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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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Repairing a vintage solid-state guitar amplifier
Hi, I'm attempting to fix a Rickenbacker TR75 GT amplifier, circa 1971.
The power amp is dead. The output power transistors have gone CE closed-circuit and power resistors in the output are open circuit. I think the previous owner shorted the output. Other components seem OK, as far as I can test them. Luckily, there's a schematic: - http://www.rickenbacker.com/pdfs/19345.pdf The 2N5988 and 2N5991 transistors are unavailable except at great expense from vintage parts suppliers. I'm not gonna pay big bucks for them, because (a) I might blow them and cry, (b) it would be cheaper just to buy a ready-made power amp module and use that. I hoped that some cheap BD441/442 transistors might be sufficient for me to find out if the rest of the circuit was OK. They are at least rated for the voltage (73 volts rail-to-rail). But putting them, and new 0.27R resistors, in the circuit and powering-up resulted in smoke. I guess their characteristics (higher gain, lower current) are too different, or there's something else wrong with the circuit. At least I only blew up a few pence/cents worth of components. Any suggestions about equivalent transistors at sensible price? Any ideas how I can try things out while minimising the chance of more fireworks? I suspect I'll end up with Plan B - replace the whole power amp with a modern one. But it would be nice to fix the original instead. |
#2
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Repairing a vintage solid-state guitar amplifier
In article .com,
wrote: I hoped that some cheap BD441/442 transistors might be sufficient for me to find out if the rest of the circuit was OK. They are at least rated for the voltage (73 volts rail-to-rail). But putting them, and new 0.27R resistors, in the circuit and powering-up resulted in smoke. I guess their characteristics (higher gain, lower current) are too different, or there's something else wrong with the circuit. At least I only blew up a few pence/cents worth of components. Any suggestions about equivalent transistors at sensible price? Any ideas how I can try things out while minimising the chance of more fireworks? The power amp appears to be direct coupled so I'd guess a few blown up transistors back down the line too. -- *Women who seek to be equal to men lack ambition. Dave Plowman London SW To e-mail, change noise into sound. |
#3
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Repairing a vintage solid-state guitar amplifier
"Dave Plowman (News)" wrote in message ... In article .com, wrote: I hoped that some cheap BD441/442 transistors might be sufficient for me to find out if the rest of the circuit was OK. They are at least rated for the voltage (73 volts rail-to-rail). But putting them, and new 0.27R resistors, in the circuit and powering-up resulted in smoke. I guess their characteristics (higher gain, lower current) are too different, or there's something else wrong with the circuit. At least I only blew up a few pence/cents worth of components. Any suggestions about equivalent transistors at sensible price? Any ideas how I can try things out while minimising the chance of more fireworks? use a variac (variable AC source) to slowly bring the voltage up. wire a light bulb in series with it. (about 100W for this case i guess) if the lamp starts to glow brightly when you bring up the voltage with no audio signal then something is still wrong. The power amp appears to be direct coupled so I'd guess a few blown up transistors back down the line too. -- *Women who seek to be equal to men lack ambition. Dave Plowman London SW To e-mail, change noise into sound. |
#4
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Repairing a vintage solid-state guitar amplifier
The power amp appears to be direct coupled so I'd guess a few blown up
transistors back down the line too. Some amplifiers of this vintage were DC couple all the way back to the pre-amp'. The only sure way to repair them was to replace every transistor on board and very carefully check all resisistors and caps' before firing up. You may also find such devices as pots to balance the output transistors bias which if not properly set will again send out smoke signals. The things really were a pain in the arse with some amps' blowing up even if the loudspeakers were left disconnected! |
#5
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Repairing a vintage solid-state guitar amplifier
for the NPN, the MJE270 looks good-- same case and higher voltage. and
48 cents! for the PNP try MJE172, a whopping 68 cents at digi-key. Do check all the other transistors... there's usually a few more with dead shorts, easily found by an ohmmeter. And do power it up gradually and check the voltages along the way to prevent another blowup. |
#6
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Repairing a vintage solid-state guitar amplifier
Yes, there is a "DC Balance" preset and a "Bias" preset.
