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Yamaha CA-800 - restoration - few questions

I have not waited that long, but it does not after 10. And the other channel is around 0 all the time. It goes up from the -36mV to +50mV and always rising
 
At 50mV the amps not going into meltdown. Yes it needs to be investigated. Could be either of TR602,3 drifting.
Suggest checking that FR on TR604-c (R619? 680 ohms). Maybe measure/track the voltage at this FR, does
it raise/fall in synch with the dc offset. Same applies to FR on TR608b (R614??? 680ohms) Hifiengine service manual
is poor quality,,,
 
I have changed those FR for pair of 1/4W resistors in series to obtain 680 ohms and 1/2W. I will check those first so. I checked the resistance and the value is correct, but as it's one of the few things I changed on the board, it's not a bad place to start.
 
Hi,

so the voltage drop accross the two 680ohm resistors R619 and R616 is stable, does not move with the offset.

I have done a bit of measuring and found out, that if I set vr601 on the bad channel to get the same voltage on base of tr602 as on the good one, I get the same Voltage (50mV) on base of tr603, but after R618 - 3.3K (emittors if TR605 and TR606) I have 0 on the good one, but over 100mV on the bad one, so TR602 and TR603 are working fine and the high offset is getting there somewhere after.

I noticed that the voltages coming in are slightly different:

Good channel: A: -0.54V, B:1.82 C: 1.82 S: 4.18
Bad channel: A: -0.44V, B:2.01 C: 2.01 S: 4.37.

Just FYI there is a slightly better visible schema of the power amp board on page 6, where the adjustment procedures are listed
 
The differences in A,B,C,S voltages is just large enough to raise questions. The voltages may become
more aligned by fiddling with VR603,4. Maybe TR604 is getting a bit tired. I think the problem is elsewhere, the
issue is dc offset rather than failure. Have you checked R626,7 4.7ohm resistors in white sleeve (I think).
 
Does "idle current " for ,both ,normal and class A adjustment work on the bad channel ?
If it does , then that pretty much eliminates A,B,C & S as and issue.
 
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TR601,602 or 603 are suspect. (Leakage) . Did C604 get replaced with a NP or BP cap ?(47µf/50vdc) Did C606 (220µf/35vdc) get installed correctly ? Check your soldering on the printed side of that amplifier assembly.
 
Regarding TR605 on the heat sink. The bevel face is the front face on the old outhouse 2SC458. On the TO-92 KSC1845, the flat face
should be on the heat sink, so the base is top most closest to the VR's. Doubt this is the issue since the other side is ok but worth checking.
Also there would be no response to the VR's, more fyi.
 
Hi, so mystery solved. I only read Avionic's note about 601 - 603 now after I fixed it and you were right. I swapped the boards just to eliminate any possibility of the problem coming from earlier stages and the problem stayed with the board. If it was any other amp I would go directly to the long tail pair if there was DC offset issue, but for some reason I didn't (I got mislead by the measurement after 603 being the same on both boards). So now I said, let's check them: swapped 602 and 603 for matched pair of 992FBUs and voila... No drift, when set to 0mV, it stays there. The original 602 had hfe of 280, 603 was 180.
I will change them on the other board as well (they have lovely black legs as well:).
 
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