KR-9400 hum, my fault somehow

[JacTaylor]

I have been overzealous in my rebuild. I had successfully performed the service bulletin, replaced caps and matched qe1 and qe2. Everything was working then one day the speaker relay wouldn't kick in. Took a look and it had one contact fused. So I replaced the relay and the caps while in the power supply. Still it wouldn't click in. I measured pin 4 and 15 on the amps and one was at 60.5v and the other was around 84.7v. I took the 84.7v amp out and tried just the one 60.5v amp and the speaker relay clicked in.

I took both of them out and looked closely at any differences and I found where qe3 pin was loose on the 84.7v, probably from trying to remove the cap next to it and never soldered it back. Soldered that in and speaker relay kicked in with both amps in. I set the bias for both at 50mV.

However, now I have a loud hum independent of aux, phono, fm am setting with the volume down all the way. Did I do that by testing with just one amp board in? I must have blown something, in the power supply???

 

[JacTaylor]

Yea, you're right. There were a couple of other suspect solder joints, too. The pin orientation is correct. ECB originally with new EBC transistors twisted around to work. Thanks rjsalvi for taking a look and helping out.

I did my gosh darn best to turn this into a large anchor weight to keep all my Digikey and Mouser invoices from blowing away. When I got into the power supply and put in the new speaker relay and changed out the caps, I unknowingly broke the positive wire to one of the two filter caps. Probably either pulling and moving it or when I reassembled it. I guess if I had more knowledge and skills, I would have checked a few more voltages and it would have been obvious. At any rate, getting back in there and reattaching that did the trick. Sounds really nice with absolutely no hum now.

 

[rickl]

Glad to read you got it working. I really like mine but it doesn't get much use. Fixed a few things and the edge cards make it so much easier to work on.

I'm hoping my 9400 will be a good match for a KW-4077 I'm bring back from the dead.

 

[JacTaylor]

The KW-4077 looks like a nice machine. I really like the 9400. Plenty of power and runs cool all day.