Ron Elliot used to have a good amp repair article but it has disappeared from his website https://www.sound-au.com/articles/index.htm
If you private message me an email listing (do not post publicly) I may send you a text of it.
I repair amps with a DVM, an analog VOM, and an FM radio for a signal generator. No scope. I detect oscillation with the analog VOM with an AC scale and a coupler cap. Significant AC voltage, no sound, probably oscillation. If AC signal goes through .390 pf couple cap, is ultrasonic. Usual couple cap is .047 uf 400 v.
I find blown transistors can read good on the diode scale at 2 v test voltage, and bad at the voltage the amp puts on them. Of course transistors & diodes bad with the DVM (short or other than .55=.75 voltage drop forwards, not infinity backwards) don't need the more complicated test. I test for leakage current with a 24 v supply, a 47kohm resistor, some clip leads. Ice=24v/47000, transistor is blown. Ice= microamps, transistor is still good.
My stable of repair parts includes MJ21194/95 TO3, MJE15032/33 TO220, MJE340/350 TO126, MPS8098/99 or 2n5401/5551 TO92. There are no replacements for TO5 parts: I bought some military surplus TO5 parts from a surplus house. Old Crown used some no longer available op amps in the front end, hope you don't need one of those.
Add newark.com to list of reliable suppliers. I use their tenma DVM now. They sometimes have TO18 transistor replacments for TO5 (EBC) which can be equivalent if heat sinked well enough.
The old Crown stuff can be reliable, but if investing in expensive speakers, an amp with DC detection & disconnect can save a $280 woofer. One amp had a bad solder joint that had fooled 3 repairmen, the last of which labeled one channel DO NOT USE because it whanged to +180 vdc so often. I repaired a couple of Peavey CS800s (one stolen) and would buy a CS800x if the freight is not rediculous. (70 lb, cannot be too far away). I'm now re-ecapping a QSC CX302 that worked great when bought but has too much old rubber to be reliable if I donate it to a church.
Basic safety, never use 2 hands with the power on or before the mains caps leak down to <2 v. >25 v from one hand to the other can stop your heart. Use an alligator clip lead on the meter negative. Wear no jewerly on hands wrists or neck, 1v at high current through metal can burn your flesh to charcoal. Wear safety glasses, solder splashes and parts can explode. Test working amps with a 75 w tungsten light bulb series the AC line to prevent exploding parts. Don't touch the mains area with the power on. Don't touch 200 v caps or any part ahead of the transformer in switcher supplies.
If you private message me an email listing (do not post publicly) I may send you a text of it.
I repair amps with a DVM, an analog VOM, and an FM radio for a signal generator. No scope. I detect oscillation with the analog VOM with an AC scale and a coupler cap. Significant AC voltage, no sound, probably oscillation. If AC signal goes through .390 pf couple cap, is ultrasonic. Usual couple cap is .047 uf 400 v.
I find blown transistors can read good on the diode scale at 2 v test voltage, and bad at the voltage the amp puts on them. Of course transistors & diodes bad with the DVM (short or other than .55=.75 voltage drop forwards, not infinity backwards) don't need the more complicated test. I test for leakage current with a 24 v supply, a 47kohm resistor, some clip leads. Ice=24v/47000, transistor is blown. Ice= microamps, transistor is still good.
My stable of repair parts includes MJ21194/95 TO3, MJE15032/33 TO220, MJE340/350 TO126, MPS8098/99 or 2n5401/5551 TO92. There are no replacements for TO5 parts: I bought some military surplus TO5 parts from a surplus house. Old Crown used some no longer available op amps in the front end, hope you don't need one of those.
Add newark.com to list of reliable suppliers. I use their tenma DVM now. They sometimes have TO18 transistor replacments for TO5 (EBC) which can be equivalent if heat sinked well enough.
The old Crown stuff can be reliable, but if investing in expensive speakers, an amp with DC detection & disconnect can save a $280 woofer. One amp had a bad solder joint that had fooled 3 repairmen, the last of which labeled one channel DO NOT USE because it whanged to +180 vdc so often. I repaired a couple of Peavey CS800s (one stolen) and would buy a CS800x if the freight is not rediculous. (70 lb, cannot be too far away). I'm now re-ecapping a QSC CX302 that worked great when bought but has too much old rubber to be reliable if I donate it to a church.
