Yamaha B2 driver board diagnosis

So this is a bit of a mind bender.

For the past few days I have focused on TR118, having appropriate base and emitter voltages per schematic, but not appropriate collector voltage. I test all surrounding circuitry R152, R148, R156, R157, R158, R 153, VR102, R160, and tried replacing all diodes D111, 115, 117, 113. I tried two different replacements for TR118 and even put original TR118 back in. Behavior did not change one bit.

Then I started playing with VR102. I think now that the problem is not that TR118 is not functioning. When I lower the value of VR102 by turning, ABS voltage of TR118 collector starts climbing (I can easily get it below -5V and counting). BUT, ABS value of NG and PG do not converge. We started with PG=16V and NG=-1V with VR102 ~ 95K. As I lower value of VR102 by turning, we can see point where PG=21V and NG=-5V. So, PG and NG seem fixed with a ABS spread of about 16V.

The only time I have seen NG and PG converge to +/- same value was when I completely removed R153 from circuit, at which point I saw +/- 19V.
 
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Any reason why I shouldn't replace these three polystyrene capacitors on these driver boards with ceramic?

I'm just replacing everything - although I will try to keep original transistors where possible.
 
I ordered replacement polystyrenes for those three locations. Thanks.

Soon every single component (except most transistors) will be new on these boards so I will know that any remaining problems are caused by transistors! As soon as my new transistor tester arrives (PEAK DCA75), I will be diving into these boards again.
 
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Polystyrene caps are sensitive components, to not damaging them during soldering you shouldn’t apply too much heat. You can also use a heatsink on the leads (e.g metal clips).
 
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Yes. It has bigger issues, giving me closer to +/- 45 volts for NG and PG. And depending on the position of VR102, it would actually smoke R149.

That's why I am just going to change everything on both boards and then re-evaluate and focus on transistors only.
 
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Well, I am having a blast. Every resistor, capacitor, and diode on these boards has been changed with new ones, and in most cases of higher quality than the originals. I left the varactor diodes since they test fine and don't really have perfect subs.

Now comes the transistors. I bought a peak DCA75 (thanks for the tip Avionic!) and can now identify and analyze these guys, and even curve trace on my PC.

I'm got both boards side by side and will remove the same transistor numbers in pairs to compare.

The thing I'm chewing on is transistors TR107-110 and TR121-124. I had hoped to put originals back in for TR121-124 since these are the work horses and I would think Yamaha carefully selected these guys. I do have a couple of bad ones, though. Interestingly, these transistors are not consistent and have wildly varying Hfe. PNPs can measure anywhere from 80-165 and NPNs can measure anywhere from 180-240. It seems these guys are degrading with age and stress, because TR121-124 all showed lower Hfe than their equivalents from TR107-110 positions. From the 16 I have, I could easily choose the best 8 and install them in TR121-124 positions on both driver boards and use the Toshiba TTC004B and TTA004B replacements for TR107-110 only. But, man, these new Toshibas measure much more consistently and the curves look much more ideal than the originals.

So, I'm torn between trying to maintain the originals and using the new replacements. I would enjoy any perspectives on this.
 
Hello

It seems these guys are degrading with age and stress, no transistors are good or bad nothing else

I am working on B2 driver boards and replace all the bad (only the bad ) transistors by original type , no sub . Everything will go back to normal function soon . V-FET replaced (one channel only) by new type too
 
use the Toshiba TTC004B and TTA004B replacements for TR107-110 only. But, man, these new Toshibas measure much more consistently and the curves look much more ideal than the originals.
The newer processing equipment make the parts a lot more consistent, that is why some mfg's can only offer one hfe grade. Good find on the Toshiba TTC004B and TTA004B but the KSC2690AY & KSA1220AY are fine as well. I used KSA1220AY and KSC2690AY to replace the blown ones on a driver pcb in a RX-900U recently.
 
Does anyone know a proper replacement for a 2SC1775A?

I had purchased the only KSC1845 that mouser have, but they aren't suitable. The 2SC1775A in this B2 measure Hfe from 750-1000, and these replacements are all measuring 350.

I am following leads on a couple of sources to try to get originals.

After full analysis, I need the following replacements:

1X 2SA810
2X 2SC1452
2X 2SC1775A
 
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