Vintage Kenwood amp repair

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The amp in question is a KAC1021

Symptoms are - blowing in line fuse, hissing sound and smoke coming from within the amp before inline fuse blows.

the output transistors were tested against a working 1021 and a few were proved to be out of tolerance, so I pulled the lot.

the power supply transistors and diodes checked out fine.

after pulling the output devices I powered the amp up to test the power supply, a few resistors in what I believe to be the pre-amp stage went up in smoke behind one of the TA2030 devices.

R77 completely burnt
R69 burnt
R79 burnt

I can match the components from my other amp, my question is, could the damage be a result of the TA2030 failure?

if so how can I test it before ordering the parts to repair?

thanks in advance


What I wanted to investigate was what may have caused the damage to the resistors and the TA2030 IC? - could the blown outputs have caused the damage or vice versa?

I can replace all the damaged components no problem, but I don't want to replace them to find they all fail again.

So if you can help Perry that would be great thank you.
The diagram for the M2A is a bit better than for the car amp. pwramp.pdf

I didn't notice earlier but the 1021 as well as the M2A use multiple rails and two different sets of outputs for each half of the waveform/signal. In the 1021, there is a single transistor for each half that operates at low power. When the signal increases above a certain amplitude, the driver IC switches on the outputs that are used for high power.
Thank you Perry and Dan for the previous replies.

The story so far I had another 1021 (call it 1021-B) which had worked fine around 12 months ago, I powered it up yesterday and it displayed exactly the same symptoms as the 1021 I attempted to repair earlier on this thread. We'll call it (1021-A)

Symptoms - amp began cycling, the amp would power up then shut down and then it began to blow my 10 amp inline test fuse.

So I took the cover off and found IC3 was hot and smelt like it was getting too hot.
This got me thinking, the dead 1021-A also had issues around IC3.

I had stopped the repair of 1021-A as I wasn't convinced the Hong Kong TA's were originals but I had kept hold of the amp.

I figured there may be a pattern here where IC3 somehow fails as it's too much of a coincidence that both amps have issues in the same area.

So I pulled IC4 off 1021-A and installed it in 1021-B in the place of IC3, I also pulled Q24 and Q26 from 1021-A and installed them on 1021-B in place of Q23 and Q25.

The amp powered up fine, no blown fuses and it sounded ok on both channels but not quite 100%. After a few minutes the volume began to roll back and the audio began to distort. Both IC3 and IC4 were giving off a lot of heat so I powered it down.

What could be causing the issue here? as I said IC3 is where it all started on both amps. Would be great if I can save at least one of these 1021's

Thanks in advance
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.