Fisher 440-T clipped wave on both channels

Fury161

Fury161

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I don't normally deal with the old Fishers but a friend dropped this guy off and asked me to have a look at it. I'll admit, I'm a bit over my head in this rat's nest! Here's what I have so far:

  • Purchased a couple weeks ago from a guy and it sounded ok at his place
  • Now it's distorted in both channels.
    • to my ears, the distortion gets worse with bass notes. Highs are still distorted but not quite as bad
  • All the contacts and pots have been thoroughly cleaned
  • I've managed to trace it through the front half of the amp
    • Pre-amp board and volume knob (pre/post) look good with a nice symmetrical 1khz sine wave
  • I get a clipped half wave on the dummy loads
1000015964.jpg

  • Looks okay at the base to q12 (2n2924), clipped as of the base of q10 (2n3638)
  • Manifests on both channels equally
  • I identified 2 200uF electrolytics in the PS that were swollen and ready to pop, i already swapped those out
  • Otherwise the unit is stock.
Since it's on both channels, I'm inclined to think it's a larger PS issue (GG voltage looks good) but don't want to start shotgunning parts. Any thoughts or common points of failure I might (probably) don't know with these units?

Thx !
 
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It may help to post the serial number of your unit. The 440T was one of Fisher's first solid state pieces, and because of this there were many changes to it during the production run. There are lots of service bulletins out there for the 440T that may be relevant to you as well depending on which version you have. I believe I have the service manual for every major revision of this receiver and would be happy to share it with you!
 
What sort of power level is this? If its extremely low it seems like the negative side is not doing anything.

What do the voltages at Q10 and Q12 look like? This looks like its biased weirdly and driving into cutoff going negative.

1769616706782.png

20000 series schematic, possibly yours is different. Also just want to confirm transistor numbering here. Input is to Q12, Q10 is driver so if the signal is bad at the base of Q12, it basically has to be bad at Q10 too.

Fury161 said:
Looks okay at the base to q12 (2n2924), clipped as of the base of q10 (2n3638)
Click to expand...
 
Last edited:
Sterling245 said:
It may help to post the serial number of your unit. The 440T was one of Fisher's first solid state pieces, and because of this there were many changes to it during the production run. There are lots of service bulletins out there for the 440T that may be relevant to you as well depending on which version you have. I believe I have the service manual for every major revision of this receiver and would be happy to share it with you!
Click to expand...
Click to expand...

Appreciate that: Serial No is: 48606c

gadget73 said:
What sort of power level is this? If its extremely low it seems like the negative side is not doing anything.

What do the voltages at Q10 and Q12 look like? This looks like its biased weirdly and driving into cutoff going negative.

View attachment 3686565

20000 series schematic, possibly yours is different. Also just want to confirm transistor numbering here. Input is to Q12, Q10 is driver so if the signal is bad at the base of Q12, it basically has to be bad at Q10 too.
Click to expand...
Click to expand...

Q12:

B: .5v
C: 23v
e: 1.0mv

Q10:
B: 23v
C: 0mv (ground)
E; 23.5V
 
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heh, this amp section has absolutely no relation to the early one. Wow. From the 47001+ manual. Are we talking about the same transistors now?

1769720260483.png

a
 
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