Author Topic: Match Pair PNP Transistor array?  (Read 1458 times)

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Offline zz28zz

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Re: Match Pair PNP Transistor array?
« Reply #25 on: September 30, 2019, 05:54:43 am »
If both channels have the same problem, yes. Example; Maybe there are two components on one heatsink that have flexed and broken traces at the same area on the PCB. 12voltvids on YouTube has many examples of this type of thing. May not be your problem, but worth it to look.

I looked around but there are no heat sinks other than the case it self. I've ohm'ed out the feedback circuits and have continuity throughout. Flexed board a little but no changes were seen.

I set up this part of the circuit in a simulator and it shows the 1k ohm resistor I have installed in series with the source leg of the audio FETs will prevent sufficient feedback. However if I connect without the resistor, the FET overheats and shorts out very quickly. Kind of a catch 22 situation it seems.
If you're not confused, you don't fully understand the situation.
 

Offline zz28zz

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Re: Match Pair PNP Transistor array?
« Reply #26 on: September 30, 2019, 06:05:15 am »


Another possible problem is too much supply ripple due to aged caps. This would also effect both channels.

After a repair one may have to adjust the standing current and maybe DC offset.

 The original transistors are low noise types with high gain and still 50 V rating. So this could be hard to find a same or better performance replacement in that combination.

All the large electrolytics have been replaced and scope on rails looks very smooth. I'm planning on adjusting idle current once amp stops blowing up FETs.  I know finding parts for these older amps isn't easy.
If you're not confused, you don't fully understand the situation.
 

Online T3sl4co1l

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Re: Match Pair PNP Transistor array?
« Reply #27 on: October 21, 2019, 12:24:23 pm »
Incidentally, a similar part is actually two matched dies:
https://zeptobars.com/en/read/2SA979-matched-pnp-bjt

It's probably a lot of fussing for nothing, about the difference between a legit part and a substitute...

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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