Oh, it all sounds too hard. Plan B beckons. If you do carry on make sure you check the bias pot for dead spots. If they go O/C they effectively switch the OP transistors hard on - BANG! You are either a brave man or an idiot trying to repair it! |
#7
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Repairing a vintage solid-state guitar amplifier
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#8
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Repairing a vintage solid-state guitar amplifier
I would try temporary insertion of much higher value resistors instead of the
0R27s e.g a couple of 1/2w 3k9. AIR One other way to disable the O/P transistors while testing is to simply short both bases together. |
#9
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Repairing a vintage solid-state guitar amplifier
In light of people's responses, I looked again at the circuit and could
see that it does all look rather fragile. Although I am an electronic engineer by education, I don't have all the test gear (Variac etc.) and stocks of components that I used to. So I have decided to take the coward's way out and replace the power amp by a ready-made module. I've been able to find one of the right rating that works with the same mains transformer as the amp already has, keeping the cost and modifications to a minimum. I just wanted to thank everyone for their help and excellent suggestions. I'll be better informed for my next project, certainly. |
#11
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Repairing a vintage solid-state guitar amplifier
http://www.velleman.be/ot/en/product/view/?id=360356
(The description of a "200W" amplifier is marketing BS. It's actually a 70-100W amp, ideal for the Rick which was originally 75W). |
#12
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Repairing a vintage solid-state guitar amplifier
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#13
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Repairing a vintage solid-state guitar amplifier
wrote:
http://www.velleman.be/ot/en/product/view/?id=360356 (The description of a "200W" amplifier is marketing BS. It's actually a 70-100W amp, ideal for the Rick which was originally 75W). Hmm CPC dont seem to stock them, I wonder how they stand up to abuse. I much regret the discontinuation of the Maplin 150watt Mosfet Amp which was perfect for resurecting old 70`s H=H and Carlsbro amps. I must have built well over 50 of those kits. Ron(UK) -- Lune Valley Audio Public address system Hire, Sales, Repairs www.lunevalleyaudio.com |
#14
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Repairing a vintage solid-state guitar amplifier
wrote in message oups.com... http://www.velleman.be/ot/en/product/view/?id=360356 (The description of a "200W" amplifier is marketing BS. It's actually a 70-100W amp, ideal for the Rick which was originally 75W). Keep in mind, he's using it as a quitar amp, not a hi-fi or PA amp. We don't mind if it clips, in fact it's a desired effect. |
#15
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Repairing a vintage solid-state guitar amplifier
Ron(UK) wrote: wrote: http://www.velleman.be/ot/en/product/view/?id=360356 (The description of a "200W" amplifier is marketing BS. It's actually a 70-100W amp, ideal for the Rick which was originally 75W). Hmm CPC dont seem to stock them, I wonder how they stand up to abuse. I much regret the discontinuation of the Maplin 150watt Mosfet Amp which was perfect for resurecting old 70`s H=H and Carlsbro amps. I must have built well over 50 of those kits. The Velleman kit is also from Maplin. I built one of the 150watt MOSFET amps about 15 years ago and it served me well for 10 years. I can't give the same recommendation for this Vellman module, as it just went up in smoke after working (with good undistorted sound coming out and everything) for 5 minutes. No doubt I did something wrong, but I'm completely at a loss to know what. Completely disheartened. Giving up on whole project. |
#16
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Repairing a vintage solid-state guitar amplifier
In article .com,
wrote: The Velleman kit is also from Maplin. Yes - but not a Maplin kit. I built one of the 150watt MOSFET amps about 15 years ago and it served me well for 10 years. I've still got several in use around the country. ;-) I really miss the genuine Maplin kits - they seemed to address holes in the market, although probably only at that time. I dunno what killed them off - perhaps the cost of CE approval, etc. -- *Some people are only alive because it is illegal to kill. Dave Plowman London SW To e-mail, change noise into sound. |
#17
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Repairing a vintage solid-state guitar amplifier
Dave Plowman (News) wrote:
In article .com, wrote: The Velleman kit is also from Maplin. Yes - but not a Maplin kit. I built one of the 150watt MOSFET amps about 15 years ago and it served me well for 10 years. I've still got several in use around the country. ;-) I really miss the genuine Maplin kits - they seemed to address holes in the market, although probably only at that time. I dunno what killed them off - perhaps the cost of CE approval, etc. Shortage of the output mosfets I believe, I found a supply of bare circuit boards at a ham radio fair rally some years ago. Ron(UK) -- Lune Valley Audio Public address system Hire, Sales, Repairs www.lunevalleyaudio.com |
#18
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Repairing a vintage solid-state guitar amplifier
In article ,
Ron(UK) wrote: I really miss the genuine Maplin kits - they seemed to address holes in the market, although probably only at that time. I dunno what killed them off - perhaps the cost of CE approval, etc. Shortage of the output mosfets I believe, I found a supply of bare circuit boards at a ham radio fair rally some years ago. 2SK135 and 2SJ50 both in stock at RS components. -- *Hard work pays off in the future. Laziness pays off now * Dave Plowman London SW To e-mail, change noise into sound. |
#19
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Repairing a vintage solid-state guitar amplifier
Dave Plowman (News) wrote:
In article , Ron(UK) wrote: I really miss the genuine Maplin kits - they seemed to address holes in the market, although probably only at that time. I dunno what killed them off - perhaps the cost of CE approval, etc. Shortage of the output mosfets I believe, I found a supply of bare circuit boards at a ham radio fair rally some years ago. 2SK135 and 2SJ50 both in stock at RS components. I dunno then, I dont think kits need CE approval especially if they aren't mains powered, I could be wrong tho. Some years ago I dismantled an old Studiomaster 1000watt mosfet amplifier with a burnt out mains tranny, and that furnished me with enough power mosfets to make about a dozen 150 watt modules up. I made up some boards of my own with double the number of output devices which proved quite sucessful too. That same Hitachi[1] based design turns up in a multitude of guitar and pa amps from the 80`s and 90`s virtually anything with the words mosfet on the front. The actual output section rarely gives trouble. (IME) [1] I believe. Ron(UK) -- Lune Valley Audio Public address system Hire, Sales, Repairs www.lunevalleyaudio.com |
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