Hi zaib4tsu,
Now you know why technicians are skilled. They aren't just parts changers. You've replaced everything in that circuit you can think of so far.
Go back to the schematic and look at your voltage readings. You have some open resistors, even if they look okay, they can be open.
You should stop everything at this point until you get a variac (not a lamp current limiter!). You will need an oscilloscope, and a good DVM. Cheap meters will not indicate bias current reading accurately when you are measuring a few mV. That and you need a soldering station, not a hobby iron. Soldering guns and line powered irons can leak AC current and damage circuitry. If you have a bad one, you can blow the new transistors.
Final question. Where did you buy your parts from? If Ebay, yank them and start over with real parts. Buy your parts from a main supplier like Digikey, or from Yamaha. Don't even think of scrounging parts from other pieces of equipment. If you used replacement brands, like NTE or ECG, yank them and discard. Use only real parts with the actual part number on them. You can substitute transistors if you know what you are doing.
I used to do warranty service for Yamaha during that time period. Many other brands too. A lot of our work was repairing equipment from bad technicians and people at home with no training. All I can say is that you should learn the basics before jumping in on a repair like this. But I can't tell you what to do with your own equipment. It's just a shame when otherwise good gear hits the trash due to service issues. Often times, your best friend and the least expensive fix is using a good technician. One who specializes in audio equipment. TV, computer and other repair people don't have the training or equipment to do a proper job. They will argue that point, but I don't fix TVs, because I'm not trained for it. Enough said.
-Chris
Now you know why technicians are skilled. They aren't just parts changers. You've replaced everything in that circuit you can think of so far.
Go back to the schematic and look at your voltage readings. You have some open resistors, even if they look okay, they can be open.
You should stop everything at this point until you get a variac (not a lamp current limiter!). You will need an oscilloscope, and a good DVM. Cheap meters will not indicate bias current reading accurately when you are measuring a few mV. That and you need a soldering station, not a hobby iron. Soldering guns and line powered irons can leak AC current and damage circuitry. If you have a bad one, you can blow the new transistors.
Final question. Where did you buy your parts from? If Ebay, yank them and start over with real parts. Buy your parts from a main supplier like Digikey, or from Yamaha. Don't even think of scrounging parts from other pieces of equipment. If you used replacement brands, like NTE or ECG, yank them and discard. Use only real parts with the actual part number on them. You can substitute transistors if you know what you are doing.
I used to do warranty service for Yamaha during that time period. Many other brands too. A lot of our work was repairing equipment from bad technicians and people at home with no training. All I can say is that you should learn the basics before jumping in on a repair like this. But I can't tell you what to do with your own equipment. It's just a shame when otherwise good gear hits the trash due to service issues. Often times, your best friend and the least expensive fix is using a good technician. One who specializes in audio equipment. TV, computer and other repair people don't have the training or equipment to do a proper job. They will argue that point, but I don't fix TVs, because I'm not trained for it. Enough said.
-Chris
Hi Chris,
Thanks for the response - but I have zero intention of giving this one to a technician. I want to learn (I already have a working one to use) and have only posted this thread since I am at the limit of my abilities and google search results. There are certainly numerous parts on the channel that could still be problematic, and I'm interested in getting better at the process of figuring it out.
If you actually read my whole post first you would have seen this:
I've actually had the other identical amp fail twice now (including an entire channel blow-up of outputs / drivers / pre-drivers / related components - through my own fault) and I've successfully repaired it each time (with help from wonderful forum members). The failure on this one seems to go a bit deeper but hopefully someone knowledgeable here can help.
Regarding the schematic - I have already consulted it and have checked all parts around the failure regions. There is an obvious problem of large negative voltages on the pre-drivers of the bad channel. It's the cause for this I can't figure out.
In any case - I would appreciate anyone helping taking the time to read my whole post.
Thanks for the response - but I have zero intention of giving this one to a technician. I want to learn (I already have a working one to use) and have only posted this thread since I am at the limit of my abilities and google search results. There are certainly numerous parts on the channel that could still be problematic, and I'm interested in getting better at the process of figuring it out.
If you actually read my whole post first you would have seen this:
I've only used (genuine) replacements verified by experienced repairmen on AK, and I've purchased my Sanken outputs (not in place yet) from chip1stop in Japan - the authorized distributor.
I've actually had the other identical amp fail twice now (including an entire channel blow-up of outputs / drivers / pre-drivers / related components - through my own fault) and I've successfully repaired it each time (with help from wonderful forum members). The failure on this one seems to go a bit deeper but hopefully someone knowledgeable here can help.
Regarding the schematic - I have already consulted it and have checked all parts around the failure regions. There is an obvious problem of large negative voltages on the pre-drivers of the bad channel. It's the cause for this I can't figure out.
In any case - I would appreciate anyone helping taking the time to read my whole post.
