Vintage Kenwood amp repair

[vincegironda]

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

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[Perry Babin]

Have you found a copy of the service manual?

 

[vincegironda]

Hi Perry,
no I don't have one.

 

[Perry Babin]

The only one I could find was $17.

 

[vincegironda]

can you send me the link and I'll buy it? or I can send you the $17 save me sending a copy to you?

 

[Perry Babin]

Your one source for service manuals

The service manual isn't yet scanned (they scan the old paper manuals as needed when an original digital version isn't available). The quality is pretty good usually. They don't work weekends (last I checked) so it wouldn't be available until late Monday or Tuesday. You can email them to confirm.

 

[vincegironda]

Thanks Perry I'll give them a try on Monday.

 

[Charles01]

Hi, you can also check at HiFi engine, they have lots of schematics, unfortunately they don't always have the one we need but worth a look. I have found many schematics there. You need to register but its free which is even better

 

[vincegironda]

Hi Perry I have a copy of the service manual now. Shall I send you a copy?

I also ordered 2 x TA2030's, all the output transistors and the blown resistors.

 

[Perry Babin]

Not unless you need help with it.

 

[vincegironda]

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.

 

[Perry Babin]

[email protected]

 

[Perry Babin]

Received the manual.

Did you check the failed TA2030s to see if the pins connected to the burned components are shorted to any other pins of the IC?

Where did you find replacement 2030s? Ebay isn't a good source. Too many counterfeits.

 

[vincegironda]

I haven't completed any further checks yet but I'll take a look later today.

The TA2030s are from an ebay seller in Hong Kong, feedback looked good and they weren't expensive.

I'll test them against my working 1021 IC's when they arrive.

 

[Perry Babin]

I doubt that the HK part will be genuine.

According to Kenwood Parts, the TA2031 is a sub. I can't find the datasheets to compare the two. It appears that kenwood parts has the 2031 but they're expensive ($35 each).

 

[Dan Mattis]

Kenwood Basic M2

I have information on the Kenwood Basic M2 parts. Please send me a message.

Dan

 

[Perry Babin]

The diagram for the M2A is a bit better than for the car amp.
http://www.bcae1.com/temp/Kenwood-BasicM2A 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.

 

[vincegironda]

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

 

[Perry Babin]

Is it possible that the bias transistors have intermittent solder connections?

 

[vincegironda]

Thanks Perry,
there were quite a few weak solder connections on a few transistors on the board with IC3 and IC4. I re soldered the lot, could one of them be defective as a result of weak solder joints?