Fisher 200-T Power Question

All readings on Q804 and Q805 on both left and right channels are roughly the same (.556 to .586V) with the exception of the Q804 bases. Q804 (Left) = Open and Q805 (Right) = .573V. I can't get to CR801 and CR802 without de-soldering the board pin connections. I can do that but haven't yet. R624 (Left) = 86.2 ohms and R624 (Right) = 84.6 ohms.

Looks like there are NTE equivalents for Q804 and Q805. I can't find anything on CR801/CR802 though. Components are marked: ZM 51B049E / B&T ABT-59E / 6735. If it's CR801 or CR802 likely going to have to do some scrambling to come up with a replacement.

I went ahead and swapped out all 4 of the 1 ohm/3 watt resistors while I was there - no change, left audio still low, weak and distorted.

Don
 
Don; Here are some of the more common FISHER #'s with Fairchild or OnSemi x-ref's. These are all available @ Mouser or Digikey. The BC107b is still available but I don't know if Mouser has it. Digikey might.

TR1002 & TR1004--->KSA1220AYS
TR1003 & TR1005--->KSC2690AYS


TR1009------>MJ 21194 Also TR1077
TR1033-34------>See 1004/1005
BC109------->2N5089
2n4062----------->mps6518

SE4010------>KSC1815YTA
2n2924-------->2n2924
2n2925-------->2n2925
2n3638-------->ksa643

www.alltransistors.com can give you xref's for the 2n4248 and others that you don't have. You won't be able to xref the FISHER #'s tho.

If ANY of the Driver transistors are socketed on the board, the sockets have to be removed.as the ksc2690 and ksa1220 pins (TO-126sized) are too large for the socket pins, as are TO-220 sized, TO-92 sized transistor legs can fit in the sockets.
 
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Was reseating / tapping tops of transistors on Left Channel Power Amplifier board just for the heck of it. When I tapped Q801 there was a loud hum in the left channel speaker that was repeatable. Hum remained as long as I kept a finger on the top of Q801. Audio Input comes in on pin 8B, goes thru R801 (5.6K), then to C801 (1uF) and then to the base of Q801 where it's supposed to read +0.7V. I'm reading about 10 - 12mV. R801 read good at 5.6K but C801's reading is only 551nF.

Checked same points on Right Channel Power Amplifier board and everything read SAT there.

Went ahead and pulled the Left Channel board and am going to go ahead and re-cap it and go from there. Will order replacement transistors but not going to swap them until I see how the re-cap turns out. It will probably be a few days before I get to it but I'll post how it turns out.

Don
 
Replaced all caps and transistors on Left Channel Power Amplifier board. No change in symptoms. At this point I think I'll start looking at the wiring to make sure I don't have a nicked wire or a bad ground somewhere. It would be nice if it was something that easy!

Don
 
While you have the board disconnected from the chassis for part replacement, check the traces visually with a magnifier and a bright light, then with a meter. You can have a fracture in the trace that separates just enough to cause an open but you can't see it unaided. That hum @ Q801 sounds suspiciously like a ground loop. Check all the ground connections to and from the board, and all the ground traces.
 
I haven't had a chance to get to it this weekend. I'm leaning to a ground problem also. As soon as I get the time I'll check the traces and wiring and see what I come up with.
 
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