Matching Transistors? Pioneer QA-800A

[sregor]

If replacement isn't a "can" (TO5) type case, make sure you get base, collector emitter correct. (diode test with meter to verify base)

 

[thefragger]

If replacement isn't a "can" (TO5) type case, make sure you get base, collector emitter correct. (diode test with meter to verify base)

10-4 on that, thanks for your help Steve!

 

[thefragger]

Well, replacing those two transistors got the amp to bias-up correctly, and it's stable! I plan on swapping out the other old parts for these ones.

I'll be keeping a couple spares taped on the inside of the case as well as four of the power transistors in there, too, just in case!

But Steve, I'm really, honestly, genuinely curious about your thought process that made you point to Q3 (and potentially Q5) as being the problem (honest-to-goodness, I wasnt to know!)... :scratch2:

 

[sregor]

:thmbsp:

Well, replacing those two transistors got the amp to bias-up correctly, and it's stable! I plan on swapping out the other old parts for these ones.

I'll be keeping a couple spares taped on the inside of the case as well as four of the power transistors in there, too, just in case!

But Steve, I'm really, honestly, genuinely curious about your thought process that made you point to Q3 (and potentially Q5) as being the problem (honest-to-goodness, I wasnt to know!)... :scratch2:

:scratch2: Basically combining "how the circuit is supposed to work" with what was measured plus a bit of experience. You were getting positive rail voltages everywhere, and no indication (burnt or open resistors and transistors) usually accompanying bad outputs. High output voltage should cause Q3 to turn on harder to bring things down to where they should be (feedback) which leads to checking voltages around Q3 to indicate where something is amiss. Being a relatively simple circuit didn't hurt either - Q3 amplifies the signal and the other transistors increase current, but not voltage.