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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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Hum particularly on clean channel builds up from nothing ,over half an hour,
and very intrusive 0.3V rms of hum over 8R speaker load and still rising. Putting a signal in Return, for PA only, is fine but opening guitar input hum returns. Owner had replaced all the valves and exactly the same hum. Hum is negligible on the downstream HTs from HT1. What sort of grounding problem increases with warmth? I've not started exploring the low voltage electros around V1 yet, I'm letting the amp cool down |
#2
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C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to hot
air. Monitoring the hum over the output load. Once force warmed and hum level risen it is reluctant to go down again from natural cooling or from freezer spray - what process is going on with in it? Now to replace by fudge fitting to in situ component leads or take the whole thing apart to replace properly? Marshall saw fit to bodge 3 resistors by cutting off the originals and flying replacement ones to the cut wires - so whats good for the goose .... |
#3
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On 3/15/2012 8:16 PM, N_Cook wrote:
C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to hot air. Monitoring the hum over the output load. Once force warmed and hum level risen it is reluctant to go down again from natural cooling or from freezer spray - what process is going on with in it? Now to replace by fudge fitting to in situ component leads or take the whole thing apart to replace properly? Marshall saw fit to bodge 3 resistors by cutting off the originals and flying replacement ones to the cut wires - so whats good for the goose .... Fudge it to verify the fault then do a proper number. Rheilly P |
#4
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Rheilly Phoull wrote in message
. au... On 3/15/2012 8:16 PM, N_Cook wrote: C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to hot air. Monitoring the hum over the output load. Once force warmed and hum level risen it is reluctant to go down again from natural cooling or from freezer spray - what process is going on with in it? Now to replace by fudge fitting to in situ component leads or take the whole thing apart to replace properly? Marshall saw fit to bodge 3 resistors by cutting off the originals and flying replacement ones to the cut wires - so whats good for the goose ..... Fudge it to verify the fault then do a proper number. Rheilly P Any ideas about the origin of the "heat ramping" effect? Allowing the whole amp to cool for 1/4 hour obviously resets the effect |
#5
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![]() "N_Cook" wrote in message ... Hum particularly on clean channel builds up from nothing ,over half an hour, and very intrusive 0.3V rms of hum over 8R speaker load and still rising. Putting a signal in Return, for PA only, is fine but opening guitar input hum returns. Owner had replaced all the valves and exactly the same hum. Hum is negligible on the downstream HTs from HT1. What sort of grounding problem increases with warmth? I've not started exploring the low voltage electros around V1 yet, I'm letting the amp cool down This might be the infamous output PCB fault. Keep metering the bias voltage from the 3 pin connector provided, (Should be 90mV both sides). If it starts to climb/runaway in sync with the hum increasing, then that's what the problem is. You will need to buy another PCB from Marshall if so. Gareth. |
#6
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If it is the output PCB, it can very quickly destroy the output tubes by
putting HT where it shouldn't, so do not put any new ones in there til you have eliminated this possibility. Gareth. |
#7
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Gareth Magennis wrote in message
... "N_Cook" wrote in message ... Hum particularly on clean channel builds up from nothing ,over half an hour, and very intrusive 0.3V rms of hum over 8R speaker load and still rising. Putting a signal in Return, for PA only, is fine but opening guitar input hum returns. Owner had replaced all the valves and exactly the same hum. Hum is negligible on the downstream HTs from HT1. What sort of grounding problem increases with warmth? I've not started exploring the low voltage electros around V1 yet, I'm letting the amp cool down This might be the infamous output PCB fault. Keep metering the bias voltage from the 3 pin connector provided, (Should be 90mV both sides). If it starts to climb/runaway in sync with the hum increasing, then that's what the problem is. You will need to buy another PCB from Marshall if so. Gareth. I will check that, I could understand with that old lino or whale hide they used to use on Fenders. Marshall fault , some chemical getting capilliary fashion down the glass fibres of the composite? |
#8
