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.

No Comments

JX3P keyboard fixing

After a quick attempt to fix my Midiverb II again (and failing) I pulled the JX3P out again to have an attempt at fixing the dodgy octave.

Thinking that it was possibly the keyboard decoder, I replaced IC45, which took a fair amount of time to do it cleanly with the desoldering vacuum.

JX3P - removing IC45

While I’ve got the board out…

Crinkly JX3P circuit board

not sure I really like the look of these crinkly paths on the back. Anyway onwards.

Changing IC45 made zero difference, so it wasn’t that. And anyway, I could see the signal appearing on the connector on the keyboard. It took me a while to work out what was going on, and it definitely made it easier yanking the keys out.

JX3P keyboard diodes

This is part of the dodgy section of the keyboard, and it looks a lot like my unhappy Poly61M from the same time. The rubbery circular things are pressed down by the keys – inside the dome is a carbon disc that presses down onto the board and completes the circuit.

One side of the switch connects to the top of the diode below it, with the other half connecting to the decoder IC45 via the keyboard connector at the far side. Pressing the switch should make the signal from IC45 appear on the other side of the switch (the top of the diode).

I can’t believe you’ve got this far, but aaaaanyway, the signal wasn’t coming up on the diodes for the broken section.

The soldered track on the far right here should have given the game away

JX3P keyboard connector and track fix

…essentially the track had failed for some reason. Scratching out the soldermask from either side of the break and soldering a wire across it sorted it. It wasn’t actually super-obvious where the break was from just looking at the track, it took me a couple of test scratches to work it out.

There was also one intermittent key which just needed the rubber switches refitting, and that was sorted too.

I still have lots of screws missing (in the JX, that is…), and the transformer hums like an electricity substation, but at least it all works now.

No Comments

Glitching Wasp keyboard fix

edp wasp inside

Bastard thing. It mostly works now, except when it doesn’t.

Ages ago, after moving house, I got my Wasp out to have a go at MIDI-fying it, and found that it would glitch when holding a note, as if it was flickering quickly between the held note and one much lower down.

Since then the Jasper clone was announced and released, I’ve got mine up and running in the last week, and that seems to work ok (although the VCA envelope sustain gets louder as a key is held down, which isn’t quite right). But the keyboard triggers absolutely perfectly, so it gives me a chance to compare it to my old Wasp.

Jasper (Wasp clone) all done bar the shouting

I noticed irregular pulses on the 4013 at IC44 on both Q and Qbar (labelled NOTE_READ and VCA_ENV_TRIG on the helpfully labelled Jasper schematic). I didn’t trust the Q-to-reset link, so I recreated that on a breadboard and linked to it, still the same. Checked the connections, my soldering might have been wonky – all OK. I replaced the CD4013 with a HCF, supposedly has Schmitt trigger inputs, so could be more forgiving of noisy inputs. Nope.

After much fretting all I had left was to strap a ceramic 0.1uF capaacitor across ground and +5V on IC44, and… it was much less glitchy. I added a couple more (fairly randomly) in the keyboard circuit, and then the glitches stopped.

And only then did I have a look at the 5V line on one of the non-decoupled ICs:

EDP Wasp - noise on the 5V rail

…which doesn’t look like it would much help. The Wasp uses a couple of NE555 timers at heart of the oscillators, and they have a tendency to smash the power rail. Only as far as I could tell the glitch wasn’t at its loudest next to the 555s, so I suspected something else.

Now I’ve put it all back together, I realise that 3.36µs works out to about 297KHz, which is in the same ballpark as the frequency of the master oscillator, which is supposed to run at 250KHz. Next time I open it up I’d be tempted to put a capacitor across 5V and ground on IC11 and see if that quietens it down.

Anyway with the glitch gone, I dared to start celebrating, and then found a different, intermittent problem – G and G# would play the notes B and C. Pressing the keyboard slightly harder seemed to bring the problem on, so it seem likely it was a dodgy solder joint, and I tracked it down to a dodgy connection between IC31 and IC30 – reflowing the joint on IC31 fixed it.

Very occasionally still F# will play a slightly inbetween lower note, but I’ve spent way too long staring at this as it is – I’m calling it done for now.

As a postscript, if you have a knackered or badly behaving Wasp, I had a look around for some help on fixing the keyboard and there was some interesting stuff in this post and comments on the Clacktronics post on the Wasp, but that’s about it. Now there’s a new generation of Wasps springing up in the form of the Jasper clone, the Muffwiggler DIY section might throw up some new help.

No Comments