Hi guys, I just finished a restoration on a beautiful 500c and have run into an issue I have not run across in over two dozen of these that I have restored. After the work was done I ran the unit for a few hours and it sounded wonderful. This unit had a big mixed bag of tubes so we replaced. New matched quad of EH7591 biased using a 10k pot installed on the rear of the chassis. New 12AX7EH phase inverters, and 12AX7LPS Sovteks in the preamp and phono positions.
After about 2 hours of use I started to notice a crackling in the left speaker. I reduced the volume and there was still a crackle with the volume all the way down. The sound became worse and I started to notice a little stream of smoke from in front of the OPT! Shut down! Put the unit on the bench and powered it up slow on the variac. Little stream of smoke seems to be coming from the PI potentiometer! I marked the position and attempted to turn the pot and it is very rough and crusty feeling. Also, the pot does not seem to turn to the extremes the other does. This unit did have a considerable amount of corrosion on the chassis so it was obviously stored in a damp location. I cleaned the pot as best as we could but still had a poor "feel" to it. Ran the unit again, played perfectly. After half hour or so crackle came back while playing but not when volume down. Seems like it is crackling when there is a demand for power. I have heard of the pots causing noise but the smoking is odd.
So...never had to replace the inverter pots. Manual shows them as 25k. I have a few parts units I can simply replace but wondering if a new part would be better. Something like this : https://www.digikey.com/product-detail/en/cts-electrocomponents/026TB32R253B1A1/CT2160-ND/203781
Or am I barking up the wrong tree??
For reference here is the underside : http://hudsonvalleyhifi.com/wp-content/uploads/2017/05/fisher500c.jpg
After about 2 hours of use I started to notice a crackling in the left speaker. I reduced the volume and there was still a crackle with the volume all the way down. The sound became worse and I started to notice a little stream of smoke from in front of the OPT! Shut down! Put the unit on the bench and powered it up slow on the variac. Little stream of smoke seems to be coming from the PI potentiometer! I marked the position and attempted to turn the pot and it is very rough and crusty feeling. Also, the pot does not seem to turn to the extremes the other does. This unit did have a considerable amount of corrosion on the chassis so it was obviously stored in a damp location. I cleaned the pot as best as we could but still had a poor "feel" to it. Ran the unit again, played perfectly. After half hour or so crackle came back while playing but not when volume down. Seems like it is crackling when there is a demand for power. I have heard of the pots causing noise but the smoking is odd.
So...never had to replace the inverter pots. Manual shows them as 25k. I have a few parts units I can simply replace but wondering if a new part would be better. Something like this : https://www.digikey.com/product-detail/en/cts-electrocomponents/026TB32R253B1A1/CT2160-ND/203781
Or am I barking up the wrong tree??
For reference here is the underside : http://hudsonvalleyhifi.com/wp-content/uploads/2017/05/fisher500c.jpg
hop-head