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![]() This might be the infamous output PCB fault. Keep metering the bias voltage from the 3 pin connector provided, (Should be 90mV both sides). If it starts to climb/runaway in sync with the hum increasing, then that's what the problem is. You will need to buy another PCB from Marshall if so. Gareth. I will check that, I could understand with that old lino or whale hide they used to use on Fenders. Marshall fault , some chemical getting capilliary fashion down the glass fibres of the composite? It is the board material itself that goes faulty. I do not know the exact mechanism, but clearly parts of it become conductive, causing absolute havoc with the bias. The output tubes often get red hot and are destroyed. Unfortunately the first thing the owner tends to do is replace the valves. Which very soon get destroyed again. (Expensive business when it eventually gets fixed properly - that would be 12 x EL34's, the output PCB, and a fair bit of labour). Keep a careful eye on the bias as it warms up, adjusting all the time to keep it less than 90mV each side. The moment you run out of bias adjustment is the moment you have pretty much demonstrated the faulty PCB, and you want to turn it off immediately. Gareth. |
#9
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![]() "N_Cook" wrote in message ... Hum particularly on clean channel builds up from nothing ,over half an hour, and very intrusive 0.3V rms of hum over 8R speaker load and still rising. Putting a signal in Return, for PA only, is fine but opening guitar input hum returns. Owner had replaced all the valves and exactly the same hum. Hum is negligible on the downstream HTs from HT1. What sort of grounding problem increases with warmth? I've not started exploring the low voltage electros around V1 yet, I'm letting the amp cool down Couple of other tips on the DSL and TSL amps: Resolder (and preferably reinforce with wire) the DIN footswitch socket. It is only held in place by the solder joints which often (always) fail in regular use. The back panel PCB mounted mains and HT fuseholders also tend to suffer from dry joints. Whip off the PCB and resolder them. 3 minutes tops. The 16 ohm speaker output is a switched socket that disables the 4 and 8 ohm sockets when used. These often get damaged, becoming a problem when the other sockets are used instead. Check, repair or replace. The speaker ohmage selector switch also tends to suffer from dry joints. (Either of the above 2 problems can destroy the output TX, output valves and other stuff, so best do it) The output mute switch is stupidly relying on the switch contact to feed the signal to the power amp, rather than being a mute. Clean it. Gareth. |
#10
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It would be nice to find a schematic for this specific variant , but only
one Google Ref for the number on the end face label JCM2-20-06 I'm wondering if there is any significance in the bodged 1/3 W R replacements R70 of 5K6 R7 of 5K6 R48 of 100K unfortunately none of these numbers agree with any JCM2000/ DSL schemas that I have. Similarly I cannot find the 2K2 HT line dropper for (a) half of V1 mentioned on these schema |
#11
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![]() "N_Cook" wrote in message ... It would be nice to find a schematic for this specific variant , but only one Google Ref for the number on the end face label JCM2-20-06 I'm wondering if there is any significance in the bodged 1/3 W R replacements R70 of 5K6 R7 of 5K6 R48 of 100K unfortunately none of these numbers agree with any JCM2000/ DSL schemas that I have. Similarly I cannot find the 2K2 HT line dropper for (a) half of V1 mentioned on these schema This site mentions people finding 220k grid resistors (R70, R7 etc) instead of the 5k6 resistors they expected, along with other details with regards to this board problem. Looks like someone may have tried to sort this problem out before, but didn't replace the PCB? http://www.lynx.bc.ca/~jc/TSL122.html Gareth. |
#12
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![]() "Gareth Magennis" Looks like someone may have tried to sort this problem out before, but didn't replace the PCB? http://www.lynx.bc.ca/~jc/TSL122.html ** The PCB material is definitely the cause of the problem. It is made from UTTER CRAP !! However - uber clever fixes involving a Dremel or mounting resistors in mid air or any similar hare brained ideas are all just re-arranging the deck chairs on the SS Titanic. If no new PCB is available from Marshall at a reasonable price - just add a bloody fan !!!!!!!!!!!!!!!!! A single 120mm, 240VAC fan blowing directly on the OP tubes from below cures the overheating / bias runaway problem completely. BTW: Marshall are a total bunch of arseholes. The Pox of the audio industry. Just like their ****ty amps. .... Phil |
#13
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![]() "Phil Allison" wrote in message ... "Gareth Magennis" Looks like someone may have tried to sort this problem out before, but didn't replace the PCB? http://www.lynx.bc.ca/~jc/TSL122.html ** The PCB material is definitely the cause of the problem. It is made from UTTER CRAP !! However - uber clever fixes involving a Dremel or mounting resistors in mid air or any similar hare brained ideas are all just re-arranging the deck chairs on the SS Titanic. If no new PCB is available from Marshall at a reasonable price - just add a bloody fan !!!!!!!!!!!!!!!!! A single 120mm, 240VAC fan blowing directly on the OP tubes from below cures the overheating / bias runaway problem completely. New PCB's ARE available from Marshall, at about £60 to the customer if I buy and fit one. Marshall tell me they sell a lot of them. I understand costs are much higher outside the UK, however. I totally agree the bodges and drilling malarky - the problem IS the PCB material. Charging the customer a whole bunch of labour to bodge a way around this problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been charged loads of money not to fix the real problem. Replacing the PCB is relatively cheap (compared with the alternative labour costs) quick and easy, and is the only logical solution here. Incidentally, 2 old faulty boards I have here and have kept, both have 220k grid stopper resistors instead of the 5k6 the schematic shows. Cheers, Gareth. |
