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Problem with Trio/Kenwood KA-80

Hi,

I have for many years owned a Trio KA-80 integrated amplifier as the heart of my hifi system. I believe this is the same as a Kenwood KA-80. It has recently stopped working. It works OK when playing through headphones. But when playing through speakers, whenever the volume is higher than very low, there is a click and the sound from the speakers stops completely. If you then turn down the volume, or the music gets quieter, there is another click and the speakers come live again. I have a feeling it is loud low frequency sounds which tend to trip it. I deduce that a relay is tripping, but I have no idea why, or why this has started to happen spuriously. Has anyone experienced these symptoms, and know the cause? Or can you suggest what experiments I should undertake to get to the root of the problem? I am reasonably technical, and have a multimeter and a scope, but I am not an audio engineer.

Thanks - Rowan



Problem with Trio/Kenwood KA-80

It seems to me that your KA-80 is more than 20 yrs. old, and at this stage parts ie. caps, semiconductors, eventually starts to become less effective.
If you hear the click again try this, without turning the volume down leave it there.
1. check the voltage at r161 or r163
2. if you get less than 3volts chances are the amp is still alright, large voltage swing ignore the following process, check for cold solder joints.
3. now turn the volume down, there will be a click again, leave it there, unhook one of your speaker's + L or +R does not matter which do you prefer, we are troubleshooting, now connect your alligator patch cable to either r161 or r163 (this is the output path) with the other end to +speaker wire, (we are bypassing the protection circuit)
4.Now remember the speaker is connected directly to the output path before the protection circuit, the speaker is now not protected, and if you could connect 1A fuse in between the patch cable going to your speaker better yet.
5. Anyhow turn the the volume up until you hear the click, once you hear the click; you can turn the volume more up and start working on your protection circuit, you have localized the problem.

Problem with Trio/Kenwood KA-80

Thank you very much for your helpful replies.

Yes, it may well be over 20 years old. On opening it up and inspecting the PCB, I see that there is some heat damage. Two resistors that I think are R93 and R94 (they measure about 520 ohms) have obviously been overheating for some time, and have charred the PCB, and are both now loose in their holes, and probably not making contact at all. I will try replacing any components that seem to have been damaged, and repair the PCB, and see what happens. If it still doesn't work after repair and adjustment, I will come back for more help. Of course, the reason that these two resistors are overheating may well be a fault elsewhere. Any thoughts on what is likely to be causing this?

Thanks - Rowan

Problem with Trio/Kenwood KA-80

Your protection relay is working. This usually means that DC is present on the audio output in one or both channels. It could be leaky transistors or that the bias is not correct; the bias controls could be dirty. Some of the capacitors in the amp could be bad also. Could also be bad solder connections.
If you clean the bias controls I recommend that you memorize where they are set, clean them, then return them to the original positions. Do this with power off.
I just looked at the schematic and your amp uses power modules for outputs; one or both may be bad. You can still clean the bias controls to see if this helps. That is a direct-coupled amp, so any problem from the pre-amp to the amplifier stage could cause this symptom; capacitors could be bad. The service manual is here in the library, but your problem may need a technician to solve it.