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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scrappy kobol-ish ads envelope

My dodgy Kobol-ish ADS envelope in a Eurorack module in my kitchen

Didn’t have any pot plants to hand, so apples from the tree in the garden had to do.

I’ve been meaning to add the RSF Kobol to my pile of started-but-unfinished projects for ages. It’d be much easier to buy the recent cheap clone, but that’s hardly any fun, and we have to do things the hard way.

Now a modern version of the SSM2040 is available, the only real sticking points are the envelopes, which are the now super-rare SSM2050 in the original. The more five volt-y 2056 is probably the closest, and that’s easier to find but they’re still really expensive.

The gods that are Alfa in Latvia have recreated the CEM3310 (as used in the Pro-One, a bunch of Oberheim polysynths, the PPG Wave, and a load of others) and that seems like a fair substitute. The circuit will need twiddling to fit it in, as the output is half of the 2050 at 5V and the inputs expect a range of negative voltages.

I needed a way of testing out the circuit changes for the 3310 before starting on drawing out the main Kobol board, and I couldn’t be bothered with sorting out a breadboard so I made a little test PCB (basically the Digisound 80-10 with the input voltage levels adjusted and the “decay” switch tacked on) with the thought that I could re-use it as a Eurorack envelope.

Here’s a sample ropey oscilloscope shot (as they all will be, sozza) of the output at minimum attack and decay.

Oscilloscope shot of my Kobol-ish ADS envelope at the fastest possible speed - 0.9 milliseconds

The minimum timing in the original is quoted at 1ms, so the 22nF timing capacitor I’ve used here is – and I can’t believe I’m saying this – really just a bit too fast, in that any waggling of the pot has no appreciable effect until about where the number 2 would be on the decay pot.

Also the maximum release time is a bit short at about 10 seconds, so I could probably do with bumping the timing capacitor size up a bit.

Obviously I’m zoomed in quite a lot here, but I was interested in that flat area at the top of the envelope after the attack finishes and before the decay starts, as it’s not like the usual Roland envelopes I’ve been looking at. Here’s a zoomed-in photo, I wanted to measure how long it lasts – 130 microseconds, ish.

AS3310 envelope flat-top

I wondered if this was down to my particular implementation of the AS3310, so while I had my Pro-One apart to try and fix the octave switching I had a poke around in there with my oscilloscope probe.

This is the Pro-One envelope at full-speed:

Sequential Circuits Pro-One envelope generated by original CEM3310, showing the fastest possible envelope shape

There’s that flat-top again, lasting for about 130 microseconds. It seems like Alfa have done a good job replicating the basic shape of the 3310 output.

While I’ve got it apart to fix something, here’s the CEM3310 in-place on the Pro-One main board.

CEM3310 envelope chips in-place inside a Sequential Circuits Pro-One

Just for a comparison, here’s the envelope shape from my 100M 140 envelope clone, set to fastest speed – ignore the timing differences, see how it hits a point and falls sharply from that:

Roland System 100M 140 envelope shape shown on oscilloscope

Alright that’s lovely, but does it make any difference to the sound of the thing being controlled?

No audio demo here this time, but I set my 100M 140 clone to the same timing as my 3310 envelope and had a listen and… they sounded about the same. Which should have been predictable really, it seemed like a tall order to be able to hear 130 microseconds.

I should mention the decay switch, as I wasn’t 100% sure what it was going to do, but it seems obvious now – it’d be better named “release”, as it enables/disables the release portion of the envelope. This sounds a bit rubbish generally, but maybe it’ll be useful in some contexts.

I thought was being Quite Clever using stripboard to bridge between the potentiometers attached to the front panel and the circuit board attached using a metal backing plate, with the standoffs glued to the metal plate and screwed into the stripboard, but it just meant I spent hours trying to work out where I’d bridged the stripboard tracks.

My Kobol-ish envelope generator resting on its side, trying to show the way the circuitboard is attached to the front panel

Next time round I’d use a chopped up pad board instead.

Either way it’s still neater than some of my previous shit builds; when Mr Beep borrowed my Polivoks filter a while back, he was a bit disconcerted to find the circuit board was attached with Blu-tack. For some reason he didn’t seem too keen to use it, I dunno.


clone experimenting

a circuit board inside a synthesiser

Just to see if I could really, and maybe also to see if a clone could sound the same as the original.

As it turned out it was too much of a pain in the arse to try and calibrate them in the same way, and I ended up being happy enough with it.

I tried to make it as similar to the original as I could, including the lovely looking but otherwise pointless bendy tracks and opamps on the wonk, although I’ve gone for multiturn presets and DIP outlines rather than the original cans, because there is a limit, shockingly.

The stripboarded TA7136P replacements are especially classy, I must get round to doing a board for those.

I’m going to try and add this as a module in my forever continuing rotary 100M project, although the chunky connectors are going to make it hard.

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