JX3P MIDI in fixing

I bought a rough old JX3P a few years back for not much.

programmable preset polyphonic

It sounds surprisingly nice – whoever had it before me had programmed in some pleasantly woozy sounding patches into bank C and D.

The chap I bought it off was upfront about the MIDI in not working, and I naïvely thought they might have just not checked the switch that selects between the PG200 programmer or MIDI in.

Haha, no.

Tried the usual swearing, different cables, different sequencers, different swearing, nothing.

Let’s have a look inside, there’s a few things wrong.

Battered JX3P internal overview

Look at this transformer! It hums like a bastard, hence this odd arrangement to try and dampen it.

Wonky JX3P transformer

Needs replacing really, but it works OK.

Something heavy must have fallen on it at some stage and smashed into the last button on switch panel, it’s still a bit wonky looking.

Wonky 16 button

Here’s some wiring to fix the cracked part of the panel switch board; good effort previous owner. The “16” switch LED nearest this calamity doesn’t work any more but on a quick check, the connections beep out fine.

JX3P switchboard extra wiring

On holding an LED to the two contacts on the back of the board it happily burst into life. Closer inspection revealed that the impact had just pushed the LED pin down and popped the joint, you can just about see it here.

16 button LED dodgy joint

Reflowing sorted it.

Let’s play the keyboard, oh dear. Well it mostly works, some are a bit intermittent, but keys 17 – 24 don’t do anything despite heavy bashing. I’m pretty sure this is a recent problem.

Usually I’d assume that the key contacts are dusty (hello Poly61 and Polysix), but it’s a bit suspicious that exactly eight keys don’t work. It turns out that the keys are grouped in eights for scanning, so I’m suspecting either wiring or the demultiplexer IC45.

But I’m not looking at all that now, I’ll do nothing else this weekend otherwise.

Here’s the DIN board.

JX3P broken DIN board

Looks alright. Let’s switch the switch a bit, my Poly61M has godawful switches. Nope, nothing.

The optocoupler is the first thing that the MIDI connector hits, so maybe that got fried. I bought a replacement PC900 then thought should really test it rather than just blindly replace it.

As far as I could tell, I was getting a signal on the other side (pin 4). Booo. Here’s the schematic for the DIN board.

JX3P DIN board schematic

The switch for MIDI enabling is grounded for when switched to memory protect and programmer, and is unconnected when set to MIDI in – that line (RX MODE) is then pulled up by the 5V line through the 10K resistor at R5 – so high = enabled. The 10K resistor was definitely getting 5V, but the other side of it read as 0V, whether or not the switch was set to MIDI in.

Having temporarily chopped the jumpers at W4 and W5 to make sure nothing was pulling it down further down the line, all that was left was the 74LS00.

For some reason I’d bought a couple of these eight years ago – no idea why – so here’s the bosted one:

Duff 74LS00

…replaced it with a socket and the new one

JX3P DIN fixed

and now it works!

Score one for methodical troubleshooting rather than just blindly swapping stuff (which is never the answer, really).

I lashed up a slightly ropey CV to MIDI converter to play the JX from my MC-4, and then belatedly realised that it’s not possible to edit the synth from the front panel while it’s accepting MIDI input. Think maybe the Kiwi3P upgrade (or more likely the Organix MIDI expansion, given the cost of the Kiwi3P) might be on the horizon, it’s a nice sounding synth.

Done for now, anyway – although I’ll come back to the keyboard soon. Ish.

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