Fisher 800c Output Tube Redplating

Hi Fi Fan

Active Member
Hi everyone,

I recently picked up a Fisher 800c that was apparently restored 2 years ago by http://aeaaudio.com

Yesterday on startup I heard a loud popping static like sound out of the right channel even when volume was turned down. Today it happened again and I shut down power immediately and swapped the tube sets from channels left and right. On startup, one of the now left channel (original right channel) tubes was redlining and very hot. Power was shut down immediately. Does this sound like just like a bad tube or could it be something more serious?

I opened up the bottom chassis and it appears that a bias adjustment has been added for each channel (blue circles with slots). Can anyone confirm based on the following photo?

Thank you.
 

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From the fact that the problem moved with the tube sets, it would indicate that a tube may be at fault. However, you need to first remove all the output tubes and use your volt meter and make sure that the signal grids at each output tube socket is still showing a negative bias voltage. If any one of them is showing much lower than the others or even a positive voltage, you may have a failed coupling capacitor between that socket and the 1/2 of the 12AX7 which is the phase inverter and sends the signal to the output tubes.

In looking at the picture you sent, there is a suspicious item at the right most output tube socket that looks like a burned resistor. You might want to check that out. Then check the schematic to see what part of the circuit it is in.

Joe
 
If the redplating tube experienced runaway after moving, then I would agree with Joe re the tube. However, looking at the pic, there seems to be some interesting stuff done to the unit, e.g. all the cathodes tied together. :idea: Anyway, try replacing the redplating tube and I think you'll be OK. The other stuff, well.........
 
From the fact that the problem moved with the tube sets, it would indicate that a tube may be at fault. However, you need to first remove all the output tubes and use your volt meter and make sure that the signal grids at each output tube socket is still showing a negative bias voltage. If any one of them is showing much lower than the others or even a positive voltage, you may have a failed coupling capacitor between that socket and the 1/2 of the 12AX7 which is the phase inverter and sends the signal to the output tubes.

In looking at the picture you sent, there is a suspicious item at the right most output tube socket that looks like a burned resistor. You might want to check that out. Then check the schematic to see what part of the circuit it is in.

Joe

Thanks a lot. I could measure the signal grids if I had a little direction on how to do it safely. I will check the service manual and research online to see what info is available.

I looked at the resistors carefully this evening with help of the flashlight. I did not see any that appeared to be burned to me (though I am not an expert).

Thank you.
 
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If the redplating tube experienced runaway after moving, then I would agree with Joe re the tube. However, looking at the pic, there seems to be some interesting stuff done to the unit, e.g. all the cathodes tied together. :idea: Anyway, try replacing the redplating tube and I think you'll be OK. The other stuff, well.........

Thank you.
 
As designed, the cathode pin was tied to ground via a small jumper. As a modern enhancement not affecting the signal, a precise 10ohm resistor replaces the jumper allowing for the measurement of bias current for each individual tube. With no other changes made to the unit, it allows for the gross "matching" of output tubes so that you could manually balance the channel by juggling the tubes.

By your question re measuring "signal," I suspect you want to be able to measure important voltages to make sure nothing is amiss. However, in the case of your output tubes, it looks like you have all your pin 5's tied together and running off to the left via a green wire. No idea why that was done and it is atypical from what I have seen. Maybe a pot back there but just don't dunno. Do us a favor if you don't mind: put up a pic of the entire underside and close ups of each quadrant. That'll give all of us a better idea of what was done, or not, which will certainly help us guide you through a pretty good analysis of your unit. No promises on how your feelings expressed in post 20 will change.
 
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As designed, the cathode pin was tied to ground via a small jumper. As a modern enhancement not affecting the signal, a precise 10ohm resistor replaces the jumper allowing for the measurement of bias current for each individual tube. With no other changes made to the unit, it allows for the gross "matching" of output tubes so that you could manually balance the channel by juggling the tubes.

By your question re measuring "signal," I suspect you want to be able to measure important voltages to make sure nothing is amiss. However, in the case of your output tubes, it looks like you have all your pin 5's tied together and running off to the left via a green wire. No idea why that was done and it is atypical from what I have seen. Maybe a pot back there but just don't dunno. Do us a favor if you don't mind: put up a pic of the entire underside and close ups of each quadrant. That'll give all of us a better idea of what was done, or not, which will certainly help us guide you through a pretty good analysis of your unit. No promises on how your feelings expressed in post 20 will change.

Thanks a lot for the help. I will post pictures shortly.

Thank you.
 
Photos as requested, thank you.
 

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Just received a copy of the invoice for work that was done by AEA.

Thank you.
 

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Hi Fi -- While it's not too hard to get a general idea of what work was done, the way it was performed makes it nearly impossible to see specifically how it was done. The original build presented a well populated but neatly laid out chassis where you could see what connected to where. After this work was performed, the power supply areas are now quite cluttered making it all but impossible to trace out the details of the actual work performed. The work may in fact be well done in terms of connections and circuit modifications made. But knowing what they are is another thing entirely.

I would agree that if the problem always followed the one tube, I'd replace it (or better, the matched pair it was part of), which will likely take care of the basic problem. Interestingly however, there seems to be no provisions made for setting the new bias controls installed (as others have noted), other than using the OPT primary resistance as a means of effecting a proper adjustment. In any event, I would strongly encourage you to either install or have installed Screen Stability resistors (it looks like someone started to at some point), as the lack of their installation when modern power supply caps were installed could very likely be the reason that the tube went south in the first place.

Good luck with it -- you're in the right place to get it all sorted out!

Dave

Ohhh -- I see it was previously modified before AEA got a hold of it, so it makes it harder to know who did what........
 
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