Kenwood ka 8004 strange problem - low rumbling noise in just one channel

[Gronard]

Hi,

I have a strange problem with my Kenwood Ka 8004 amplifier. When all the filters are set to off there is a low rumbling noise in the right channel. When I switch on the 20 hz low filter the noise is gone and the amp is quiet.

When I switch the filter back to off the relay will switch off for a second and switch back on again and the noise is back.

I have already recapped it and have replaced the transistors.

Any have an idea of what could be the problem?

Best regards,

Bernard

 

[mrk229]

Bernard,

When did you recap and replace transistors? We need a little more information to help you pinpoint the cause. The more basic info the better.

Did the noise appear immediately after you did the work, or has it been working fine for a period of time and then it started acting up?

If it was after you did the work go back over everything very carefully. Pay particular attention to the cap polarity. The protection relay opening and then closing is indicative of some kind of DC spike going on when you switch. Is there a nice pop just before the relay opens?

 

[Gronard]

Hi,

Thanks for your reply. I bought this amp a few months ago and it had a lot of issues.
A lot of crackling noise on both channels and some distortion.
So I decided to do a full recap and change all the small transistors.

All of the issues have been solved except for the noise in the right channel.

I have done some more tests:

The filter has 3 switches:
- low 20Hz
- low 40Hz
- high 7KHz

All switches in the off position -> amp works with noise in the right channel.

When I switch "low 20Hz" or "low 40Hz" or both to the ON position the noise is gone and the sound is great.
When I switch either of them back to off there is a loud pop and the relay cuts out for about one second and comes back on and the noise is back.

When I switch "high 7KHz" on and ff there are no problems (no pop or relay).

I did not replace the small tantalium caps on the filter board only the electrolytic.

Attached the schematic and some pictures of the filter board.

Thanks,

Bernard.

 

[mrk229]

Thanks for the info. If that were my KW I would reflow every joint on that board, I'm not saying that is the problem but a lot of those joints look iffy.

How is it noisy? Without digging deep in the parts list it looks like you replaced three e-caps? DBL check the polarity of those. And if you would let us know which ones you replaced.

I almost always universally replace tantalum caps of that age. Most of them are small values and you can easily use film caps to replace them

I suspect a leaky cap.

 

[Gronard]

Thanks.
I checked the caps I changed and the polarity is correct. I will reflow all the soldering on this board. You are right the soldering quality and this board is really bad. A bit strange because the quality on all the other boards is good.
I have ordered the replacement parts and will send an update when they are installed.

 

[dr*audio]

I always replace tantalum caps. Usually you have to use electrolytic caps to replace them, but if the value is 1uF or less you can use polyester film caps. Your problem is consistent with a leaky cap, leaking DC.

 

[Gronard]

Thanks for the help @mrk229 and @dr*audio it was indeed a bad tantalum capacitor (C14 on board X12-1040).
I also finished the recap and replaced some transistors. No more noises only sweet music

It was well worth the effort, it is an excellent sounding amp.

 

[dr*audio]

Glad I could help.

 

[mrk229]