Hello!
I am new to the forum and am seeking some assistance. I have rebuilt a Thomas organ, model 821. The organ was produced in the late 60's and early 70's and boasted Solid-State design. Its 2-channel amplifier uses a dual section audio-taper potentiometer, which is rotated by a mechanism linked to the organ's expression pedal, and governs the amount of signal fed into the input section of the amplifier. Over time the pot's resistive windings become worn and dirty and the end result is scratch and static heard through the amp. The OEM pot, being an Ohmite 10K dual with an inch-long, 1/4" in diameter shaft, is hard to find these days. The last one I replaced came from Ebay and cost $40.00. Sadly, it was NOS and lasted about a year. I am hoping to get help in designing some type of dual-channel voltage controlled gate, if you will, that would 1) limit the amount of signal passed to the amplifier and 2) be controlled by a single- or dual-section pot by adjusting a small voltage rather than the raw audio.
Any ideas would be welcomed!
Thank you,
Bruce Nelson
Toledo, Ohio

 

Where in the world are you.........I may have a couple decent used units, or suggest that you could cull one or two from salvaged stereo multiplex units, which is my source............those can be found some in units employed as channel balance or tone control, and don't tend to be used very much, and could be just what the doctor ordered.

as an afterthought, I don't know if what I am thinking may be too light duty -- as they have a carbon-strip resistive element, and the one you describe sounds as though it may be wirewound.

 

One way is to use an analog multiplier or a variable gain amp such as this to control the volume. You need one gain control element for each channel. The gain is controlled by a DC voltage which can be provided by a standard single potentiometer connected to the pedal.

 

Hello,

You could build a VCA using a LM13700, like the one on this page:
http://www.musicfromouterspace.com/analogsynth_new/OLDIESBUTGOODIES/VCA/dualvca-1.html

This is one of the schematics from "Music from outer space":
http://www.musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&VPW=1910&VPH=750

On the menu on the left you will find a lot of entries.

Bertus

 

This arrived at this site a few days ago.
Interested?

 

Some great ideas here. Thank you!

 

I like the J-fet idea. Nice and simple circuit. Any idea what the nuber is on the J-Fet?
I know I've got about 24 volts available and I'm guessing this'll run on 12 to 18 or so.
Again, thanks for the schematic!
Bruce

 

2N5459 or 2N5486 were listed by the person who posted that circuit. (I think it was AG.)