Hi zaib4tsu,
Sorry, I missed some of your first comments. Late night posting while tired.
Do you have :
I don't intend on being harsh, but I have trained technicians for years and run a large repair facility for 16 years before I sold it. You might not like what I have said, but I'm only being truthful and realistic. No matter how determined you might be, it is possible that your desires don't line up with your abilities.
Learning. So what have you learned so far by "shotgunning" this amplifier? You replaced all components, which hasn't taught you much. To actually learn, you needed to troubleshoot down to the actual fault. Not to throw parts at it in hopes that it would be fixed.
Not knowing what your substitutions were, have you made certain that the pinouts of the replacements are the same as the original parts? I mean, have you made certain, not just asked if the subs were okay?
You're the one learning, not the experienced people around who are trying to help you. I'm trying to lead you to the answer, and if you follow my previous suggestion in the post before this one, you would probably have fixed this amplifier already. Note: To check components accurately, they must be disconnected from the circuit.
-Chris
Sorry, I missed some of your first comments. Late night posting while tired.
That's fair enough. But, be aware that you can't do everything just because you want to. You might be further ahead to put this one aside and replace it. Come back to it in a few years after you have learned more. Forcing yourself will possibly end up with a junk amplifier. Most people starting out have trouble with soldering and PCB damage.
Okay, but you haven't taken my advice yet.
Do you have :
- oscilloscope
- good DVM
- Soldering station
- Variac
- Source of sine waves
- distortion analyser
I don't intend on being harsh, but I have trained technicians for years and run a large repair facility for 16 years before I sold it. You might not like what I have said, but I'm only being truthful and realistic. No matter how determined you might be, it is possible that your desires don't line up with your abilities.
Learning. So what have you learned so far by "shotgunning" this amplifier? You replaced all components, which hasn't taught you much. To actually learn, you needed to troubleshoot down to the actual fault. Not to throw parts at it in hopes that it would be fixed.
Not knowing what your substitutions were, have you made certain that the pinouts of the replacements are the same as the original parts? I mean, have you made certain, not just asked if the subs were okay?
You're the one learning, not the experienced people around who are trying to help you. I'm trying to lead you to the answer, and if you follow my previous suggestion in the post before this one, you would probably have fixed this amplifier already. Note: To check components accurately, they must be disconnected from the circuit.
-Chris
I am not hugely offended by your generic (and quite presumptuous) responses, but I posted this thread in hopes of meeting someone who may happen to have a lot of specific experience with this particular amplifier (A-700 or its similar brother the A-1000).
At least some suggestions on which specific resistors or non-electrolytic capacitors might be prone to failure in this particular amp given the locations (that I provided) of the bad voltages and the specific failed components would help me a lot. I could then test them with the testing tools which I have available. I know this forum has many kind members who offer this type of advice.
I respect that you have many more years of experience and have access to much more expensive tools than I. It's also possible I misconstrued the title of this website ("diy Audio"), my apologies if so.
I would kindly appreciate if you could just let my thread run its course.
At least some suggestions on which specific resistors or non-electrolytic capacitors might be prone to failure in this particular amp given the locations (that I provided) of the bad voltages and the specific failed components would help me a lot. I could then test them with the testing tools which I have available. I know this forum has many kind members who offer this type of advice.
I respect that you have many more years of experience and have access to much more expensive tools than I. It's also possible I misconstrued the title of this website ("diy Audio"), my apologies if so.
I would kindly appreciate if you could just let my thread run its course.
A preview for you of our upcoming Netiquette section.You are not the center of cyberspace
There are many people on the forum who will be willing to offer help. This is mostly done in the spirit of good will and helping the hobby.
...be gracious and try to understand the help that is being offered.
Hi zaib4tsu,
I am trying to help you. However, I'm not going to troubleshoot this for you because you need to know so much more. You haven't said anything about the equipment you have access to, and truthfully, you can not fix this amplifier without this equipment.
I worked on many of these amplifiers and have a lot of experience with this and many other models.
I'm trying to reach you in some way to make you understand. I told you flat out what to check, and you have not done it yet. So if you aren't willing to put in some effort, you can sink for all I care. Helping you isn't possible.
Second paragraph in post #2. Get off your duff and check these things, or don't you even have a meter?
-Chris
I am trying to help you. However, I'm not going to troubleshoot this for you because you need to know so much more. You haven't said anything about the equipment you have access to, and truthfully, you can not fix this amplifier without this equipment.
I worked on many of these amplifiers and have a lot of experience with this and many other models.
I'm trying to reach you in some way to make you understand. I told you flat out what to check, and you have not done it yet. So if you aren't willing to put in some effort, you can sink for all I care. Helping you isn't possible.
Second paragraph in post #2. Get off your duff and check these things, or don't you even have a meter?
