poly61m deathsmell fix

I’d been working on a track and moved a bunch of stuff around, including my battered Poly61M. After recording I turned everything off for the night. And mistakenly left the Korg on.

I woke up at about 3am thinking “what’s that godawful smell?”

Dead Poly61m safety capacitor on PSU board

Yeah, that looks wrong doesn’t it?

Mmmm, 30-plus years of dust. In the middle we have a bulging Rifa 33nF paper capacitor. It doesn’t smell very nice.

The synth still works, but worryingly the power switch doesn’t actually turn it off any more, so the capacitor has failed short.

I found the recommended replacement according to the Rifa datasheet – here’s the bottom of the old burned-up one on the left and the new one on the right.

Poly61M burned capacitor

…and now it works fine again: boring fix, sorry everyone.

Here are some random photos of the inside. I had to take the keyboard out to get the power supply board out, gahhh

Poly61M PSU board

Here’s the main voice board, socketed SSM2056 and all

Poly61M main board

And this is probably the main processor board – there’s another board wonkily attached for MIDI on the left which I cleverly didn’t get a photo of.

Poly61M processor board

Yes, the 80s:

Poly61M hot pink

The electrolytic capacitors in the power supply could probably do with replacing – as could the tactile switches on the front panel – but that’s a job for another day.

Oh and the keyboard barely works as well, which is standard with these. I’ve been in it before to try and fix it but they always die again, it’s one of those inherently shit designs. Good thing it’s the M version and I can control it externally.

I managed to threaten a few keys into action by banging seven bells out of it – here’s me absent-mindedly stepping through some presets and CLICK-CLICK-CLICKING the half-dead buttons.

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spi4teensy3 and 74hct595 shift registers

Look at this godawful mess. Look at it.

It’s a few surface-mount 74HCT595s chained from a Teensy 3.6, and it took me too long to work out why it wasn’t working.

More to remind myself as much as anything – don’t declare the clock and data pins as output pins. Like this:


void setup() {
  spi4teensy3::init();
  pinMode(10, OUTPUT); // attached to pin 12 of the 595
  pinMode(13, OUTPUT);
  pinMode(11, OUTPUT);
}

Nooooo. ‘Cos then it doesn’t work.

This’ll do the trick – Teensy pin 10 is acting as our slave select pin in this case:


void setup() {
  spi4teensy3::init();
  pinMode(10, OUTPUT); // attached to pin 12 of the 595
}

And then the lights come on, kaboom.

Here’s some panicky twiddling of a BCR2000 talking to a DIY monosynth through the above mess (plus a bonus DAC). Pitch and gate currently isn’t MIDI’d up but I’ll get there.

The synth bit sounds way better since I sorted the sawtooth waveform out and tracked down the resistor that was out by a factor of 100 in the filter… everything sounded like RAHHHHHHHMAXVOLUMERAHHHH all the time until I chopped that out. And not in a good way.

With lots of parameters being voltage-controlled, maybe it could have presets and a way to edit those sounds and extra software-generated LFOs and envelopes and MIDI program changes and a pony. It’ll be fun working out how on earth to actually do that.

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Roland System 700 716 mixer clone

Roland System 700 716 mixer clone front panel

It looks good, but it’s not very good.

Don’t bother trying to make a PCB of this one, you could stripboard it in half-an-hour or less, I’d imagine. As on the original it’s tacked onto the end of the sample and hold PCB.

Notice no screws for the slide potentiometers. The slide potentiometers are bunged on a bit of veroboard (at a jaunty angle to the strips), with acrylic spacers screwed to the board and glued to the front panel using JBWeld, which is as shit as it sounds.

I had a couple of failures before I got it to stick. I’ve also tried using some special/horrific acrylic cement, which totally failed. There would be a photo of the back of it here but it’s just embarrassing.

Since then I’ve reverted to making PCBs for the slide pots, with screws in the middle of the pot throw, which works much better, and helps reinforce the front panel.

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