KA-87 Stereo Integrated Amp Repair: Help with Schematics, circuit ID, bias trimmers

Good afternoon, everyone! I am trying very hard to comingle all of the stuff I've read about amplifier circuits and apply it to a diagnosis and repair of a Kenwood KA-87 that sounds nice for 10 minutes and resets into protection. Turn off to cool down for 5 minutes and it will repeat the 10 minute working->heat->shutdown cycle.

I am having difficulty locating a schematic/service manual that could really help me identify two trimmers that may help adjust current or bias to reasonable levels.

This was a wonderfully working amp until two days ago when I took it apart to clean the accumulation of dust and crap off the PCB, De-Oxit the front controls (badly gunked balance slider was the rehab target), and tackle my first attempt at DC bias measurement and adjustment. Welp, of course it didn't go smoothly. I disconnected the AC line, wrapped a small screwdriver with a blue shop towel to clean the crap off the board, and ZAP!! Most of the way through, the screwdriver arc-welded itself to R41 due to some residual charge. It was a spectacular lightshow! You can see the repair I had to make to the resistor above the right-side trimmer, soldering a new lead extension to the end that was welded to my favorite TV repairman screwdriver. The repaired resistor measures the same as its sister next door. Why not just replace the resistor? I have $8.57 in my account until Tuesday, so I'm stuck home reading and teaching myself about stereo repair.

So, in the cleaning, resitor repair, etc., the two trimmers pointed out in the picture are no longer for sure in a calibrated position and I would love to know exactly what it is they trim. As I understand it, if the DC bias at the speaker terminals is off and those trimmers are not related then it will require faulty component removal, checking, and replacement. So, I'm crossing my fingers that they are related. I found a copy of the service manual available for download for $4.99 from an unusual site, but I don't want to spend 58% of my empire's wealth on something that may be a scam. I also believe the only people who should be paid for a service manual is Kenwood, an authorized distributor who will give me a printed copy, or a collector selling me a copy, not some Belarusian online PDF database. I digress.

So, first priority is asking nicely if anyone happens to have a service manual for a KA-87 so I can get to know the circuit sectors better and see what Japan has to say about calibration. Secondly, I would welcome any pro-tips on what my next move is, service, measurement or knowledge-wise. The bands on those damned old blue resistors are nearly indistiguishable to go in and check component values blindly. Would DC measurement at the speaker outs interest you? Is there something else to suspect is causing the thermal anomoly? Thank you so much in advance for any advice and guidance.

Your pal,
Jon Darby

set your meter on dc mv ..then measure voltage at speaker terminals ..if it goes OL turn meter to dc volts ..report back with reading ..
also see the large white resistors by the heatsink ? they have 3 prongs on top . measure dc mv accross the outer ones ..report back .
please don't adjust anything anything yet until we know what we are doing .. it might be the case i have a manual but could take some time in looking for it .. closer pictures might help in determining what the trimmers are for ...

google ...its googles fault honest ..

Thank you so much for the quick reply!!

No, that is a different amp, Mine is a KA-87, that manual for a Kenwood A87. Similar names with different guts.

(1. Left Channel Speaker Out) OK, warm amp, Left Channel tested first, initial reading is about -.2V, not mV. Flucuates minutely up to -.55V over two minutes and holds, then climbs to -.7V, holds for two minutes, climbs (or falls, if we're speaking sinsusoidally) to -1V, then -1.5V, so I shut it down

(2. Right Channel Speaker Out) Amp off, heat felt above the heatsink. Connect leads to Right Channel -amp off, seeing a falling residual charge starting at -1.7V that falls to 0.00
-.6V, pretty steady for 30 seconds, then rapid climb to -2.3V, which it unstably holds for 30 seconds before shutdown.

(3. Right Resistor measurement) -.392V, two minutes later, it was at -.422V

(4. Left Resistor measurement) -.116V, two minutes later, -.125V


I can tell that the right side is noticeably hotter than the left, but both are pretty hot. I would love to provide better pictures, the flash mucks it up a bit, so give me an idea of what to focus on and I'll get at it.

Thank you so much for your assistance

Some bare topped centerfolds for you to look at.

I see the part number of the white resistors is MPC725 with ".22ohmK + .22ohmK" (in that order, .22 O K, not .22K Ohms of the front, but at rest they both measure identically like this:

1.2ohms
/ \
{| | |} outside leads

1 ohm
/\
{| | |} left and middle

1 ohm
/\
{| | |} middle and right leads

Some things about electronics I just don't have a handle on yet. No .22Kohms like advertised, but identical readings from both components

Those are the emitter resistors; set the idle current/bias by measuring the voltage across them for each channel. The internal layout of your KA-87 is very similar to the that of the KA-76, whose service manual is available here: http://elektrotanya.com/kenwood_ka-76.pdf/download.html

Oh, man, thank you, Leestereo, for your pattern matching, they are very similar and I'm going to get back to it in a couple of hours when I get home. My God, I spent so many hours looking at the circuits up close last night that as soon as I saw the schematic it was like the Matrix, values laying on top of mental memories of what each component looked like. Whew! That was a cheap thrill. Anyhoo.

I can't wait to get this wonderful little amp back to work, I'm catalogueing a few blunderous boo-boos on my part that rattled a few fundamental safety chains in my head. This kind of stuff is why me and my Volkswagen Beetle have such a happy symbiotic relationship.

there should be 3mv across emitter resistors ..
are they marked as 0.22 ohms on the resistors ?
your meter may have a large error try holding leads together and make a note of reading then deduct from the one ohm your are reading . this will be more accurate .maybe the meter battery needs changing .
even if they were 1 ohm the present reading in mv is way too high .