-Chris
https://www.hifiengine.com/manual_library/yamaha/a-700.shtml
If you don't have a hifiengine account, I also tried to upload it here:
http://docdro.id/v98ONxr
If you can suggest what specific points or components I should test and with some explanation I would appreciate it. The relay is not clicking.
I have an iron, a meter (fluke 117), a component tester. I can get my hands on a scope but would prefer to try everything possible before having to do that.
This is the transistor that is overheating:
An externally hosted image should be here but it was not working when we last tested it.
Thanks kindly
No I do not, if you have the time I would like to learn how, and then where/how to take measurements.
The amp has switchable Class A (so if the switch is not engaged it will never engage the high bias)... default operation is Class AB. For testing purposes I've just always left the switch turned off, which should already disengage the "Auto A" circuit components.
However maybe you mean something else.
Thanks again for your time
Have you already looked at my voltage measurements btw?
I wonder if they narrow down where the problem might be?
Complementary transistor pairs A129/C127 are both -20V on B/E legs when they should be close to 0V. A113/C115 pair have -20V on C leg when it should be around 1V.
(from comparing bad to good channel: A129/C127 = A130/C128, and A113/C115 = A114/C116)
I wonder if they narrow down where the problem might be?
Complementary transistor pairs A129/C127 are both -20V on B/E legs when they should be close to 0V. A113/C115 pair have -20V on C leg when it should be around 1V.
(from comparing bad to good channel: A129/C127 = A130/C128, and A113/C115 = A114/C116)
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Try the following (or go to the beach);
An externally hosted image should be here but it was not working when we last tested it.
Things may change when 'variables' are removed.
Good Luck!
EDIT:
The supplied voltage measurements suggest that 'something' is wrong. Vbe should be approximately +/-0V666!
Some of your supplied measurements suggest Vbe=0V. N.B. Many of the manufacturer supplied voltages marked on the circuit diagram are incorrect - +/-70V @ the emitters of TR139 & TR141 would ensure total destruction of ALL output components (and surrounding areas)!
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I will throw in my 2 cents I am far from a expert, if you have a fluke multi meter its pricey but a worthy investment, I am suspecting a faulty resistor or small cap, the variac is a very worthy investment it might even power the amp up at a very low voltage and let you find the bad part, by finding out whats hot and whats not lol, anyway
There are a few things that you can do to narrow down a problem without even applying power or removing anything, employing just a multimeter:
If a resistor measures high in circuit, it's toast.
Use the diode test. If a diode or transistor p-n junction measures dead short, verify what's in parallel but generally expect it to be toast, too. Same for C-E, electrolytics or other caps (ceramics or film are rarely dead but sometimes pigs do fly).
Visually inspect the soldering, old Yamahas are notorious for bad joints. Also watch out for the Fixing Glue of Death around big caps and the like, which if turned black has an appetite for component legs. I think this model is old enough to potentially still be affected... made '83-'84, IIRC it was up to '84-ish.
Transistors can be tested a bit more thoroughly out of circuit, but even a part that looks OK there may still be bad - for one thing, you generally won't be subjecting it to the full C-E voltage it sees in circuit. Tubes are no different.
Some parts like the capacitor in the feedback loop have a tendency to be damaged in a fault condition.
Complex DC-coupled feedback circuits are pretty nasty to troubleshoot because the fault can be almost anywhere, and overlooking one single bad diode or something can make things blow up again.
I will second the advice against shotgunning parts and for sticking with reliable component suppliers. Buying from random Chinese suppliers on eBay is like playing the lottery.
I'd say diode test the parts you removed, and if you find any dead ones, consult the schematic on what else they may have taken with them. Form a theory on what may have gone wrong in the first place. Did a transistor just die out of the blue, or was the output stage overbiased?
Didn't look at your voltages now, have to go mount /dev/bed0 RSN.
If a resistor measures high in circuit, it's toast.
Use the diode test. If a diode or transistor p-n junction measures dead short, verify what's in parallel but generally expect it to be toast, too. Same for C-E, electrolytics or other caps (ceramics or film are rarely dead but sometimes pigs do fly).
Visually inspect the soldering, old Yamahas are notorious for bad joints. Also watch out for the Fixing Glue of Death around big caps and the like, which if turned black has an appetite for component legs. I think this model is old enough to potentially still be affected... made '83-'84, IIRC it was up to '84-ish.
Transistors can be tested a bit more thoroughly out of circuit, but even a part that looks OK there may still be bad - for one thing, you generally won't be subjecting it to the full C-E voltage it sees in circuit. Tubes are no different.
Some parts like the capacitor in the feedback loop have a tendency to be damaged in a fault condition.
Complex DC-coupled feedback circuits are pretty nasty to troubleshoot because the fault can be almost anywhere, and overlooking one single bad diode or something can make things blow up again.
