of the schematics: http://www.davidreaton.com/PDFs/Adve..._schematic.pdf The cap that blows is C313. It is a 100mfd 35volt cap It is the filter cap connecting the negative rail in the lower channel to ground. It is on the far lower right of the schematic, just above the separate, isolated schematic of the 15 volt regulator circuit. The voltage on the negative rail is as it should be. The polarity of the cap was correct. The corresponding cap in the other channel does not blow. Both caps have the negative connected to the same source from the power supply and both have their positives connected to ground. In other words, the negs of both caps are connected together to the negative rail from the power supply and both positives are grounded, yet one cap blows and the other doesn't. The cap blew with a sharp crack and the rubber plug at the bottom blew out. The cap that blew is not shorted. It still acts like a cap should when measured with an analog ohm meter. I installed another, new cap of equal value and it started heating up, so I cut the power. I'm not a pro tech, but fairly knowledgeable and good at logic and such. Any insights? -- Einstein forgot to carry the two In many years of repairing this stuff, and replacing electrolytics that have failed any way from passively to explosively, I have only ever found three things which cause their demise in the way you have described. 1) Excess voltage across them 2) Incorrect polarity of voltage across them 3) Excess current through them 3) is obviously the most difficult to diagnose, as it really requires a 'scope to see what's going on. However, given that this cap is effectively in parallel with the main filter cap for the output stage negative rail, and the one for the other channel, which is also connected to the same place for all intents and purposes, it's hard to see how there could be excessive ac across one of them on its own. Have you tried running the amp without the offending cap in ? It should almost certainly run normally without it, as small caps in that position are usually just for 'belt and braces' decoupling purposes. I know that you have stated several times that the cap is in correctly, but are you *absolutely totally completely and without question* sure ? Noted that you replaced the caps a couple of years back and have not had trouble in the intervening period, but that is not necessarily an indication that all was well. I have seen equipment where caps have been fitted backwards by the manufacturers, and have lasted a long time before finally failing. If you are basing your latest replacements which are failing, on the way you took out the first failed one - or even based on the way that the board is marked, because I've seen them silkscreened backwards as well - I would say please check very very carefully, go away and have a cup of coffee, then come back and check again. It's easy to convince yourself that you are seeing something you actually aren't. It has happened to me many times over the years ... I too would like to know what you eventually find, as the circumstances you are describing are a bit of a conundrum. If it does come down to something you have done, please tell us. I'm sure no-one will think any the less of you. If we're honest, we've all been there d;~} Arfa Arfa, an update as requested and as promised. First, a repost of my reply to William Sommerwerck posted today, 9/11/08 at 2:11pm so that everything is in one post: ---------------------------------- Both output pairs are pulled. I am reading 20 volts AC to ground on each of the stepped down leads from the transformer and 40 volts AC between them. The positive rail is reading 26.3 volts DC to ground and the negative rail is reading 26.7 volts DC to ground after going through the bridge rectifier and the filter caps. They are reading 53 volts between them. The replacement for the electro cap that blew is reading 26.7 volts across its terminals. It connects the negative rail to ground. Positive side is grounded. It is no longer heating up when power is on. With power on and about to take some voltage readings across the empty output tranny terminals on the CB, I noticed some tiny sparking going on at the edges of the trace leading to the collector terminal of Q317, with occasional tiny puffs of smoke (see schematics): http://www.davidreaton.com/PDFs/Adve..._schematic.pdf I removed as much solder as possible from the traces around all output terminals with a solder wick and cleaned up the board with switch cleaner. Sparking appears to have stopped. Began taking various voltage measurements at same points in right and left channels. Discovered huge variation on voltage across C-E terminals with output transistors removed. Right side had about 26 to 27 volts across the C-E terminals of each output transistor. The left side had no voltage across C-E terminals of one output transistor and about 52 to 54 volts across the C-E terminals of the other output transistor. Also had different readings across C-E terminals of the two pairs of differential transistors at the input. Right side had about 26-27 volts across C-E terminals of each transistor and the left side had about 7 volts across the same terminals. Disconnecting the base of Q302 resulted in both differential transistors on the left reading identical to the right side across the C-E terminals. I traced it down to where D304 connects to the base of Q302 thru a 20K resistor, R336. I reconnected the base of Q302 and disconnecting the cathode side of D304 and got the same results as disconnecting the base of Q302. Now it's just a matter of going through the same procedure until I find the faulty component. Disconnecting the base of Q302 had no effect on the voltage measurements of