Kenwood KA880D channel dropouts

[miljoneir]

Hi everyone.

I am trying to restore an old 80's Kenwood amp. Model KA880D.
It has issues with random loss of signal from either the left or right channel, moments where the volume suddenly drops and goes distorted.
I was able to fix lets's say 50% of the drop outs, by resoldering the volume pot as the solderings were broken at the back of the board.

Now I found out that the amp can play perfectly by using the 'CD DIRECT' button. The manual is somewhat vague about what it does, but playing with this mode on works fine for hours on end without dropouts. Good as a partial solution, but this way the amp is limited to only one input (CD).

Still, many of these drop-outs remain when not using this mode.
I thought the ‘CD DIRECT’ was something like a preamp bypass. However, when looking at the service manual I figured that it only bypasses the channel switches, the balance knob, mono switch and high pass filter switch (see image attached).
Does anyone have an idea which of these parts might be causing those dropouts? The solderings of these components look fine be me. I already tried cleaning the balance pot with Kontakt 60 but this didn’t improve anything.

Thanks in advance,



(Input jacks are on the left, output on the right,
line straigth switch toggles the eq which is on the bottom but not visible on this screenshot - probably not related)

 

[northpaw]

Sounds like you still have a dirty switch or pot, or one of those pots possibly is bad if cleaning does not calm it down. It could be any of the ones you mention, since they work on both channels.

To try to pin it down, while music is playing, very gently nudge/play with each switch/pot (not through their full motion, just enough to rock it) one at a time, and listen for sensitivity to those movements. in any case, I would clean the switches and pots again (I prefer a simple no-residue contact cleaner for switches because it can be used liberally without making a mess), and exercising them well. Make sure you are getting product directly into the switch contacts, or wiper area for the pots; usually you can find a small hole or seam in the switch/pot body into which you can place the straw from the contact cleaner spray.

 

[avionic]

Slide switches are near impossible to get clean without disassembly..

I thought the ‘CD DIRECT’ was something like a preamp bypass. However, when looking at the service manual I figured that it only bypasses the channel switches, the balance knob, mono switch and high pass filter switch (see image attached).

These switches you listed would be the switches to concentrate your cleaning efforts.

 

[avionic]

These are what slide switch contacts look like on the interior. Note the plastic pockets that cover the slider clips. These impede the application of contact cleaner unless you disassemble the switches .
https://audiokarma.org/forums/index.php?threads/real-dirty-pictures.244784/

 

[miljoneir]

Thank you all for the responses. I figured that it is the 'CD Direct' button itself that was acting weird. I tried my best to get Kontakt spray into the switch housing, but in the end it seemed to have worked. I hope this works as a permanent solution, as replacing that switch doesn't seem easy (It's 12 + 4 solderings keeping it in place.) For now, I will simply not touch that button anymore!

Slide switches are near impossible to get clean without disassembly..

Luckily I didn't have to mess with those sliding switches. The amp has one, but it is used only for setting the recording out, which I don't use. It seems to hold up for now.