I will second the advice against shotgunning parts and for sticking with reliable component suppliers. Buying from random Chinese suppliers on eBay is like playing the lottery.
I'd say diode test the parts you removed, and if you find any dead ones, consult the schematic on what else they may have taken with them. Form a theory on what may have gone wrong in the first place. Did a transistor just die out of the blue, or was the output stage overbiased?
Didn't look at your voltages now, have to go mount /dev/bed0 RSN.
Thanks everyone for the suggestions!
I am away from the amp for 3 weeks but I'll make a list and try everything when I return, and post updated voltages if anything changes.
I am guessing that if lifting R213 corrects the wild voltages, the problem is in the DC servo section (related to IC104); and if lifting D147 fixes it the problem is in the variable class A section (related to IC105). If lifting both of those produces no change then the problem is outside both sections. Have I understood that correctly? If the problem is in either of those sections would it be likely that the relevant IC is to blame - or unlikely since the wild voltages are only on one channel?
I already have a fluke meter (117), and from my basic knowledge I suspected the same as you - a failed resistor or small cap (non-electrolytic since I've replaced all those). Transistor A113 is overheating like I said, but the cause is elsewhere since it's tested good (and I've replaced to be sure). I'm not sure how a variac would help isolate the problem in this case - maybe you could explain? I can keep my eye out for one.
I understand the in-circuit tests (and associated parallel complications) regarding transistors/diodes/resistors, however could you explain how you test caps in-circuit? Can you also share which capacitor/s are in my feedback loop (if any non-electrolytics)? Do you think there's a tendency for the green film caps parallel to burnt fusistors to fail? For example C191 (which I haven't tested) next to dead R301. I also don't know how to test ICs.
I have already addressed most of your general suggestions, but just so they don't keep getting repeated:
I am away from the amp for 3 weeks but I'll make a list and try everything when I return, and post updated voltages if anything changes.
Thanks so much for that. Saved your pic.Try the following (or go to the beach);
An externally hosted image should be here but it was not working when we last tested it.Things may change when 'variables' are removed.
EDIT:
The supplied voltage measurements suggest that 'something' is wrong. Vbe should be approximately +/-0V666!
N.B. Many of the manufacturer supplied voltages marked on the circuit diagram are incorrect - +/-70V @ the emitters of TR139 & TR141 would ensure total destruction of ALL output components (and surrounding areas)!
I am guessing that if lifting R213 corrects the wild voltages, the problem is in the DC servo section (related to IC104); and if lifting D147 fixes it the problem is in the variable class A section (related to IC105). If lifting both of those produces no change then the problem is outside both sections. Have I understood that correctly? If the problem is in either of those sections would it be likely that the relevant IC is to blame - or unlikely since the wild voltages are only on one channel?
I appreciate the 2 cents!
I already have a fluke meter (117), and from my basic knowledge I suspected the same as you - a failed resistor or small cap (non-electrolytic since I've replaced all those). Transistor A113 is overheating like I said, but the cause is elsewhere since it's tested good (and I've replaced to be sure). I'm not sure how a variac would help isolate the problem in this case - maybe you could explain? I can keep my eye out for one.
Thanks, I've reflowed every solder joint already (with fresh solder) so that is OK. Traces seem OK (have been checking with meter in continuity mode). Can you explain how to test the servo opamp (IC104 I think)?
I understand the in-circuit tests (and associated parallel complications) regarding transistors/diodes/resistors, however could you explain how you test caps in-circuit? Can you also share which capacitor/s are in my feedback loop (if any non-electrolytics)? Do you think there's a tendency for the green film caps parallel to burnt fusistors to fail? For example C191 (which I haven't tested) next to dead R301. I also don't know how to test ICs.
I have already addressed most of your general suggestions, but just so they don't keep getting repeated:
- I didn't blindly shotgun the channel, anatech just assumed this during his unhelpful ego trip. I really do appreciate how helpful everyone else has been!
I tested for dead parts first, shorted/failed parts were:
TR139/TR141 (outputs)
TR135/TR137 (drivers)
TR135/TR137 (predrivers)
TR113/TR115 (predrivers?) from memory
TR125 (bias transistor) from memory
R111, R113, R301 (fusistors)
Please let me know if this tells you anything combined with my voltages. - I only went and swapped the rest of the parts since replacing the above didn't fix it (and I suspected, wrongly, that some dynamic misbehaviour of a semiconductor might be the issue). I diode tested everything further that I removed and nothing else was shorted to my knowledge. The bad voltages didn't change after replacing the remainder.
- Solder joints all checked / reflowed
- Parts all genuine (from mouser), nothing from ebay
- Part matches verified by multiple sources, and I have used them before in my other identical amp with no problem.
- Glue removed, corrosion removed, and all electrolytics replaced
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