I don’t usually take on general repairs, but a customer whose TASCAM 4-track I serviced messaged me about an issue in his Boss SP-303. He said the microphone input knob stopped working. He was willing to do an initial diagnostic and opened it up, and saw that the potentiometer had been damaged. Likely from an impact. The metal part became detached from the plastic base.

If you search the web for this part, you’ll find it’s hard to find. It’s out of stock at the usual suppliers like Syntaur. I thought I was able to find a substitute on Mouser and ordered, it but upon inspection, it was a different design.

Gluing it failed, unfortunately. The potentiometer read a resistance across the ends (~240K ohms), but nothing on the wiper.

I then discovered there was enough room to mount a potentiometer on the hole.

The one I had though was a little to big, so I ordered some smaller ones from Amazon. I had to order all those just for the 250K pot.

When they arrived, I quickly realized I did not need to mount it on the hole. The pins could simply be bent to turn it into a right-angle potentiometer. And the structural pins could be bridged with some thick solid 20AWG “bus bar” wire. I also put some electrical tape under the pot to be extra safe that nothing would short circuit (I don’t think it’s necessary though).

At this point, I thought I was done, but upon testing at high volumes, I noticed some intermittent noises when I was using the pot. It turns out that since it’s a metal pot, touching it is the same as touching the ground of the machine, and I think the body acts like an antenna introducing noise into the circuit. I think this is why a lot of audio gear use plastic potentiometers, because they act like insulators. If I were to do this again, I’d like for a plastic potentiometer.

Thankfully this noise issue is easily resolved by adding a knob to the potentiometer.

Other loose ends

Whenever you have a unit like this on the bench make sure to check for failed solder joins. On this one, I found a couple RCA jacks that needed to be touched up, and also the chasis grounding connections had worn down. I added more solder to these and put it all back together.

Here’s the finished product.

Some useful products mentioned in this post

• Knob: Adafruit Orange Micro Potentiometer Knob – 4 pack
• Potentiometer: 100 pcs Vertical Small 10% Tolerance 15mm Shaft 9mm Potentiometer Assortment Kit for PCB Mount 1K, 2K, 5K, 10K, 20K, 50K, 100K, 200K, 250K, 500K, 1M, Linear, logarithmic, Reverse Log
• Bus Bar Wire: Jameco ValuePro 20 AWG Solid Tinned-Copper Bus Bar Wire 100 Feet
• Glue: Gorilla Original Gorilla Glue, Waterproof Polyurethane Glue, 2 Ounce Bottle, Brown,

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