Pilot 210 Scratchy volume pot

[Kalamazoo]

Pilot 210 Scratchy volume pot. DC on the POT?

Just finished replacing the selenium rectifier, filter caps, and all the non- ceramic Caps in my Pilot 210 preamp. Thanks to all the posts here over the years, my lurking was very helpful as I finished the task.

Both prior to and after the recap, the 4 ganged volume pot is "scratchy". Sprayed it first with tuner cleaner/ lube. Gave it a second shot with non lube precision electronics cleaner then again with the cleaner lube. Still scratchy.
But here is my theory. I do not think it is a bad pot. I have noted lots of threads in various forums referring to their SP210 having a "scratchy pot" I think from the way it sounds and acts that, rather than a bad pot, I have "DC on the POT Syndrome"- DC crackle which changes if I touch the pot using me as a big capacitor) without even turning it.

I have fixed this problem in a lot of simpler guitar amps with new coupling caps. Here the major cap replacement has not fixed it, but there are a gazillion ceramic caps and old resistors I have not touched, and would rather not.... Since this preamp receives its' power from the 232 amp, not yet refurbed, there are even more potential errant DC sources one could envision, so this is not a simple Champ amp fix for me. If I must I can live with it as it sounds very nice and is only an issue when touching or turning the Volume pot.


I hope first I stir some sympathetic "me toos" from the forum, as many may have the problem and mistakenly start searching for what may be an unobtainium pot replacement. I would even better love to hear from the pros here if there are any common fixes like a small cap off the pot to ground etc. Thanks for bearing with the long post!

 

[Kalamazoo]

Here is a gut shot and schematic

Unrelated to the problem in the post, but of possible interest to others:
The Skinny filter cap was restuffed with 2 Nichicon 47uf caps. The fat Black cap was emptied then I attached a terminal strip to mount the 1000, 220, 220 caps to the far right. I used a 2.7 ohm 7 W dropping resistor between the new diodes and the 1000uf cap to bring down the DC voltage to 10.1 V, near the schematic spec of 9.5V. The replaced caps are a combination of Orange Drop 716s and Mallorys.

 

[Tom_r]

Thanks for that info Kalamazoo. I have one of these to recap. I'm also building the separate power supply from one of the other threads .... just in case my SA-232 and SA-260 aren't finished yet!

Unrelated to the problem in the post, but of possible interest to others:
The Skinny filter cap was restuffed with 2 Nichicon 47uf caps. The fat Black cap was emptied then I attached a terminal strip to mount the 1000, 220, 220 caps to the far right. I used a 2.7 ohm 7 W dropping resistor between the new diodes and the 1000uf cap to bring down the DC voltage to 10.1 V, near the schematic spec of 9.5V. The replaced caps are a combination of Orange Drop 716s and Mallorys.

 

[markn2wae]

ANY DC leakage into the pot circuit WILL cause noises even with a CLEAN pot.

Noises coming from you holding the knob or shaft means the shaft is losing ground contact with the threaded bushing, spray a little cleaner around the shaft where it goes inside the control.

Mark T.

 

[Kalamazoo]

By George, That was it!

Thank you Mark! I had hit them a bit before but a robust dual cleaning of the shaft entry point to the pot with the Precision electronics spray followed by the cleaner/ lube has eliminated virtually all the noise

 

[uenjoymyself]

Nice one Mark!

 

[Kalamazoo]

Thanks Tom

Thanks for that info Kalamazoo. I have one of these to recap. I'm also building the separate power supply from one of the other threads .... just in case my SA-232 and SA-260 aren't finished yet!

Glad to contribute my small share Tom. I am sure there are other ways to redo the diode and filter section but I chose this route to keep the original grounding and wiring scheme. I hate to outguess the original designers and introduce potential (pun) new ground loops. From my vintage guitar amp hobby, I try not to drill any new chassis holes during restorations.

I have added photo above to show how I attached the terminal strip to the emptied Black can cap. I then just JB welded the can back together and fitted the black paper cover back on.

BTW, you had asked on another thread where one could obtain the 11 pin female amphenol socket for your planned 210 power supply, and I saw no reply. Check "uliaru" on eBay. He obviously has a business of tearing apart old organs, and always has a variety of plugs and sockets. He was my source for an 11 pin male and female 11 pin amphenol combo. I love old organs, but that seems to be their fate in the age where only the Hammond B3 seems to be a protected species.