Hi all, this is a very hard one. For me...
I'm servicing for a friend a set of Kenwood L-07M power amps. (Mark I). Unfortunately, and for unknown reason in my knowledge they keep failing. First, when he bought them he had both amps running and after 2 1/2 days, one amp blowned its fuse. Looked into it and found few outputs and one driver short. Replaced them, look for other parts in the amp cct and everything was looking right. He brought it back, had it to play for a few hours and it did the same thing? I then decided to replace outputs; I've used MJ21193 and 21194 for replacement and replaced all lytics caps for ELNAs. He brought it back again and ...failed again after a day of use. But this time, only the fuse was burned, no shorted output. Probably these new O/Ps are stronger then the original ones.
What is causing that? Can it be some diodes? Does it starts oscillating?
Anyhbody worked on this amp had problems? I'm quite desperate and don't know where to go now? How come the amp can work for several hours before failing?
I have the SM. Joined some schematics.
I'm servicing for a friend a set of Kenwood L-07M power amps. (Mark I). Unfortunately, and for unknown reason in my knowledge they keep failing. First, when he bought them he had both amps running and after 2 1/2 days, one amp blowned its fuse. Looked into it and found few outputs and one driver short. Replaced them, look for other parts in the amp cct and everything was looking right. He brought it back, had it to play for a few hours and it did the same thing? I then decided to replace outputs; I've used MJ21193 and 21194 for replacement and replaced all lytics caps for ELNAs. He brought it back again and ...failed again after a day of use. But this time, only the fuse was burned, no shorted output. Probably these new O/Ps are stronger then the original ones.
What is causing that? Can it be some diodes? Does it starts oscillating?
Anyhbody worked on this amp had problems? I'm quite desperate and don't know where to go now? How come the amp can work for several hours before failing?
I have the SM. Joined some schematics.
Check and make sure the bias cct is working and set to the proper value. As it worked for a bit and then failed again does sound like thermal runaway.
All voltages, bias current, DC balance are set properly not to say perfectly. The amp doesn't heat almost at all. Get a bit warm after some hours at not too much volume. Normal.
Can it be the "triple" diodes in the bias cct?.
To be honest, my friend has 4 of these amps and except for one they are doing the same thing...
I thought replacing the lytics would solve the problem. Being the only aging parts.
Are diodes aging also?
Can it be the "triple" diodes in the bias cct?.
To be honest, my friend has 4 of these amps and except for one they are doing the same thing...
I thought replacing the lytics would solve the problem. Being the only aging parts.
Are diodes aging also?
also make sur there are no frayed speaker wired shorting the outputs.
Each time an amp fails, damage can propagate further and further back, so if all faulty parts are not replaced, further failures is a fact.
Each time an amp fails, damage can propagate further and further back, so if all faulty parts are not replaced, further failures is a fact.
Each time I opened the unit for repair I made sure ALL components on the amp board were tested. All resistors all diodes. Can't be any speaker wire, the amps were installed and not moving at all.
Electrochap,
Check the zobel network Rf35/Rf37 and Cf9. If any of the components are gone or out of spec, the amplifier may be oscillating leading to the output destruction. It happened to me once with my Nikko amplifier, after replacing the outputs, it was good for some time and output were gone later. I found the capacitor in the zobel network is open.
Thanks
Check the zobel network Rf35/Rf37 and Cf9. If any of the components are gone or out of spec, the amplifier may be oscillating leading to the output destruction. It happened to me once with my Nikko amplifier, after replacing the outputs, it was good for some time and output were gone later. I found the capacitor in the zobel network is open.
Thanks
Surprised no one has asked what sort of loads these amplifiers are connected to.. Using highly capacitive long speaker cables or speakers whose load impedance falls below the safe minimum (over some range of frequency) for the amplifier could cause these sorts of problems.
Another common cause is lack of ventilation, causing the amp to overheat and blow up.
Most ppl dont realize this and shove the amp into a hole not much bigger than the amp itself with no air holes what so ever.
Or put other equipment on top that doesent have tall enough feet to leave enuf room for hot air to come out.
Most ppl dont realize this and shove the amp into a hole not much bigger than the amp itself with no air holes what so ever.
Or put other equipment on top that doesent have tall enough feet to leave enuf room for hot air to come out.
Quite a puzzler. Normally new electrolytics (esp. Ce3/Ce4) should get things stable again.
Not sure how one would troubleshoot a flaky triple diode. I suppose a bulb tester, a dummy load with voltage divider and a scope would come in handy.
Not sure how one would troubleshoot a flaky triple diode. I suppose a bulb tester, a dummy load with voltage divider and a scope would come in handy.
Why not visit your customer and find out what the amplifiers are driving?
Something caused the initial and subsequent amplifier failures.
The fact that one of the amps was returned with a blown fuse while the output transistors have survived might indicate that there is a load impedance problem.
Something caused the initial and subsequent amplifier failures.
The fact that one of the amps was returned with a blown fuse while the output transistors have survived might indicate that there is a load impedance problem.
SPeakers on these are Spendor BC3s now. They were driving QUAD electrostatics before. We tried them on many different speakers.
Before getting to me, he had another set of L07M that the same problem. I also replaced outputs a few times with same results. He then bought another set who were coming "in the box", "with the books" pretty cleaned and serviced.
funny note: one unit was getting more hot then the other when he bought them... After some troubleshooting I've found, in the bottom board of the amp, in the voltage regulation cct a 5532 opamp (or a 4558 I don't remember, not important). It was a 8 pins IC that was replacing an old TO92 ( is it TO92 looking like flying saucer?) and the tech didn't bother to verify the pinout which was wrong and was causing all sorts of wrong voltage, unable to adjust the 63 volts, 1rst thing to align in the manual... Good tech...
