Trouble shooting and repair help - No low frequency output from stereo reciever.

[skismatik]

I have a 15 year old Kenwood KR-A5070 stereo component receiver. It has suddenly stopped producing low frequency output. I can hear the high end of the human voice and up but nothing lower. I've tried multiple inputs and both output channels. (Can be used w/ two pairs of speakers.) I've tried different speakers. I've visually inspected for blown/burnt components. I've tested resistance across the bass adjustment pot.

I have downloaded a low quality service manual w/ schematics from this location:
Kenwood KRA4070, KRA5070 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

I've run out of ideas. Anyone have any idea what could cause this sort of behavior?

 

[unclejed613]

you have dried out electrolytic caps in the power amp.. particularly the ones used for DC offset stabilization in the diff amp of the amplifier board. these are usually about 100-470uf caps between the base of the inverting input and ground. if they are dried out enough to change value, you will lose bass response from the amp.

the diff amp is a pair of transistors with the emitters tied together. one side is the noninverting input, which is where the audio input goes to. the other side is the inverting input. this is where your feedback goes to. there is usually a rather large (compared to others around it) cap, usually between 100 and 470uf, and usually the voltage rating is 50V or more, that goes between the base of the inverting input transistor and ground. it will have a resistor in series with it, usually somewhere between 1-3k.

btw, did i mention i hate working with .rar compressed files? this is the 21st century, we don't need to fit stuff on floppies anymore....

 

[canadaelk]

No low-end on both channels or just one?

 

[skismatik]

Thank you both for the replies.

canadaelk

No low-end on both channels or just one?

both channels - uniformly

unclejed613
I believe you're right, dried electrolytic has been a popular response and the solution I plan to pursue. You seem to have a very clear idea of which caps need to be replaced. As I mentioned earlier I've found the **broken link removed** but I don't have the skill/experience to make much sense of it. Would you mind having a look and giving me an idea which caps need to be replaced to fix the described symptoms? I've temporarily posted the **broken link removed**

Should I consider replacing all the can caps in the box or would that be overkill?

Thanks again.

 

[skismatik]

Are these the ones?

unclejed613
My best guess is the highlighted C205-6 are the culprits. Am I right?
**broken link removed**
**broken link removed**