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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MC-4 power switch horror

Alright, horror is a bit dramatic. Bear in mind I love my MC-4, maybe not quite to the extent that I want to be buried with it when I die, but you know.

The softly glowing power switch had gone wonky, with one side slightly pushed in. I didn’t really think anything of this until the other day when I went to switch it off and it fell in completely, and then started flipping off and on as if it was panicking…

…and then I started to panic, and lunged for the mains switch.

Then the switch fell out completely – here’s the button with the two broken bits.

Broken MC-4 power switch, showing the pivot points having snapped off

Clearly one of the plastic (not delicious) orangey bits had fallen off at some stage, so it was just a matter of time until the other one broke. The bobbley bits are the parts that the switch face pivots on, they clip into holes in the on the inside of the body of the switch:

Bosted power switch

…which is looking pretty manky after forty years or so.

I glued the pivot points back onto the switch in the hope of getting it working again, but I couldn’t balance the contacts and springs and squeeze the switch face into the body it without hearing it all coming adrift.

I didn’t fancy taking a guess at how it should go back together when 230V would be bouncing around inside, so I started looking for a new switch, more out of hope – and found this from RS for a panel cut-out of 30x22mm:

Brand new replacement power switch - shame it's not amber-colour, but can't have everything

Not the right colour, but maybe the right size?

Time to pull the old switch body out, and it was a bit of a fight. I took a photo of the markings on the side of the original in case anyone comes across a warehouse full of the things – marked T100 16A 2WI XII, and also T 65 C UND.LAB JNC LIST.

MC-4 original power switch markings

I made a note of all the connections before desoldering:

Making a note of the connections on the old bosted switch

After carefully heatshrinking the connections, and doing a bit of testing with a multimeter, it fitted in place perfectly:

Replacement switch in place - fits perfectly

Flip the switch, nothing goes bang, it glows…

New power switch glowing on in the murkiness

and success. (Still a shame about the cracked screen, nothing to be done about that really).

Switched-on and now working ok, phew

I’ll keep hold of the original amber switch in case I manage to finagle it back together.

While we’re here, let’s have a look inside. First time I’ve actually had it open, most things usually succumb to the screwdriver in the first week or so – here’s the transformer/noise filter/power supply board.

Transformer, noise filter, and power supply board

Here’s the main board, the Sharp version of the Z-80A in the middle-top, 16K of Mitsubishi-flavour memory on the left and 10K of EEPROMs including the system program on the right.

Roland MC-4 main board, including Z-80A CPU, memory, EEPROMs, and supporting circuitry

I went off into a little dream for a bit thinking, ooh actually, it seems like there’s actually 6K of memory address space free for extra code – maybe we could add some extra functionality.

Even if that’s technically possible, I realised it wouldn’t be much fun actually finding somewhere to put the extra ROMs and wiring up the additional select lines from the ROM address decoder at IC60.

And then desoldering the EEPROMs, installing sockets, reading the EEPROMs, disassembling the code, re-learning how to write Z-80 assembly (not that I was ever very good at it) and then about a billion other things.

It would be easier just to recreate it, like I’ve been talking about for the last ten years or so. Maybe this scare will spur me on.

Here’s the extra 32K of memory on the expansion board, with the connector on the right leading from the main board, and the connector on top leading to the digital cassette interface at the back.

MC-4 RAM board, including 32K of extra RAM and the digital cassette interface circuitry

This MC-4 has a serial number that works out to being built (according to the handy Roland/Boss serial number decoder site) in September 1981, and the date codes on the ICs on the main board all line up with that, but expansion board ICs are dated 1982/83, which is weird.

I would have wondered if mine was an MC-4A (with 16K of memory) later expanded to an MC-4B (with 48K and the digital cassette port) but for the fact that the serial number plate clearly says it’s a B. Maybe the extra RAM board chips were failing and another one was subbed in.

Here’s the back of the front with the lovely period wiggly lines.

Wiggly lines: MC-4 front panel circuit board from the back

Now it’s all back together and working happily.

So anyway, if you’ve got an MC-4 still in use, go easy on the power button.

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fixing the armstrong 626

I’d been using my dad’s old hi-fi tuner-amp without any problems for nearly thirty years, but recently it developed a super-loud low-pitched hum on the outputs, of the type that makes you go – shit, turn it off quick.

It’s been knocked about a bit over time so no photos of the outside, just the inside – if it was in decent nick it’d look like the example here. Here’s the inside from the top:

Inside the Armstrong 626

Lots of brown resistors! Barely any ICs! Some weird retro-futuristic green transistors! Complicated wiring!

Once I’d worked up the confidence to turn it all back on again, I pulled the output from my Soundcraft mixer all the way down: still humming. Turned the volume down on the Armstrong: slightly quieter, but still loud.

Straight-away I was thinking it would be due to a capacitor somewhere, but it’s easy to jump to conclusions, so thought I’d try and take measurements here and there.

The service document/schematic could be a lot clearer for a basic idiot such as me, but I managed to find the stereo amplifier inputs to check with my multimeter, and measured a load of AC there (with the output level from the external mixer set to 0).

The main internal rail voltage on the Armstrong is a chunky 82V, and I measured some ripple on that. Here’s the actual power supply board – the black slightly pointy blobs on the outside of the pair of green blobs are the diodes in the rectifier:

Armstrong 626 power supply circuit board

Actually it would have been a lot easier just to measure the voltage on the main smoothing capacitor; speaking of which, what’s this here?

Ancient 3300uF 100V DC capacitor in-place in Armstrong 626, leaking crystallised electrolytic

That brown-ish crystalline blob that is growing out of the top of the capacitor just next to the blue wire looks very suspicious. OK, given that we’re seeing ripple on what should be a nice flat DC rail and the capacitor looks dicey, let’s just replace it.

Here’s the old one versus the new one – it’s definitely not as pretty a colour:

The massive old 1974-era 3300uF capacitor vs a much smaller 2022-era device of the same rating

…but that sorted it.

Not the most exciting, but I was glad that this one was a boring easy fix.

The two fat 4000uF output caps are obviously the same vintage and will also need replacing. The cap I changed was apparently the first component change it’s needed in nearly fifty years, which is good going.

Really I should celebrate by listening to Radio 3 FM stereo broadcasts, which is what I think it was mostly made for, but recently it’s just been pure Intergalactic FM disco fetish all the way. Poor old Armstrong.

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