Problems with Kenwood L-07M power amp.

[electrochap]

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.

[Tekko]

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.

[electrochap]

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?

[Tekko]

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.

[electrochap]

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.

[routhun]

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

[kevinkr]

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.

[Tekko]

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.

[sgrossklass]

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.

[Frank Berry]

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.