Pioneer A88-x with Bad Channel

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Joined 2013
You can't buy originals on Ebay. The fakes there will do you no good, either.

2SK129A = dual JFET, probably similar to 2x 2SK117 or 2x 2SK170, both obsolete, better hope it's not blown.
2SA992 = KSA992
2SA1845 = KSC1845
2SA1145 and probably 2SB560 = KSA1013
2SC2705 and probably 2SD438 = KSC2383

And for outputs, again:

2SA1265 = FJA4210
2SC3182 = FJA4310

All Fairchild parts. Mouser has them all.

And don't forget to check the emitter resistors too, they tend to blow up along with output transistors.
 
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tomharrison1

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Joined 2015
I checked Q1 and Q2 last night. Q1 looks good, but Q2 measurements are off. Q2 itself may be ok, but the voltages coming in and out are off, so something surrounding it may be bad. See diagram I have attached for the voltages I should see in green, and the voltages I am seeing, in red.
 

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tomharrison1

Member
Joined 2015
Hi Craig, I guess there are a number of reasons for oscillating, but with what I have provided so far, what might be the cause?

Also, I have 2 different issues going on here:

Left channel on Voltage Amp Assy looks good, but R115 on Left Output Assy is getting hot (I checked its voltage last night and it was at .9V, while R114 on right was at .06V).

Second issue is with Right Channel and bad voltages throughout the Voltage Amp Assy.
 
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llwhtt

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Joined 2008
Is that .9V AC or DC? If AC it's probably much higher but the frequency will be so high your meter won't be very accurate. If it's DC then you have a major offset problem. But if it is DC the resistor won't get hot because of C39/41. There should be NO DC continuity thru C39/41.

As far as the wrong voltages at Q1/2 the only place the negative voltage can come from is thru the FET itself, pull it and check it. Do you have +30VDC on R6,8,10,12?

Craig
 
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llwhtt

Member
Joined 2008
Q1/2 are dual FETs so BCE don't work. It's Drain, Source, and Gate. Google "testing FETs".

So your voltages are DC, you have DC offset and if the resistor is hot you oscillation on top of that. This is going to be a fun one. You might think about pulling the fuse on one channel and work on one channel at a time.

Craig
 
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Mooly

Mooly

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You can't read a CCD (constant current diode on a meter). If its a 4.5ma device as Craig and Jaycee and llwhtt suggested then you should see up to, but not more than around 21 volts across R18. If the reading is less then its not necessarily the diode at fault, the FET's could be biased off more than they should be.

A 0.00 reading on the meter could be OK depending what range you are on as the diode is simply a junction fet configured as a current source.

https://en.wikipedia.org/wiki/Constant-current_diode
 
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