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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midiverb II fettling

Alesis Midiverb II front panel

I bought a Midiverb II a while back, and when it arrived, it was dead.

It was advertised as working so I should’ve sent it back, but it was thirty quid and I thought I’m a super–DIY-electronics-pillock who totally knows what he’s doing – I’ll have this fixed before bedtime.

Four years later, and the Midiverb had fallen into a tiny gap between the guitar amp I got when I was fifteen and an Ikea shelving unit, still waiting for me to resurrect it.

Here it is in its dead state:

Midiverb dead state - all LEDs on, nothing on the numeric LED display

The dust! You’d think I’d have given it a bit of a wipe before taking a photo.

And inside – the board takes up about three-quarters of the rack.

Overview of the Midiverb II circuit board

The big square chip with the sticker on it is the magic Alesis reverb chip, and the chip to the left is the DAC.

The ICs above are for memory, and on the left-hand side there are some old-fashioned silver-topped TL084 quad op-amps, some of which might be used for output.

I’m guessing because there’s no schematic available, which makes it more exciting/fruitless, although the Midiverb III service manual might be similar in some ways. Anyway, for the Midiverb II this page on wolzow.com gives a far better impression of the insides than I ever could.

Here’s a random picture of the power regulation bit – the Alesis external PSU is a 9V AC adaptor, which gets rectified into +12V, -12V and +5V.

Closeup of Midiverb II power supply

I optimistically replaced the capacitors in this section a while back, plugged it in, and when it still didn’t work I thought, oh well, that’s me fresh out of ideas.

Looking at the date codes on the ICs this seems like it was probably a good thing to do anyway.

I rechecked the +5V power supply after reading this post on anlage-e – seems like some of the decoupling capacitors go bad after a while – no shorts, seemed fine.

Recently I plugged it in again and boringly got the same result, all LEDs on, no LED display. Maybe something to do with the microprocessor? I wondered if the reset line on the 80C31 wasn’t going high – measured it on the oscilloscope, it seemed to be blipping high when plugging the MVII in.

Alright, next thing – clock on the 80C31, it should be ticking along at 12MHz, what have we got?

FLAT LINE. (I should’ve taken a photo, although it wouldn’t have been very interesting). At this point I dragged the probe slightly, LEDs flickered, and the front panel 7-segment display burst into life.

Tears of joy.

I worked out that if I flicked the resonator next to the 80C31 hard enough with my finger it would jump-start the clock – restarting the Midiverb would produce the reset signal and get it going.

This wasn’t much of a fix, so I replaced the resonator with a brand new 12MHz crystal.

And it’s been working happily ever since. Here’s a picture of it under my Quadraverb, on the infinitely large patch 29.

Here’s an edit of the second thing I did with it, which might be an actual track eventually. Soz about the quality, it was onto cassette tape…

The main high-ish sound is the System 100 102 expander through patch 29 on the Midiverb, the bonky bass is the 100M clone, drums are a 606, and the pad-ish synth at the end is the Poly-61M. All running off the MC-4.

Anyway, what have I learned from this? The main thing is that digital things aren’t totally unfixable. Also, the lack of a service manual isn’t the end of the world.

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