Fender Rhodes Janus Preamp Volume Control

[nevetslab]

This past week I was brought two Fender Rhodes Control Panels, one being the Janus Preamp, which needed a volume control. I pulled it apart, unsoldered the pot, then pried the cover off to see if it was just a cleaning issue....CTS pot,1/2" long 3/8-32 threaded bushng, plastic shaft, serrated for the Push-On Knobs. When I got the cover off and looked close at the carbon track, I could see thru the gouges on the carbon over a portion of the arc. Checked it with the ohmmeter, and, sure enough....it went open circuit in that region. I looked thru what Fender parts were on hand across the aisle, but as expected, no such luck finding THAT part. While the cover indicated 5k Audio Taper, it measured 20k. I searched thru various sources on the internet, and ended up on Mouser, ordering a Bourns PDA 24 series pot, 25k Audio taper, solder lugs, 1/4" shaft, 3/8" threaded bushing. Hoping to make that work, expecting I'd use wires off the lugs to mate with the PCB solder pads.

Pots came in yesterday, so I first pried the cover off, just on the slightest chance I could swap out the shaft/rotor & bushing, but, fat change....not CTS-compatible parts.

Measuring across the plastic serrated shaft, it was 0.240, and the metal shaft was 0.249. I measured the push-on knob, to see if there was enough material to drill and tap for a set screw. Nope...not enough material, and I didn't want to use a different knob.

The shaft, being aluminum, made me wonder how to cut serration grooves into it. Not having a mill, let alone the appropriate cutting tools, I dismissed that thought. But, in my collection of carbide deburring tools, I grabbed the tapered triangular one, and took a few strokes across the length of the round shaft, as though I was whittling the end of a branch. Ah....this works. So, I set about whittling the shaft down, rotating the control as I went, also making use of a small tapered file to assist. Took better part of half an hour of whittling the aluminum shaft, but succeeded in getting a rough-textured shaft to grip the push-on knob.

Then, noticing the solder terminals were long enough to modify into PCB mounting terminals, and could splay the middle terminal forward to match that PCB pattern of the CTS pot, I shaped those to fit the PCB, and got it to fit like it was made to go into the unit. The mounting bushing was a touch short, but, as the stock CTS bushings have excess length, everything fit just fine. A friend of mine's father used to tell his sons "if you can't cobble, you ain't worth a damn".

[Helmholtz]

While the cover indicated 5k Audio Taper, it measured 20k. I searched thru various sources on the internet, and ended up on Mouser, ordering a Bourns PDA 24 series pot, 25k Audio taper

Just curious: What made you choose a 25k instead of a 5k pot? (Probably doesn't make much difference, though.)

[nevetslab]

Just curious: What made you choose a 25k instead of a 5k pot? (Probably doesn't make much difference, though.)

Where the pot was in the circuit, following an op amp and feeding Baxendall-style tone controls, it didn't look to make any difference. Plus, that's what was available. And the damaged pot measured 20k, though perhaps with the voids cut thru the carbon track over time by the wiper, perhaps that's why it increased from the original value. Worked just fine in the circuit.

100W Suitcase & Janus 1 preamp.pdf

[Helmholtz]

..though perhaps with the voids cut thru the carbon track over time by the wiper, perhaps that's why it increased from the original value.

That's what I thought. Pots that get much use always increase in resistance over time. I've had wah-wah 100k pots that measured over 300k and volume pots of vintage strats often are above 300k while the tone pots still are around 250k.