Basic safety, never use 2 hands with the power on or before the mains caps leak down to <2 v. >25 v from one hand to the other can stop your heart. Use an alligator clip lead on the meter negative. Wear no jewerly on hands wrists or neck, 1v at high current through metal can burn your flesh to charcoal. Wear safety glasses, solder splashes and parts can explode. Test working amps with a 75 w tungsten light bulb series the AC line to prevent exploding parts. Don't touch the mains area with the power on. Don't touch 200 v caps or any part ahead of the transformer in switcher supplies.
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Hi indianajo,
In all fairness, you don't have the equipment you actually do need. You would never be authorized for warranty service on even simple amplifiers with what you have, never mind anything more complicated.
Yes, a brain and experience can carry you a ways, but you need answers you are sure of, and you are not. There is no argument here, I get stuff all the time fresh from "service shops" where the distortion "was okay because they can hear it". Well, some over 1 % THD, clearly failing spec. Most failing spec by the way. Many oscillating, on and on.
If you don't have sufficient equipment, you shouldn't be allowed to work on these things. Period. I'm not trying to pick a fight, but this has been proved time and time again. If you aren't aware of something, everything looks just great. You need the equipment to be aware of things.
In all fairness, you don't have the equipment you actually do need. You would never be authorized for warranty service on even simple amplifiers with what you have, never mind anything more complicated.
Yes, a brain and experience can carry you a ways, but you need answers you are sure of, and you are not. There is no argument here, I get stuff all the time fresh from "service shops" where the distortion "was okay because they can hear it". Well, some over 1 % THD, clearly failing spec. Most failing spec by the way. Many oscillating, on and on.
If you don't have sufficient equipment, you shouldn't be allowed to work on these things. Period. I'm not trying to pick a fight, but this has been proved time and time again. If you aren't aware of something, everything looks just great. You need the equipment to be aware of things.
I understand UK & EU do not allow consumers to repair their appliances. Every used appliance has to go to the dump or a factory authorized repair shop. Perhaps Canada mothers its citizens also. USA is still a free country. I find amps with 1% distortion like my ST70 sound a lot *****ier than 0.1% HD amps on grand piano, cymbals, tinkly bells.
I do not sell my services, I give them away. I've upgraded several geriatric organs & amps donated to various churches, without the $XXXXX the pros charge. $1000 entry fee to repairing my amps & speakers would have left me listening to a $20 radio with a 2" speaker most of the years when I made 1/3 the median salary. I still can't afford a $80000 Allen organ, but I repaired a silent one that went for ~$15000 in 1980.
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You speak two different languages in two different universes.
Chris is a repairer approved by several brands with expensive equipment and skills to use it, you repair with the means at hand and experience and I am sure that even in the middle of the dessert with a small spoon and a yaw you will be able to repair everything, these are two different universes.
I quite agree with Chris, most of the time on simple devices and known schematics, I do without advanced test materials, but when the amp is more complex or simpler but each component is important, I bring out the heavy artillery to check what I'm doing because an oscillating amp can work very well for a few weeks or months and in certain conditions until the conditions or use change and then the magic smoke appears.
No one is wrong, no one is right, just two different universes.
Chris is a repairer approved by several brands with expensive equipment and skills to use it, you repair with the means at hand and experience and I am sure that even in the middle of the dessert with a small spoon and a yaw you will be able to repair everything, these are two different universes.
I quite agree with Chris, most of the time on simple devices and known schematics, I do without advanced test materials, but when the amp is more complex or simpler but each component is important, I bring out the heavy artillery to check what I'm doing because an oscillating amp can work very well for a few weeks or months and in certain conditions until the conditions or use change and then the magic smoke appears.
No one is wrong, no one is right, just two different universes.
And no one forbids repairs in Europe/UK or in the world (from what I know), everyone does what they want, it's only on cars and trucks that it's becoming more and more more complicated in Europe with pollution standards, for example, with my vehicle, I can no longer enter most of the regional capitals, I will nevertheless keep this vehicle which I can repair endlessly.