#14
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On Fri, 16 Mar 2012 20:56:53 +1100 "Phil Allison"
wrote in Message id: : just re-arranging the deck chairs on the SS Titanic. *guffaw* Gotta remember that one! |
#15
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![]() "Gareth Magennis" "Phil Allison" "Gareth Magennis" Looks like someone may have tried to sort this problem out before, but didn't replace the PCB? http://www.lynx.bc.ca/~jc/TSL122.html ** The PCB material is definitely the cause of the problem. It is made from UTTER CRAP !! However - uber clever fixes involving a Dremel or mounting resistors in mid air or any similar hare brained ideas are all just re-arranging the deck chairs on the SS Titanic. If no new PCB is available from Marshall at a reasonable price - just add a bloody fan !!!!!!!!!!!!!!!!! A single 120mm, 240VAC fan blowing directly on the OP tubes from below cures the overheating / bias runaway problem completely. New PCB's ARE available from Marshall, at about £60 to the customer if I buy and fit one. Marshall tell me they sell a lot of them. I understand costs are much higher outside the UK, however. ** It's not just the cost in dollars or whatever - availability is crucial too. Even common Marshall power and output transformers ( the ****ty Indian made one ones that sit the lams right on the chassis) are regularly out of stock here in Australia for months on end. New output stage PCBs are not even on the radar !!!!!!!!! I totally agree the bodges and drilling malarky - the problem IS the PCB material. Charging the customer a whole bunch of labour to bodge a way around this problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been charged loads of money not to fix the real problem. Replacing the PCB is relatively cheap (compared with the alternative labour costs) quick and easy, and is the only logical solution here. ** See above. The logical solution is the use a bloody FAN !!!!!!!! Takes less than 1 hour to fit one to any combo model and the job is DONE !! An IEC inlet mounted on the lower back panel supplies AC power to the fan and it is up to the owner to use an extra IEC lead to run it. ..... Phil |
#16
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![]() "Phil Allison" wrote in message ... "Gareth Magennis" "Phil Allison" "Gareth Magennis" Looks like someone may have tried to sort this problem out before, but didn't replace the PCB? http://www.lynx.bc.ca/~jc/TSL122.html ** The PCB material is definitely the cause of the problem. It is made from UTTER CRAP !! However - uber clever fixes involving a Dremel or mounting resistors in mid air or any similar hare brained ideas are all just re-arranging the deck chairs on the SS Titanic. If no new PCB is available from Marshall at a reasonable price - just add a bloody fan !!!!!!!!!!!!!!!!! A single 120mm, 240VAC fan blowing directly on the OP tubes from below cures the overheating / bias runaway problem completely. New PCB's ARE available from Marshall, at about £60 to the customer if I buy and fit one. Marshall tell me they sell a lot of them. I understand costs are much higher outside the UK, however. ** It's not just the cost in dollars or whatever - availability is crucial too. Even common Marshall power and output transformers ( the ****ty Indian made one ones that sit the lams right on the chassis) are regularly out of stock here in Australia for months on end. New output stage PCBs are not even on the radar !!!!!!!!! I totally agree the bodges and drilling malarky - the problem IS the PCB material. Charging the customer a whole bunch of labour to bodge a way around this problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been charged loads of money not to fix the real problem. Replacing the PCB is relatively cheap (compared with the alternative labour costs) quick and easy, and is the only logical solution here. ** See above. The logical solution is the use a bloody FAN !!!!!!!! Takes less than 1 hour to fit one to any combo model and the job is DONE !! An IEC inlet mounted on the lower back panel supplies AC power to the fan and it is up to the owner to use an extra IEC lead to run it. .... Phil OK, I see that in your situation that makes a lot more sense than in mine. Any thoughts on the 220k resistor anomaly? Someone stuffed the component insert machine with the wrong resistors? Bit of a disaster, this PCB. Gareth. |
#17
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I should be able to get back to it in a couple of hours. Only just got
