Fisher 500c Channel Issue and Distortion. I'm stumped

[Zepman69]

I have a 500C that has had a lot of work done in the past, new caps, adjustments etc.

Anyways, when I moved into my last apartment, the first time I took it out, only one channel seemed to work until the volume was around 11 o'clock. You could still hear some faint noise in the left channel, but it would really turn on at 11. When I DO get both channels working, I get some pretty bad distortion.

I opened the bottom panel up last night and did some tests on the volume pot. There appears there are no dead spots, but the resistance peaked about 75% of the way, and dropped off on both sides. That seems weird to me.

This happens with all inputs. If it isn't the volume pot(which I was planning on removing and rebuilding this weekend), what else can I check? Output tube voltage when the amp is turned on?

 

[gadget73]

How were you measuring the pot exactly? That doesn't make sense unless you had the probe from a loudness tap to the wiper instead of end to wiper.

Check the current draw on the output tubes, and check the power supply voltages would be my first suggestion if nothing looks obviously damaged underneath.

Can you post pics of the underside to show what has been done so far?

 

[Zepman69]

How were you measuring the pot exactly? That doesn't make sense unless you had the probe from a loudness tap to the wiper instead of end to wiper.

Check the current draw on the output tubes, and check the power supply voltages would be my first suggestion if nothing looks obviously damaged underneath.

Can you post pics of the underside to show what has been done so far?

With the unit flipped upside down, the output side facing you and the face-plate away, I was measuring the left terminal and middle terminal. I also check both end terminals, and got a reading.

I'll see if I can get some pictures tonight. I am going to dig into it tomorrow.

 

[larryderouin]

Checking them in circuit can give you weird readings as you are reading the rest of the circuit to boot. Disconnect the wires from the pot 1st, test and reconnect). Take plenty of pictures from different angles of the wiring and how they are attached to the volue pot BEFORE doing anything else.