Hi indianajo,
What is important is the expectation of the person who's equipment you are repairing is. Even at no charge, which is commendable, the person who owns the equipment believes and expects everything to be done correctly, and that the equipment is in fact operating as the manufacturer intended. Otherwise they wouldn't allow you to work on it. Wouldn't you agree?
The proper operating performance of the equipment isn't the issue at all. Sure I work on equipment that doesn't measure great, but it still has to meet whatever specs the manufacturer indicates it should. Otherwise it's broken. I also listen to tube equipment (and I design tube amps as well as solid state), some simply doesn't sound very good overall. I have repaired several ST-70 amps that were "improved", and sounded sort of okay. After properly servicing them they sounded much better, measured better and customers noticed. They also stopped failing.
The fact is very simple. A listening test does not ensure the equipment is working properly, in fact you can have a great deal of distortion way above where it should be and it may sound "fine" on a listening test. Depends on your speakers too. You have to face the fact that without proper test equipment, there are several things you simply cannot know about the equipment.
Yes, I make several repairs that don't require much in the way of test equipment. However, once done I measure the performance and I sometimes find other real problems.
What is important is the expectation of the person who's equipment you are repairing is. Even at no charge, which is commendable, the person who owns the equipment believes and expects everything to be done correctly, and that the equipment is in fact operating as the manufacturer intended. Otherwise they wouldn't allow you to work on it. Wouldn't you agree?
The proper operating performance of the equipment isn't the issue at all. Sure I work on equipment that doesn't measure great, but it still has to meet whatever specs the manufacturer indicates it should. Otherwise it's broken. I also listen to tube equipment (and I design tube amps as well as solid state), some simply doesn't sound very good overall. I have repaired several ST-70 amps that were "improved", and sounded sort of okay. After properly servicing them they sounded much better, measured better and customers noticed. They also stopped failing.
The fact is very simple. A listening test does not ensure the equipment is working properly, in fact you can have a great deal of distortion way above where it should be and it may sound "fine" on a listening test. Depends on your speakers too. You have to face the fact that without proper test equipment, there are several things you simply cannot know about the equipment.
Yes, I make several repairs that don't require much in the way of test equipment. However, once done I measure the performance and I sometimes find other real problems.
Unfortunately, (or fortunately depending on how one looks at it) people like Indianajo and myself are often the last resort for most of what we “service”. When its either we do what we can or it ends up in the dumpster. If the client was going to spend big money to get an amp fixed he would have already done so. Various reasons - maybe just dont have it, ain’t gonna pay $800 when a new one is $200, proper replacement parts are unavailable, or perhaps was told it was too far gone and just junk it. I’ve rebuilt my share of PA amps and blown home stereos when all I had was a multimteter, uncalibrated single trace green screen, and a dim bulb. My experience was that when operating points are where they are supposed to be, replacement parts are CAPABLE of performing adequately (SOA, speed compatibility, gain, noise, matching), and it doesn’t misbehave with a full signal under normal load it’s as “fixed” as it’s going to get. I may or may not be attempting to fix a DESIGN issue at the same time. And I’ve run into plenty. Still true today, even with more test equipment available. I still don’t have a distortion analyzer, noise figure rig, spectrum analyzer, 1 GHZ analog scope or .0001% distortion sine wave generator that’s frequency calibrated to an equal fraction of a hertz. I live with what my DSO, un calibrated Tek analog boat anchor scope, sig gen/freq counter, variac AND dim bulb, half a dozen multimeters, and various home brew test jigs can do. If somebody is worried whether their amp will meet .000x% and 112dB S/N they can always take it to someone who can verify that. They are going to want real money for it. As long as I’m not forced to use 4 MHz MJ15024’s where unobtainium ring emitter or LAPT types used to be there is usually no good reason for it NOT to meet spec if normal behavior is observed.
Well, another view.
Most people are aware most servicers are hacks destroying equipment.
The replacement cost is not what you can buy a used one for. That is the biggest fallacy. Great, you bought one in questionable condition that might work.
