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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CV-to-MIDI stripboard thing

That stripboard project is more or less done, although there’s a bit of debugging still to do. It’s a sort of a mini-Roland OP8M – maybe an OP2M, effectively, allowing the MC-4 to control a MIDI synth.

I did a demo of it for a mate, I think he was a bit mystified. Here it is – I got slightly carried away, as usual.

It’s the usual MC-4 syncing a TR-606, with the out-of-tune big black modular bass synth being the System 100M clone, and the parpy MIDI’d up thing being a JX-3P.

It takes CV1, CV2, gate, and MPX from the MC-4. (MPX is a digital signal and is either on or off for the duration of the step).

One switch selects the octave for the base note, and the other switch is for selecting what happens when it receives an MPX signal from the MC-4.

Position one of the MPX switch selects sending CV1 to a different MIDI channel (so you might get a different sound), position three sends CV2 to the same channel (so you get a second note), and in the middle it’s effectively off and just uses CV2 for velocity.

My JX3P doesn’t do velocity, and is only responding on MIDI channel 1 in the video above, so we get this muting effect when switching to “cv1 diff ch”.

I fret about the timing, as always. I’d buffered the CV signal before it hits the ADC to get a low impedance, but it still takes a fair number of samples before I get something reliable. My Poly61M definitely seemed a bit late when triggered from it, but there’s every chance the MIDI in that is just slow.

Anyway it’s working well enough for now, although there’s a minor bug that needs sorting. It should give me different options when I do my long-winded MC-4 jams.

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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.

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