the output transistors in the left channel, but disconnecting D304 did. Reading across the C-E terminals of one output tranny on the right channel with D304 disconnected, the digital meter read about 27 volts and then began drifting steadily lower, similar to reading a large cap with an ohmmeter. When I switched the meter to the C-E terminals of the other output tranny on the right side, it read higher than 27 volts by as much as the first tranny's voltage diminished when it was read. And that's where it stands. Sorry for not reading and replying to every post, but when I got enough insight from what I did read to see where I needed to go, I stopped reading and went to work. Will report all findings and methods used. -------------------------------------- Update: Narrowed down the search to the upper section of the right channel as seen on the schematics at the link above. Following the logic outlined above, I reconnected everything and disconnected the base of Q308 and turned the power on to check voltages discrepancies outlined above. With the unit on its side and probing the underside of the circuit board, smoke began rising from the top side opposite me. I cut power and found the following fried components: Q308 Q310 R326 R328 R324 Pulled all the fried components and checked. Q308, Q310, R326 and R328 were toast and I replaced them. R324 was still good, but replaced it. Everything except the output trannies are in place and the voltage discrepancies between the two channels have disappeared. I will now install the output trannies and give it a go. Output trannies are back in. Speakers hooked up. Tuner tuned in. Fingers crossed. And here goes... power up... power down, got smoke. Output trannies on the left side got hot, but held. The drama continues. Will pull output trannies on left side and check right side for okayness. Spoke too soon. Three of the output trannies fried and the fourth probably would have if I had left the power on longer. I have never given up on a piece of gear until now. It almost seems like the power supply might be the problem seeing as how the outputs on both channels fried, but who knows. It reads okay. I've been at this thing for about four days straight now and I am totally fried behind it. I've resurrected stuff the local vintage stereo store has given up on, but this took me to my limits and left me out to dry. If anyone wants this pup, with schematics, minus a couple of output trannies, for the price of postage, email me at jack6128 at gmail dot com Je suis fini. -- Einstein forgot to carry the two |
#31
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
I was partly right once, so perhaps the luck (ahem) will continue.
Barring a bad part, it sounds -- as someone else suggested -- that there's something wrong with the board, or a solder joint. Lemme tell you a story... Back in 1980, I assembled an Integrex kit and had problems with one of the op amps overheating. I spend almost an hour checking voltages, resistances, etc, trying to figure out exactly what was causing the problem. It then occurred to me to turn the board over and look carefully. Viola! Two traces had a solder bridge. (Integrex was cheap and didn't provide a solder mask.) |
#32
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
I've been at this thing for about four days straight now and
I am totally fried behind it. I've resurrected stuff the local vintage stereo store has given up on, but this took me to my limits and left me out to dry. Be patient. Put the unit aside for few days and come back to it. Don't expect to solve every problem instantly. When your mind is fresh again, you're likely to figure it out. |
#33
Posted to rec.audio.tech,sci.electronics.repair
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One channel on stereo amp blows filter cap on negative rail
snip Output trannies are back in. Speakers hooked up. Tuner tuned in. Fingers crossed. And here goes... power up... power down, got smoke. Output trannies on the left side got hot, but held. The drama continues. Will pull output trannies on left side and check right side for okayness. Spoke too soon. Three of the output trannies fried and the fourth probably would have if I had left the power on longer. I have never given up on a piece of gear until now. It almost seems like the power supply might be the problem seeing as how the outputs on both channels fried, but who knows. It reads okay. I've been at this thing for about four days straight now and I am totally fried behind it. I've resurrected stuff the local vintage stereo store has given up on, but this took me to my limits and left me out to dry. If anyone wants this pup, with schematics, minus a couple of output trannies, for the price of postage, email me at jack6128 at gmail dot com Je suis fini. -- Einstein forgot to carry the two I do sympathise old boy. In years past, I never gave up on anything either, but I must admit that these days, sometimes I do, out of both a need to be commercial and keep the wife off my back for not earning any money, and out of sheer frustration. I think that latter condition is something that creeps up on you with age. Suddenly, some things are more important than not letting some inanimate object beat you ... That said, I reckon that this baby has got to be worth one more go if - and *only* if - you can lay hands on a variac, so you can wind up the input power to make the rails just high enough to take sensible measurements, but low enough to stop the ouput trannies releasing their magic smoke ... As William says, give it a break over the weekend, then come back to it with fresh eyes (and a variac !!) on Monday. If you do, carry on keeping us updated. It's an interesting saga, if a little frustrating for you. Arfa |