around to building a Dexion support frame so I can toss these sorts of amps around with gay abandon, and not risk the bottles. The valve set that are in here are all 2009 , he took out the new ones and replaced these used ones. They look brand new except for simple dragon's teeth marks, ie not repeatedly moved around. The Russian markings are quite visible and there is no sign of overheating of the bases , still light brown , and the pcb around looks as new. But the first thing I will do is check for o/p bias drift, then heating the pcb with hot air and a 2Gohm megger and then remove V1 and see if hum returns then replace and monitor DCs around V1 on heating , No mention of that Marshall Forum page about hum but it could be these pcb problems around V1 the immediate problem |
#18
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![]() "N_Cook" wrote in message ... I should be able to get back to it in a couple of hours. Only just got around to building a Dexion support frame so I can toss these sorts of amps around with gay abandon, and not risk the bottles. The valve set that are in here are all 2009 , he took out the new ones and replaced these used ones. They look brand new except for simple dragon's teeth marks, ie not repeatedly moved around. The Russian markings are quite visible and there is no sign of overheating of the bases , still light brown , and the pcb around looks as new. But the first thing I will do is check for o/p bias drift, then heating the pcb with hot air and a 2Gohm megger and then remove V1 and see if hum returns then replace and monitor DCs around V1 on heating , No mention of that Marshall Forum page about hum but it could be these pcb problems around V1 the immediate problem The "Hum" I have associated with the PCB problem is ripple due to the huge currents the output valves are taking from the PSU. One pair of valves inevitably draws way more current than the other pair, which again manifests as a hum. The more current the valves draw, the hummier it gets, until things start glowing red, then things start to break and the hum is gone. Yours may not actually have this PCB problem, but the bodged in grid stoppers make me suspicious. Gareth. |
#19
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Gareth Magennis wrote in message
... "N_Cook" wrote in message ... I should be able to get back to it in a couple of hours. Only just got around to building a Dexion support frame so I can toss these sorts of amps around with gay abandon, and not risk the bottles. The valve set that are in here are all 2009 , he took out the new ones and replaced these used ones. They look brand new except for simple dragon's teeth marks, ie not repeatedly moved around. The Russian markings are quite visible and there is no sign of overheating of the bases , still light brown , and the pcb around looks as new. But the first thing I will do is check for o/p bias drift, then heating the pcb with hot air and a 2Gohm megger and then remove V1 and see if hum returns then replace and monitor DCs around V1 on heating , No mention of that Marshall Forum page about hum but it could be these pcb problems around V1 the immediate problem The "Hum" I have associated with the PCB problem is ripple due to the huge currents the output valves are taking from the PSU. One pair of valves inevitably draws way more current than the other pair, which again manifests as a hum. The more current the valves draw, the hummier it gets, until things start glowing red, then things start to break and the hum is gone. Yours may not actually have this PCB problem, but the bodged in grid stoppers make me suspicious. Gareth. main bias results monitoring every 5 minutes for the first 15 minutes rate of increase is falling passing 10 to 15 minutes in one side rising 1.0mV per 5 minutes and the other side 1.5mV per 5 minutes starting from that 15 min reading of 76.9mV and 68.2mV should be about 90mV over an hour . locally heating with hot air (low setting) for 20 seconds the voltages shot up 5 or so mV but soon dropped back to where they were , unlike the problem around V1 it would seem. now for the other preliminary tests |
#20
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No E number of board supplier found but I would say, by trying to press a
needle into the board, it is epoxy rather than polyester composite. Megger showed nothing untoward With no V1 in place , the grid socket pins measure about 10 or 20mV DC, wave hot air over the valve base and the readings shoot up to 100 to 200mV, just like applying a magic wand. The cathode lines have 2/3 orders of magnitude lower resistance to ground so any such effect not so obvious there These are the small bottles not the hotter big bottles, something to do with that metal shield plate for them? I'm assuming the effect is at the valve base through board , hot pins rather than where the anode dropper leads pass through the pcb, and passing through rather than along the surface . I will explore this, as hard wiring the HTs to isolated valve bases ,only, is quite different to hard-wiring all HT traces. Now what is the physics /chemistry of all this ? I assume something hygroscopic is grabbing moisture then forming a conductive salt that stays within the micropores of the surface of the glass fibres, but why temperature increasing the conductivity , what salt has highly temp dependent conductivity, we're only talking 50 degree C or so |
