I had been experiencing pitch instability on my 258J VCOs. It seems that the frequency pots had become scratchy, because by rotating them a few times the instability would stop. I decided to replace the old Alpha pots and went looking for a replacement. I found Bournes model PTD901-2015F-B104 that’s a drop-in replacement. The shaft and bushing are the same size as the Alphas, although the bushing is a little shorter. It’s even about the same cost as the Alphas. Downside is the PC board pins that are hard to solder to.
258J with Bournes pots
While I was at it, I looked into adding a voltage regulator to isolate the power to the frequency pots, like the way it’s done on the Dannysound Cali Oscillator. Turns out that the 258J pots here are powered by +V and 0V, so only one 12 volt regulator was needed to drop the pot voltage down from the +15V rail. I had some LM7812 regulators around and there was prototyping space left on the board, so it was easy. The 3-volt lower maximum voltage (3 octaves) meant I needed to adjust the initial frequency trimpot. I set the new range of the pot be from 3Hz to 10KHz.
12V regulator added to board
The soldering iron smush on the green capacitor happened seven years ago, during the initial build.
You might or might not have already figured this by now (just found your site), but an easy way to wire up pcb mount pots is to solder them to a small piece of proto board to which you can also attach your wiring. Nice site btw!
Hi Serdin. Thanks for your comment.
I’ve heard of pot chicklets. https://www.thonk.co.uk/shop/pot-chicklet-pcbs/
Those would be good for an initial build, though I’ve actually never used them myself. This upgrade replaced pots with solder lugs with pots with PC mount pins. I wanted to reuse the existing wires to the board, so the quickest way was just to ‘lay solder’ the wires to the pins. This is harder when you have two wires attaching to the same pin. I suppose that could be handled with pot chicklets or a piece of protoboard, if you planned it in advance by a change in the wiring plan (for instance running a +V wire from the board to each pot instead of jumpering from pot to pot on the panel).