When you service something, you service it to the level of performance it was intended for. Yes, you can improve some things. However there is never any excuse as to why proper service isn't done. I'm sorry, there isn't. Incorrect service can take a repairable unit and make it scrap, possibly destroying speakers in the process. By the way, some speakers burn quite well, so you can burn a house down and kill people. I have seen this happen (not the kill people, but a house being burnt).
There is never any excuse to work on something when you can't do a proper job. A misinformed person needs educating, not hack work that can be excused. If parts are not available, you either properly engineer a modification, or the unit can't be fixed - by you.
If someone is too cheap to repair something, encourage them to put it in the hands of someone who will. Everyone wants something for nothing.
Now for my final comment on value. Got an old good product that costs money to repair. Okay, look at the current replacement value for something of like performance and build quality. Hmmm, all of a sudden that expensive repair is worth it.
Most people are aware most servicers are hacks destroying equipment.
The replacement cost is not what you can buy a used one for. That is the biggest fallacy. Great, you bought one in questionable condition that might work.
When you service something, you service it to the level of performance it was intended for. Yes, you can improve some things. However there is never any excuse as to why proper service isn't done. I'm sorry, there isn't. Incorrect service can take a repairable unit and make it scrap, possibly destroying speakers in the process. By the way, some speakers burn quite well, so you can burn a house down and kill people. I have seen this happen (not the kill people, but a house being burnt).
There is never any excuse to work on something when you can't do a proper job. A misinformed person needs educating, not hack work that can be excused. If parts are not available, you either properly engineer a modification, or the unit can't be fixed - by you.
If someone is too cheap to repair something, encourage them to put it in the hands of someone who will. Everyone wants something for nothing.
Now for my final comment on value. Got an old good product that costs money to repair. Okay, look at the current replacement value for something of like performance and build quality. Hmmm, all of a sudden that expensive repair is worth it.
I've been watching both the OP's recent threads, and it's fantastic to watch. I've learned a lot. It's their journey, but I can empathize and echo a few things.
As a consumer, generally, we haven't the foggiest of clues as to whether something is repaired to specifications or not. If it didn't make music when it entered the shop, but makes music when it leaves, is it "fixed"? I like to consider myself reasonably well-informed, but with the exception of automobiles and associated "authorized repair centers", I don't know of much else that (as a consumer) can give someone confidence that they've chosen a 'reputable' tech for their audio gear. From my own perspective, I got into acquiring some additional test gear for a few reasons;
1) to try and fix a few things that personally I wouldn't have paid to get fixed, but can be used as learning tools and not go directly to the dumpster
2) to see if my DIY builds were actually performing the way the designer(s) intended. I couldn't live with my old ears just being the judge. If I didn't like an amplifier that others seem to love, did I build it wrong?
Today... in my area, I (minutes ago) did an Internet search for "electronics repair shop". I found no one (based on advertising on the web alone) to whom I'd take any of my stereo gear. Asking the manufacturer for authorized warranty repairmen for certain brands used to be a nice option, but now they seem to be all but dried up. We are quickly moving toward disposable electronics it seems. That's not intended to be divisive or controversial. It's one person's observation.
So, to see someone that not only wants to repair a piece of equipment and is willing to learn and aspiring to be a tech... is inspiring to me. I'm fortunate that a close friend's father is part of a group that sets up a table locally at a 'Flea Market' each month to repair everything from microwaves to TVs etc. All that is asked is a donation. The volunteer repair team is made up largely of retired techs from local industries. Sitting beside them and watching how they methodically go through a device to troubleshoot and repair is a wonder in and of itself.
Cheers
As a consumer, generally, we haven't the foggiest of clues as to whether something is repaired to specifications or not. If it didn't make music when it entered the shop, but makes music when it leaves, is it "fixed"? I like to consider myself reasonably well-informed, but with the exception of automobiles and associated "authorized repair centers", I don't know of much else that (as a consumer) can give someone confidence that they've chosen a 'reputable' tech for their audio gear. From my own perspective, I got into acquiring some additional test gear for a few reasons;
1) to try and fix a few things that personally I wouldn't have paid to get fixed, but can be used as learning tools and not go directly to the dumpster
2) to see if my DIY builds were actually performing the way the designer(s) intended. I couldn't live with my old ears just being the judge. If I didn't like an amplifier that others seem to love, did I build it wrong?
