unexciting pro-one octave switch fix

the back of a Sequential Circuit Pro-One synthesiser, showing the logo and the jack inputs and outputs

A few months back the octave switch on the first oscillator of my Irish theme pub synth, the Sequential Circuits Pro-One,¬†started to miss out certain octaves when switching. It’s the switch with a black base and and manky off-white stalk towards the top in this photo:

Pro-One circuitboard oscillator section

I had a look at the schematic and hoped it might be a dead logic IC or something, as I wasn’t sure if the switch would be available forty years later. Also, what kind of switch is this?

A section of the Pro-One schematic, showing the octave switch for oscillator A

I poked around with my multimeter, and boo, it was the switch. It wasn’t making the connection on particular footage settings.

I had a search around and it turns out it’s a double pole, four position rotary slide switch, and according to the excellent A to Synth blog, it’s actually still available in the form of the C&K R20407RN02Q from Farnell or Mouser for about ¬£6 or so.

Although I say that now, I didn’t find out it was still generally available until after I’d ordered a replacement on eBay from a seller in Spain for twenty quid. Curses – but a grudging hats off to the guy in Spain for marketing his listing well, I guess.

I wanted to desolder the original switch cleanly in case I could take it to bits and fix it but it…disassembled itself.

The old rotary switch, having undergone emergency disassembly, removed from the circuitboard

Whups. This was due to a bit of mild violence when trying to get it off the board. At least we can get an idea of how it works – those silver discs slide into cutout sections either side of the base of the shaft, and connect adjacent pins.

It looks almost as if I should be able to rebuild it, but I think I’ve trashed the tiny plastic clips that hold the black top of the switch in.

It took me far too long to get it off the board, despite my desoldering tool.  More as a reminder to myself; the tactics should be:

  • add some fresh solder to the pin
  • press the tool over the pin
  • wait until it obviously melts
  • press the button, waggle the tool over the pin,  and suck all the solder out for longer than you think
  • check that the pin is no longer attached to the hole, but pushing the pin the edge of a screwdriver or something to see if the pin moves, and therefore is no longer attached to the edge of the hole
  • add some fresh solder if it’s still attached, and try desoldering again
  • once you’ve finished, clean all the old solder out of the solder sucker

Here are the desoldered holes:

Showing the octave switch having been desoldered from oscillator A

And here’s the newly soldered switch – being careful to align pin 1 on the switch with pin 1 on the board:

Showing the new octave switch in-place on the circuitboard

And it works.

I mean I am glad, it’s just kind of a bit boring.

Time to put it back together. Thanks for checking it over Wendy:

"Wendy - 12/2/81" written in marker on the base of the Pro-One

To finish off I gave it a bit of a clean-up, and reglued some of the previously-Araldited panel standoffs that had broken, and now it works nicely and looks good again, despite the lack of side panels. I could get some replacements but it wouldn’t fit in the space next to my MC-4, so it stays like this for now.

Pro-One all screwed back together and looking shiny (but still minus its wooden sides)
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