They can't heat; they are on the floor in the middle of the living room ( and they don't get "warm" either). And sometimes, they blow without playing anything, just powered on. Also, I hope the way these are built, this size of amp for 150 Watts they could drive quite low loads...
Before getting to me, he had another set of L07M that the same problem. I also replaced outputs a few times with same results. He then bought another set who were coming "in the box", "with the books" pretty cleaned and serviced.
funny note: one unit was getting more hot then the other when he bought them... After some troubleshooting I've found, in the bottom board of the amp, in the voltage regulation cct a 5532 opamp (or a 4558 I don't remember, not important). It was a 8 pins IC that was replacing an old TO92 ( is it TO92 looking like flying saucer?) and the tech didn't bother to verify the pinout which was wrong and was causing all sorts of wrong voltage, unable to adjust the 63 volts, 1rst thing to align in the manual... Good tech...
They can't heat; they are on the floor in the middle of the living room ( and they don't get "warm" either). And sometimes, they blow without playing anything, just powered on. Also, I hope the way these are built, this size of amp for 150 Watts they could drive quite low loads...
Kenwood L-07M II Issues - AudioKarma.org Home Audio Stereo Discussion Forums, probably the best way to get them working.
Yop I'm looking into that now. But it is more on the MK II unit but at where I am now(and price of these small caps is not an issue) I have nothing to. Loose.
I'll also look at this "Zobel" cct too. Thanks for these suggestions.
I'll get my results here in a few days.
I'll also look at this "Zobel" cct too. Thanks for these suggestions.
I'll get my results here in a few days.
Bias changing on L-07m?
Wondered how you made out and what you found on your L-07m. I also have one amp exhibiting a similar symptom. I have it on the bench currently, no load, and when powered up cold, the bias as measured ( should be the 25mV) starts out too low, 16-18mV, then gradually creeps up over time as the amp heats up. I can adjust it down, to a point, but then will suddenly "jump" to 120mV or more as I get too close to the end of adjust range. It does appear that something in the circuit has the wrong temp co. Anyway, if let alone, it will eventually have too high bias and pop the fuse. All other parts seem to be fine, and the amp work fine if I could keep trimming the bias to keep it low ?!
I have replaced the 3 element diode on the driver trans, as it appeared to read a forward voltage of around 0.7V or so - too low. The new part looks the same but has different number. There are 3 of these in the bias control circuit, one on the drive transistor, the other two are just on the PCB close by. I suspect these parts, but not sure about repair as this diode - STV3HY is unobtainium. SO far I have replaced the drive transistors, 3 element diodes with "supposed" replacements, and the 500ohm adjust pot.
The amp turns on and is fine - for a while. Then the bias eventually cannot be set down to 25mV, and the amp will overheat and pop the fuse. If these 3 element diodes are really the culprit, we need a source for real replacements.
Wondered how you made out and what you found on your L-07m. I also have one amp exhibiting a similar symptom. I have it on the bench currently, no load, and when powered up cold, the bias as measured ( should be the 25mV) starts out too low, 16-18mV, then gradually creeps up over time as the amp heats up. I can adjust it down, to a point, but then will suddenly "jump" to 120mV or more as I get too close to the end of adjust range. It does appear that something in the circuit has the wrong temp co. Anyway, if let alone, it will eventually have too high bias and pop the fuse. All other parts seem to be fine, and the amp work fine if I could keep trimming the bias to keep it low ?!
I have replaced the 3 element diode on the driver trans, as it appeared to read a forward voltage of around 0.7V or so - too low. The new part looks the same but has different number. There are 3 of these in the bias control circuit, one on the drive transistor, the other two are just on the PCB close by. I suspect these parts, but not sure about repair as this diode - STV3HY is unobtainium. SO far I have replaced the drive transistors, 3 element diodes with "supposed" replacements, and the 500ohm adjust pot.
The amp turns on and is fine - for a while. Then the bias eventually cannot be set down to 25mV, and the amp will overheat and pop the fuse. If these 3 element diodes are really the culprit, we need a source for real replacements.
STV3HY replace
Thanks, however these are the replacements I used. They do not have the same part # as original diodes, and of course the thermal issue remains...
Thanks, however these are the replacements I used. They do not have the same part # as original diodes, and of course the thermal issue remains...
L-07m update
OK, looks like a bit more complicated... I tested the thermal diodes with hot water and monitored the change in Vbe with a controlled current. As far as I can tell, all the ones I have are pretty similar, match expected values, and should adjust.
Another strange symptom is if I touch the speaker outputs or wiggle the rear housing, the bias adjust voltage suddenly jumps to 120 140mV. Sounds like connection issue, but for the life of me I cannot locate a possible culprit. The closest I have determined is the copper ground strap seems to need a REALLY good solder point. Additional soldering here seems to have helped reliability a bit.
I will be going through this in more detail & report.
Incidentally, jiggling the bias adjust PCB itself does nothing - no change in voltage, the "issue" is somewhere else...Not sure if this relates to the original thread problems, but seems suspicious...
OK, looks like a bit more complicated... I tested the thermal diodes with hot water and monitored the change in Vbe with a controlled current. As far as I can tell, all the ones I have are pretty similar, match expected values, and should adjust.
Another strange symptom is if I touch the speaker outputs or wiggle the rear housing, the bias adjust voltage suddenly jumps to 120 140mV. Sounds like connection issue, but for the life of me I cannot locate a possible culprit. The closest I have determined is the copper ground strap seems to need a REALLY good solder point. Additional soldering here seems to have helped reliability a bit.
I will be going through this in more detail & report.
Incidentally, jiggling the bias adjust PCB itself does nothing - no change in voltage, the "issue" is somewhere else...Not sure if this relates to the original thread problems, but seems suspicious...
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