Today... in my area, I (minutes ago) did an Internet search for "electronics repair shop". I found no one (based on advertising on the web alone) to whom I'd take any of my stereo gear. Asking the manufacturer for authorized warranty repairmen for certain brands used to be a nice option, but now they seem to be all but dried up. We are quickly moving toward disposable electronics it seems. That's not intended to be divisive or controversial. It's one person's observation.
So, to see someone that not only wants to repair a piece of equipment and is willing to learn and aspiring to be a tech... is inspiring to me. I'm fortunate that a close friend's father is part of a group that sets up a table locally at a 'Flea Market' each month to repair everything from microwaves to TVs etc. All that is asked is a donation. The volunteer repair team is made up largely of retired techs from local industries. Sitting beside them and watching how they methodically go through a device to troubleshoot and repair is a wonder in and of itself.
Cheers
You report that all output devices are short circuit.
Have you measured each output device removed from the amplifier?
If not, your conclusion is not valid.
Have you measured each output device removed from the amplifier?
If not, your conclusion is not valid.
Hi ItsAllInMyHead,
Yes, and I don't want this to get dragged too far off topic.
I think it is excellent to fix stuff for yourself. It's a great way to learn and you do need a few things in order to learn by knowing what is going on. But as Clint Eastwood would say "a man has to know his limitations". That's all fair. I do draw the line at repairing something beyond real simple stuff for anyone else because you do open yourself up to liability - legally. When you do fix something for another person, you do owe it to them to dfo it right, or tell them it's beyond you. That includes hanging a picture if you can't do that. That's life.
You are absolutely correct. People will use stuff until they can't. Either it stops working, or sounds too bad to listen to in this case. So as long as it makes reasonable noise, people think it has been done correctly. I have had a few of these come in because people were curious and it is amazing what people will accept for performance. Once it is fixed properly, they can hear the difference and are themselves shocked as to what they accepted. Some folks are much pickier, but many are happy if it makes sound and they don't know any better. That's how all these hacks get away with having a good reputation. The general population are starting to wise up (took long enough).
I just saved an amplifier after an "expert" with a web site and good reputation caused a ton of damage. He didn't fix it, but it took an additional 9 hours to repair his damage on what was a 2 to 3 hour service job. It was a tough problem, but this guy was into two power amp boards where the fault wasn't, and I can see he was panicking. He tore up traces, installed parts backwards and incorrect parts ... he even removed a zener in a regulator allowing it to run full B+ to circuits not designed to run at those voltage levels. So he is an extreme example.
A small mistake in a bias circuit can blow up an amplifier, maybe even cause the thermal to open in the power transformer. It doesn't take much to cost someone else a lot of money, intended or not. If it's you, you learn.
Yes, and I don't want this to get dragged too far off topic.
I think it is excellent to fix stuff for yourself. It's a great way to learn and you do need a few things in order to learn by knowing what is going on. But as Clint Eastwood would say "a man has to know his limitations". That's all fair. I do draw the line at repairing something beyond real simple stuff for anyone else because you do open yourself up to liability - legally. When you do fix something for another person, you do owe it to them to dfo it right, or tell them it's beyond you. That includes hanging a picture if you can't do that. That's life.
You are absolutely correct. People will use stuff until they can't. Either it stops working, or sounds too bad to listen to in this case. So as long as it makes reasonable noise, people think it has been done correctly. I have had a few of these come in because people were curious and it is amazing what people will accept for performance. Once it is fixed properly, they can hear the difference and are themselves shocked as to what they accepted. Some folks are much pickier, but many are happy if it makes sound and they don't know any better. That's how all these hacks get away with having a good reputation. The general population are starting to wise up (took long enough).