#21
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For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with electrical isolation , as well as all that hard wiring and intermediary isolation/ mounting points. Looks like too much of a work up in comparison to paying Mr Marshall for a replacement board. I'm just wondering if this particular board , only 1 google ref, that they will not have a replacement |
#22
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![]() "N_Cook" wrote in message ... For those thru-board valve bases it looks as though you would have to replace with standoff types to retain structural integrity but with electrical isolation , as well as all that hard wiring and intermediary isolation/ mounting points. Looks like too much of a work up in comparison to paying Mr Marshall for a replacement board. I'm just wondering if this particular board , only 1 google ref, that they will not have a replacement I think replacements are generic, i.e. many different models share the same output PCB. Its the number of channels and other such stuff that are the variants. Problem with repair, is that you might repair your V1 symptoms, but 6 months later it comes back with the massively more disappointing bias destruction problem. Or vice versa. The whole PCB is possessed, it needs to be exorcised. It would be interesting if you could establish the physics and chemistry behind all this though! Cheers, Gareth. |
#23
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N_Cook wrote:
Hum particularly on clean channel builds up from nothing ,over half an hour, and very intrusive 0.3V rms of hum over 8R speaker load and still rising. Putting a signal in Return, for PA only, is fine but opening guitar input hum returns. Owner had replaced all the valves and exactly the same hum. Hum is negligible on the downstream HTs from HT1. What sort of grounding problem increases with warmth? I've not started exploring the low voltage electros around V1 yet, I'm letting the amp cool down Well, that's where you will find the problem, almost for sure. Hum that increases over time is very likely to be bad electrolytic caps. Jon |
#24
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Gareth Magennis wrote in message
... "N_Cook" wrote in message ... For those thru-board valve bases it looks as though you would have to replace with standoff types to retain structural integrity but with electrical isolation , as well as all that hard wiring and intermediary isolation/ mounting points. Looks like too much of a work up in comparison to paying Mr Marshall for a replacement board. I'm just wondering if this particular board , only 1 google ref, that they will not have a replacement I think replacements are generic, i.e. many different models share the same output PCB. Its the number of channels and other such stuff that are the variants. Problem with repair, is that you might repair your V1 symptoms, but 6 months later it comes back with the massively more disappointing bias destruction problem. Or vice versa. The whole PCB is possessed, it needs to be exorcised. It would be interesting if you could establish the physics and chemistry behind all this though! Cheers, Gareth. Why did the mod-bods choose to isolate the grid pins rather than the anode pins? Anyone know if they use mineral filler (cheaper) in the epoxy of epoxy pcb manufacture. Like the use of calcium carbonate in the epoxy bulking/fixing of toroidal transformers or even car body repair filler. This morning I made a test cell of some calcium carbonate and water to thick paste consistency in an inch wide plastic bottle cap. Resistance across a diameter about 60K. Waft a low-setting hot air gun over it and resistance drops to about 2K, now rising again. So moisture/condensation can get into the edges and component holes of such a pcb and the glass dutifully conduct it capilliary fashion. Perhaps a cure might be a low oven bake of a day at 105 deg C of a populated board , assuming nothing comes to grief at that temp and then some sprayed on conformal coatinf along all edges and component leads, but that would not get to the prime source of problems , inside /under the valve bases and those pcb holes |
#25
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Jon Elson wrote in message
... N_Cook wrote: Hum particularly on clean channel builds up from nothing ,over half an hour, and very intrusive 0.3V rms of hum over 8R speaker load and still rising. Putting a signal in Return, for PA only, is fine but opening guitar input hum returns. Owner had replaced all the valves and exactly the same hum. Hum is negligible on the downstream HTs from HT1. What sort of grounding problem increases with warmth? I've not started exploring the low voltage electros around V1 yet, I'm letting the amp cool down Well, that's where you will find the problem, almost for sure. Hum that increases over time is very likely to be bad electrolytic caps. Jon I can see run away bias , less neagative grids with temperature rise being caused by conductive epoxy. But as I say (for the moment) this amp PA seems ok, but where is the hum coming from around V1? some conductive path from the heaters? |
#26
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For an owner who knows he has such an amp, this would be worth doing.