#34
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One channel on stereo amp blows filter cap on negative rail
I do sympathise old boy. In years past, I never gave up on anything
either, but I must admit that these days, sometimes I do, out of both a need to be commercial and keep the wife off my back for not earning any money, and out of sheer frustration. I think that latter condition is something that creeps up on you with age. Suddenly, some things are more important than not letting some inanimate object beat you ... That said, I reckon that this baby has got to be worth one more go if - and *only* if - you can lay hands on a Variac, so you can wind up the input power to make the rails just high enough to take sensible measurements, but low enough to stop the ouput trannies releasing their magic smoke ... As William says, give it a break over the weekend, then come back to it with fresh eyes (and a Variac !!) on Monday. If you do, carry on keeping us updated. It's an interesting saga, if a little frustrating for you. One of the reasons I've enjoyed helping is that you've explained exactly what you've done, which makes the process fun -- and educational, too! PS: I have several working Advents. Let me know if you want me to check the voltages on a "good" one. I'm willing to do it "live", in real time, if we can find a time suitable for both of us. |
#35
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One channel on stereo amp blows filter cap on negative rail
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#36
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One channel on stereo amp blows filter cap on negative rail
On Fri, 12 Sep 2008 02:36:23 +0100, "Arfa Daily"
wrote: snip Output trannies are back in. Speakers hooked up. Tuner tuned in. Fingers crossed. And here goes... power up... power down, got smoke. Output trannies on the left side got hot, but held. The drama continues. Will pull output trannies on left side and check right side for okayness. Spoke too soon. Three of the output trannies fried and the fourth probably would have if I had left the power on longer. I have never given up on a piece of gear until now. It almost seems like the power supply might be the problem seeing as how the outputs on both channels fried, but who knows. It reads okay. I've been at this thing for about four days straight now and I am totally fried behind it. I've resurrected stuff the local vintage stereo store has given up on, but this took me to my limits and left me out to dry. If anyone wants this pup, with schematics, minus a couple of output trannies, for the price of postage, email me at jack6128 at gmail dot com Je suis fini. -- Einstein forgot to carry the two I do sympathise old boy. In years past, I never gave up on anything either, but I must admit that these days, sometimes I do, out of both a need to be commercial and keep the wife off my back for not earning any money, and out of sheer frustration. I think that latter condition is something that creeps up on you with age. Suddenly, some things are more important than not letting some inanimate object beat you ... That said, I reckon that this baby has got to be worth one more go if - and *only* if - you can lay hands on a variac, so you can wind up the input power to make the rails just high enough to take sensible measurements, but low enough to stop the ouput trannies releasing their magic smoke ... As William says, give it a break over the weekend, then come back to it with fresh eyes (and a variac !!) on Monday. If you do, carry on keeping us updated. It's an interesting saga, if a little frustrating for you. Arfa If you don't have a variac, put a 60 watt light bulb in series with the hot side of the AC in. If the light glows brightly, your amps are still drawing too much current or there is a power supply fault. Once you find the problem, the light will hardly glow after the initial currrent rush and the bulb will usually keep the outputs from blowing if there is still a problem. Chuck |
#37
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One channel on stereo amp blows filter cap on negative rail
if the -22 supply has failed, due to a blown diode or fuse, and if the
output transistors have shorted, then the -22 rail will be pulled positive and that will blow the capacitor. Also note that the schematic has errors on it, for instance the output transistors have emitter and collectors reversed! Hope you didnt wire the new ones in like the schematic!! |
#38
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One channel on stereo amp blows filter cap on negative rail
Ancient_Hacker wrote:
if the -22 supply has failed, due to a blown diode or fuse, and if the output transistors have shorted, then the -22 rail will be pulled positive and that will blow the capacitor. Also note that the schematic has errors on it, for instance the output transistors have emitter and collectors reversed! Hope you didnt wire the new ones in like the schematic!! I thought that at first too but if you zoom to 300% you'll see the emitter marks where they belong and the 'squiggles' on the collectors may be a 1970's version of heatsink. You'll see the same marks on the collector of Q312 indicating it is thermally coupled to the heatsink with the outputs. Way back in the days when I worked on stereo gear I had a unit with very bizarre distortion. It turned out to be a transistor with a 'leak' which on a curve tracer showed up as a transistor in parallel with a resistor from E to C. If Tiger has a similar quirky part it may explain why he's having such a bad time with that amp - which is pretty straightforward and decent for it's day. G² |
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