I just saved an amplifier after an "expert" with a web site and good reputation caused a ton of damage. He didn't fix it, but it took an additional 9 hours to repair his damage on what was a 2 to 3 hour service job. It was a tough problem, but this guy was into two power amp boards where the fault wasn't, and I can see he was panicking. He tore up traces, installed parts backwards and incorrect parts ... he even removed a zener in a regulator allowing it to run full B+ to circuits not designed to run at those voltage levels. So he is an extreme example.
A small mistake in a bias circuit can blow up an amplifier, maybe even cause the thermal to open in the power transformer. It doesn't take much to cost someone else a lot of money, intended or not. If it's you, you learn.
@968driver I am not making any strong conclusions, I am new here, so I am just posting what I do know with my limited knowledge and trying to learn and figure out where to go next.
I have been a little tied up and have to catch up on the thread, but I have not removed them as of yet. I did purchase a nice heath kit transistor tester so I am waiting to get that and will test them once installed and then once removed, because, why not 🙂
I have been a little tied up and have to catch up on the thread, but I have not removed them as of yet. I did purchase a nice heath kit transistor tester so I am waiting to get that and will test them once installed and then once removed, because, why not 🙂
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Thank you for your kind words. Take advantage of that group! I will be honest the techs in my area, two that I know of are very backed logged. So, like you mentioned finding one close let alone competent can be difficult! I do love this vintage hifi so I really am driver to learn as much as I can and keep this alive. I even thought it would be nice once I retire to have a small business and between nice and when I retire, why not learn and practice as much as I can 🙂
I have to say @anatech has been probably one of the kindest people i have ever encountered (and I do not mean that to be demeaning to anyone else here) I just mean he has given me so much of his time in the last few days answering my questions that I am already eternally grateful! And also, to everyone else, even if differing opinions, it is nice to see the different sides and there is so much to learn from the conversations i have been seeing on all of my posts, so thank you everyone so far!!!
Also, I am currently on the hunt for a nice scope but also gathering some other equipment. I have a nice list put together and a little money to play with (thank you work for the bonus) haha, so I will post my progress and hopefully one day when I get a nice bench put together I can share a photo!
Thank you all
Well in 9/2020 the local housepainter for cash carried off every bit of electronics I owned, including the DVM, the analog meter, the soldering iron and the FM radio. All electronic components, resistors, capacitors, transistors, IC's, all leaded parts, wire, connectors. If I had had $5000 in test equipment it would have all gone to the fence too. On top of $30000 in musical instruments, LPs, CDs, DVD's, mechanic tools. Took 4 jeep cherokee loads over 3 days. The broken scope was left because the top was off it, but the scope probes were stolen.
If you load up your test room, better invest in a 24 hour armed guard. One who hasn't taken a bribe from the syndicate. The thieves out at my country property use diesel truck/semitrailers and acetelyne cutting torches. The owner of Weld-rite had his burglar alarm equipped garage cut through the metal roof with a gasoline 12" abrasive saw.
One more tip on the Crown D150. You probably can't buy the front end IC. Best of luck.
If you load up your test room, better invest in a 24 hour armed guard. One who hasn't taken a bribe from the syndicate. The thieves out at my country property use diesel truck/semitrailers and acetelyne cutting torches. The owner of Weld-rite had his burglar alarm equipped garage cut through the metal roof with a gasoline 12" abrasive saw.
One more tip on the Crown D150. You probably can't buy the front end IC. Best of luck.
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I have been a hardware developer for a lifetime and my impression from professional repair guys is that their knowledge often is quite limited so they do not understand all the details of the circuits they try ro repair. (Anatech excluded whom I think is really competent). But the vast majority has some experience, are unwilling to learn more and without deep insight things are fixed in a poor manner. And this is independent of the equipment they use. My point is that you need a ton of knowledge and this enables you to fix appropriately without a ton of sophisticated equipment.
this is not technical, but my daughter and I built these speakers for her dorm room when she went off to college. If you notice, they are driven by your same amp. It sounded great.
Sounds like you got ripped off pretty bad, maybe the wrong side of town? No offense but I am pretty sure I am not the only guy investing in test equipment so I think I will hold off on the armed guards for now, I will start with some home insurance and make sure and include my possessions 🙂
Thanks for the tip on the crown, I will do some research on that IC!! Hoping for the best!