Obtaining or making an amp sized heavy duty polythene bag that can be resealed easily and some sachets of activated silica gel crystals and a large jam/pickled gherkin jar to keep the oven acivated ones in, until use. After each use of the amp , place amp while still warm prefereably ,in the bag with a fresh sachet from the storage jar. |
#27
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![]() "N_Cook" wrote in message ... Gareth Magennis wrote in message ... "N_Cook" wrote in message ... For those thru-board valve bases it looks as though you would have to replace with standoff types to retain structural integrity but with electrical isolation , as well as all that hard wiring and intermediary isolation/ mounting points. Looks like too much of a work up in comparison to paying Mr Marshall for a replacement board. I'm just wondering if this particular board , only 1 google ref, that they will not have a replacement I think replacements are generic, i.e. many different models share the same output PCB. Its the number of channels and other such stuff that are the variants. Problem with repair, is that you might repair your V1 symptoms, but 6 months later it comes back with the massively more disappointing bias destruction problem. Or vice versa. The whole PCB is possessed, it needs to be exorcised. It would be interesting if you could establish the physics and chemistry behind all this though! Cheers, Gareth. Why did the mod-bods choose to isolate the grid pins rather than the anode pins? Anyone know if they use mineral filler (cheaper) in the epoxy of epoxy pcb manufacture. Like the use of calcium carbonate in the epoxy bulking/fixing of toroidal transformers or even car body repair filler. This morning I made a test cell of some calcium carbonate and water to thick paste consistency in an inch wide plastic bottle cap. Resistance across a diameter about 60K. Waft a low-setting hot air gun over it and resistance drops to about 2K, now rising again. So moisture/condensation can get into the edges and component holes of such a pcb and the glass dutifully conduct it capilliary fashion. Perhaps a cure might be a low oven bake of a day at 105 deg C of a populated board , assuming nothing comes to grief at that temp and then some sprayed on conformal coatinf along all edges and component leads, but that would not get to the prime source of problems , inside /under the valve bases and those pcb holes I'd be willing to donate one of my faulty PCBs if you sincerely thought it might help you to further establish the mechanisms involved here. My email reply address is valid. Gareth. |
#28
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Gareth Magennis wrote in message
... "N_Cook" wrote in message ... Gareth Magennis wrote in message ... "N_Cook" wrote in message ... For those thru-board valve bases it looks as though you would have to replace with standoff types to retain structural integrity but with electrical isolation , as well as all that hard wiring and intermediary isolation/ mounting points. Looks like too much of a work up in comparison to paying Mr Marshall for a replacement board. I'm just wondering if this particular board , only 1 google ref, that they will not have a replacement I think replacements are generic, i.e. many different models share the same output PCB. Its the number of channels and other such stuff that are the variants. Problem with repair, is that you might repair your V1 symptoms, but 6 months later it comes back with the massively more disappointing bias destruction problem. Or vice versa. The whole PCB is possessed, it needs to be exorcised. It would be interesting if you could establish the physics and chemistry behind all this though! Cheers, Gareth. Why did the mod-bods choose to isolate the grid pins rather than the anode pins? Anyone know if they use mineral filler (cheaper) in the epoxy of epoxy pcb manufacture. Like the use of calcium carbonate in the epoxy bulking/fixing of toroidal transformers or even car body repair filler. This morning I made a test cell of some calcium carbonate and water to thick paste consistency in an inch wide plastic bottle cap. Resistance across a diameter about 60K. Waft a low-setting hot air gun over it and resistance drops to about 2K, now rising again. So moisture/condensation can get into the edges and component holes of such a pcb and the glass dutifully conduct it capilliary fashion. Perhaps a cure might be a low oven bake of a day at 105 deg C of a populated board , assuming nothing comes to grief at that temp and then some sprayed on conformal coatinf along all edges and component leads, but that would not get to the prime source of problems , inside /under the valve bases and those pcb holes I'd be willing to donate one of my faulty PCBs if you sincerely thought it might help you to further establish the mechanisms involved here. My email reply address is valid. Gareth. A day on of drying out and my test cell showed 150K, warming easily brought it down to 10K. Tried a sample of calcium carbonate as it is , powder straight from the open bag a decade old at least , and no response to a megger. My chemistry failed. 30 percent HCl showed no fizzing with the powder. Warming up and a lighted taper extinguished in the tube. Ground off a sample from this Marshall board and the same. But then tried a tube of HCl on its own and same extinguishing of taper. I'll stick with the day job. |
#29
