Help with 2285 (non B) Rebuild

You can verify the preamp by using headphones to test, can't hurt to verify bias as well.
Haven't done any work to my 2285 except rebuilding the power supply so can't comment further on your issue.
 
I used my multimeter on diode function to measure the transistors. I'm not exactly sure how to read these.
Transistor measurements are as follows while in circuit:
2SA747A
Collector(+) + Pin 1 = .531
Collector + Pin 1(+) = .531
Collector(+) + Pin 2 = .428
Collector + Pin 2 (+) = 0.0
Pin 1(+) + Pin 2 = .480
Pin 1 + Pin 2(+) = .408

2SC1116A
Collector(+) + Pin 1 = .001
Collector + Pin 1(+) = .001
Collector(+) + Pin 2 = .499
Collector + Pin 2(+) = .378
Pin 1(+) + Pin 2 = .498
Pin 1 + Pin 2(+) = .379

2SA747A
Collector(+) + Pin 1 = .530
Collector + Pin 1(+) = .0
Collector(+) + Pin 2 = .427
Collector + Pin 2(+) = .0
Pin 1(+) + Pin 2 = .480
Pin 1 + Pin 2 (+)= .403

2SC1116A
Collector(+) + Pin 1 = .0
Collector + Pin 1(+) = .0
Collector(+) + Pin 2 = .498
Collector + Pin 2(+) = .378
Pin 1(+) + Pin 2 = .498
Pin 1 + Pin 2 (+)= .376

I also took them off the heat sink and measured them:
2SA747A
Collector(+) + Pin 1 = .0
Collector + Pin 1(+) = .0
Collector(+) + Pin 2 = .439
Collector + Pin 2 (+) = 0.0
Pin 1(+) + Pin 2 = .498
Pin 1 + Pin 2(+) = .0

2SC1116A
Collector(+) + Pin 1 = .0
Collector + Pin 1(+) = .0
Collector(+) + Pin 2 = .0
Collector + Pin 2(+) = .469
Pin 1(+) + Pin 2 = .0
Pin 1 + Pin 2(+) = .482

2SA747A
Collector(+) + Pin 1 = .0
Collector + Pin 1(+) = .0
Collector(+) + Pin 2 = .442
Collector + Pin 2(+) = .0
Pin 1(+) + Pin 2 = .493
Pin 1 + Pin 2 (+)= .0

2SC1116A
Collector(+) + Pin 1 = .0
Collector + Pin 1(+) = .0
Collector(+) + Pin 2 = .0
Collector + Pin 2(+) = .386
Pin 1(+) + Pin 2 = .0
Pin 1 + Pin 2 (+)= .386

Is there a reason I am getting readings when the transistors are in circuit as opposed to out of circuit? Is this indicative of improper mounting?
I reused the mica sheets beneath the transistors and only used a small amount of compound.

Thank you all again for your help.
 
In response to testing with headphones, the preamp is functional only on the left side (the side with the unmodified amp board). It also has quite a bad hum.
 
If you move the good module over to the right side (leaving the bad one out, you should get sound. That would prove the pre-amp ok, the hum is most likely because you have the bottom cover removed.
 
The preamp board works just fine. I have one dead 2SC1116A. I will order a new one and have it sent with my next Mouser order.
 
Do I need to match the output transistors?
Since I'm replacing one, should I replace all of them in the channel or all eight in the unit?
 
Back
Top Bottom