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Fisher TA800 left channel problem

N

NickL

New Member
OK I'm at a dead end here. I have finally completed the resto of my TA800 that I had started years ago with the help of Jim McShane but it got shelved when my first kid was born. Anyway fast forward ten years and it seems to have gone perfectly for the right channel which sounds utterly amazing, but the left channel is giving me trouble. It's slightly weaker and has some significant distortion. The lows are flabby and the highs scratchy.

Here's what I've done to try and isolate the problem to no avail: 1. Swapped power, preamp, and tone tubes 2. Checked all amp section resistor values and solder joints. 3. Cleaned volume, balance and loudness switches (which did improve their function but had no effect on the sound) 4. Tried/swapped different inputs, speakers and cables. 5. Cleaned power tube sockets.

I tried listening for the distortion with the monitor switch on and I think it sounds clean but it's so quiet I can't be sure.

Sidenote: I've got this hooked up to B&W 602s and the unit is plugged into a variac set at 115v.

Any ideas? Thanks in advance
 
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I was going to say, Swap V12 and V17 (Tone control amp tubes.) see if the problem follows the tube. If so replace the tube.But you already did that. Clean the hell out of the BASS and TREBLE CONTROLS. Clean, Scrub, and re-tension all the 12ax7 tube pin sockets. Look at the tone control PEC's Check at the input of C102-C103. Very may well be PC5 is flaky (this is a Brown PEC, 7 legged packaged electronic component) It looks like a lumpy rectangle. They can be replaced with a perf board with 1% tolerance parts. I'd replace both. They have a tendency to go out of spec, and can cause problems like what you have. BTDT with a TA-600, a 550-T, and a 700-T.
 
I'd verify voltages first, compare side to side even if it doesn't match the schematic exactly. If nothing stands out there, see if you can inject signal directly at the volume control. That will isolate the problem to the preamp or the power amp section. If its good with signal fed at the volume pot, the issue is between the input jacks and the vol pot, otherwise its between the pot and the speaker terminals. Basically preamp vs power amp.
 
larryderouin said:
I was going to say, Swap V12 and V17 (Tone control amp tubes.) see if the problem follows the tube. If so replace the tube.But you already did that. Clean the hell out of the BASS and TREBLE CONTROLS. Clean, Scrub, and re-tension all the 12ax7 tube pin sockets. Look at the tone control PEC's Check at the input of C102-C103. Very may well be PC5 is flaky (this is a Brown PEC, 7 legged packaged electronic component) It looks like a lumpy rectangle. They can be replaced with a perf board with 1% tolerance parts. I'd replace both. They have a tendency to go out of spec, and can cause problems like what you have. BTDT with a TA-600, a 550-T, and a 700-T.
Click to expand...

Thanks for the response! OK I found no discrepancies with tube voltages, so i checked the voltages on caps C102 and C103 and they did not match. One side of each measured almost zero but the other sides read .235 for the right one and 1.75 for the left one (so I assume those are the output sides). If I understand this correctly those should both read as close to .22 as possible seeing they are .022uf caps? Also my PC4&5 appear to just be individual components on strips and not really a "package", but seeing as how I'm learning as I go I could certainly be wrong. I'm attaching a pic of the side that reads 1.75 (brown cap with "472")

FisherTA800C103

FisherTA800C103

  • Media owner NickL
  • Date added
PEC6 with C103
 
Confirm what they are supposed to be, 0.22 vs 0.022 won't act the same. Those old "brown turd" caps did sometimes break loose internally and go open circuit though. Possible thats what you have going on. I'm not sure which cap in the pic are the ones you mention, the only number I can read is 472 which should be a 0.0047 cap.

a 716p orange drop is a fine sub for the brown caps though. I might source them from Mouser or Digikey, probably be cheaper.
 
voltage and uf dont work like that .
to know the correct voltage at that point its best to look at the schematic first .
 
C102 & C103 are .022uf 250V mylar (either the black or brown epoxy caps). Follow lead 4 of the Left side PEC past R96 (39K 1/2W) to C102. This cap is a better fit for the tight spaces in the TA-800. And it sounds pretty much the same as the O.D. (at least to me.) Plus it's cheap. $1.03ea plus shipping. Get as many parts as you can so the shipping is not so much a large percentage. https://www.mouser.com/ProductDetai...=/ha2pyFaduiVS0B2Fm0Rtqe36v4nu6k50Nk8qHP1EU4=
 
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