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My test cell now measures 500M cold or warm , so perhaps not calcium
carbonate |
#30
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Now trying a test cell mix of NaCl and calcium carbonate, as no convenient
source of calcium chloride to see if a mix of those , via deliquescence, will go thermally conductive without addition of liquid water. Until anyone gets more info I will go with chloride contaminated calcium carbonate as the suspect pcb filler As nothing is learnt by handing over money to Mr Marshall, modded this main board. As this 06 variant is almost a google-whack I doubt an exact replacement board is available ,off the shelf, anyways. Missing opto devices etc Now I've worked out how to make a 5mm diameter hollow end face cutter , to go in a Dremmel. Once the main board is disconnected ( how many connections?) it is now a simple matter of cutting away a neat hole of pcb around the grid socket pins. And while at it, did so to all 12 grids ECC83 and EL34 and hard wired with silicone sleeving , so all grids are now isolated from the pcb. Now no trace of any hum over normal background even over an hour and also no sensitivity at all from blowing hot air around V1 or the other valves. Just leaves the mystery of what the exact hum intrusion mechanism was , something to do with DC+AC to V1 and V2 heaters? Now all that is left is to get more history from the owner, why and what Marshall had farted about with when returned to them before, other than the all too obvious cut and fly 1/3 W resistor replacements. |
#31
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In article , N_Cook wrote:
Now I've worked out how to make a 5mm diameter hollow end face cutter , to go in a Dremmel. Diamond-tipped hollow drill bits/saws are commonly available (they're used for punching holes in ceramics, gemstones, tiles, etc.) and they work quite nicely on PCB material... they make nice pad-cutters. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#32
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Dave Platt wrote in message
... In article , N_Cook wrote: Now I've worked out how to make a 5mm diameter hollow end face cutter , to go in a Dremmel. Diamond-tipped hollow drill bits/saws are commonly available (they're used for punching holes in ceramics, gemstones, tiles, etc.) and they work quite nicely on PCB material... they make nice pad-cutters. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! Nothing so grand as that, I will look out for those, but I was unaware of them. Hollow cutting tool Particularly for Marshall conductive epoxy pcb boards To isolate valve base pins by cutting into the board around a pin. A steel sleeve , this one dimensions internal 4mm and external 5.3mm . Cut eight small cuts on one end to make a castellated form with Dremmel and .6mm disc. Grind a rake angle behind each cutting edge. Place a rod inside the other end to mount in a drill chuck. Start at an off axis angle and bring up to axial after first cutting. Leave soldered joint in place. As this was actually perhaps a roll pin rathe rthan a sleeve, not a solid ring in plan, an axial join line along its length. Found a use for a Dremmel mandrel where the screw is sheared off inside the stem. With free hand grinding with the mandrel rotating in a sleeve and grinding against a disc brought the diameter of the mandrel support down to a tight fit inside this sleeve. Then a plastic filler for the other end and fitted in a Dremmel. Perhaps more reliable , repeat with a good mandrel , find a longer screw, and some compressible silicone sleeving to grip the inside of the sleeve like those Dremmel sanding cylinder holders. Not necessarily punch through as will be ragged anyway. With a dart point excavate around while desoldering the pin . Owner had previoiusly returned the amp to Marshall for changing those resistors and general maintainence only. He has exactly the same amp and components back , only difference is a few holes in the pcb and wiring path a bit different. |
#33
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I just realised the ideal starting form for such a tool would be one of
thise jeweller's screwdriver nut spinners. Grind back the internal hex form, castellation cuts and cut through the stem to mount in a chuck |
#34
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20 percent NaCl with calcium carbonate seems to show the undesired affect
over a few days so maybe the Marshall contaminated boards have 1 or 2 percent contamination. Pitty no Chinese? E number on their boards so we could tell if they turn up with other makers products, but perhaps not with valve voltages. So after a few days the dry test cell showed about 500M and over a few hours dropped to 80K or so ,maybe the Megger high V across the probes accelerating this effect , and light heating with hot air , drops to 40K or so, returning to 80K or so soon after. Much the same today as yesterday |
#35
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![]() "N_Cook" wrote in message ... As nothing is learnt by handing over money to Mr Marshall .............. I knew you wouldn't buy one. ![]() Good luck. Gareth. |
#36
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Marshall didn't tell the owner about changing those resistors . Recent hum
problem developed over about 10 hours total of gigs until it became too intrusivce and embarrassing |
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