KA-7100, only get sound with Tone Defeat engaged

[Gobstopper]

Hello Everyone,
I recently started working on a KA-7100 that I found on the sidewalk several years ago. I used a DBT and it started up okay. Initially sounded pretty scratchy and dropping out, but Deoxit on switches and pots + manipulation seemed to clean it up nicely. The one thing I found is that sound only plays if tone defeat is engaged. If I switch it to "off", then there is a horrible loud noise in both channels(can't hear music anymore). Looking at the schematic it would seem that something is wrong upstream of Qi5 and Qi6 (where it enters the tone controls). I just started testing voltages(haven't tested every corresponding point with a labeled voltage on service manual), and so far the only thing that seems a bit off is the voltage at the (base of Qi10)/(Collector of Qi8) (reads -11V instead of -10V). The collector and base of Qi10 read a bit more in spec(-11.25V instead of -11@collector and 12.6V instead of 12V@emitter) Should I keep testing voltages in that area, or is there a more likely source causing the problem?

I'm not a full fledged pro, but I have refurbished several units in the past (mostly pioneers and one marantz)

 

[kwayde]

I'm not familiar with that amp, but since the problem is affecting both channels and your readings are good, I would think the board may be good and the problem is coming from elsewhere. Have you checked the switch itself?

 

[Gobstopper]

Thanks for the suggestion. I haven't checked it directly, got too excited that it was working at all I suppose. What's the best course of action, check continuity across the different poles in the two positions?

 

[HPMguy]

Clean the switch first, bad contacts can make all sorts of noise. Then check Ci23 and Ci24, they are the coupling capacitors and if bad can make all sorts of noise too.

That unit is old enough for all the smaller capacitors to be dying of old age

 

[Gobstopper]

Switch seems okay. I checked resistance between middle set of pins and those above and below. Depending on switch position resistance was either 0 ohms (more or less) or ~15K ohms for both combinations. Contacts were previously cleaned as much as possible without removing from board. I'll check the caps next. I was planning on doing a full recap on most of the boards but wanted to resolve the issue before going in full bore.

 

[HPMguy]

Very likely those 3.3uF caps have several to many ohms ESR. May be leaking too. That was the case on a Technics SA-120 I fixed up for my brother -- tone coupling caps were fried.

 

[rickl]

The one thing I found is that sound only plays if tone defeat is engaged.

while HPMguy might be correct that both coupling caps are bad, i'd try cleaning the bass, treble and other switches in the tone circuit. I know my 7100 needs cleaning about every year.

if you do replace ci23/ci24 use 3.3uF WIMAs (MKS2xxx, 50v) will be perfect with 5mm lead spacing.

 

[Gobstopper]

Hi everyone, thank you all for the suggestions. I cleaned the pots last night, and today it doesn't shriek at me when I disengage the tone defeat! It's only coming in on one channel now, and it sounds tinny and crackly, but I think some more cleaning plus a recap will take care for most of it. I had already ordered some Nichicon UKLs for ci23/24, I checked my stock of caps, but I don't have any 3.3uf Wimas. Hopefully the UKLs will suffice. I'll update this post after the cleaning and recap. After this unit I only have one more left to possibly work on; a Pioneer Sx-650, but it has power packs and some ICs, so not sure I'll be able to do much.

 

[HPMguy]

UKL will be fine. Film caps are better, but mainly because they last much longer. Slightly lower noise and no harmonic distortion too, but if you don't want to wait the UKLs won't disappoint.

Over a buck apiece for those Wima's, too. I seem to use a huge number of them, also 1uF films, usually Kemet R82s. Every receiver or amp I work on seems to have a dozen or so of them!