[sdiy] Sequential Pro-One repair report

Tom Wiltshire tom at electricdruid.net
Mon Apr 18 18:34:50 CEST 2016

I had some trouble with my Pro-One yesterday. Having been working fine, it started occasionally playing the wrong note when a key was pressed. Over the last week it had done this intermittently, but yesterday this became unbearable since the whole keyboard went funny and wouldn't come back to life.

My initial thought was the cable between the j-wire keyboard and the main board, since that is the weak link on this synth, and I've had trouble with it before. They cut a corner and used DIP sockets and DIP ribbon headers instead of proper connectors. So, time to disassemble it and have a look.

However, unplugging the keyboard lead me to discover that playing the preprogrammed scales from the sequencer also gave incorrect notes. This mean that either the uP was sending bad data to the DAC, or the DAC wasn't working correctly.

Removing both these chips showed considerable corrosion on the pins, particularly down one side of the uP. From the "unusual" smell and stickiness, I suspect mouse urine from a creature sitting on the control panel. Gack!

Cleaning the pins and sockets up and reassembling sorted out the keyboard and sequencer problem, but while putting the synth together, I noticed that it wouldn't operate with the cover tipped up vertically. After some experimenting, I realised it was losing power when the board was tipped up. Checking for cracked traces or loose components revealed nothing, but eventually I spotted that one of the spring contacts inside the power connector from the transformer had broken off. With the board horizontal, it fell into place and made a contact. While I hadn't moved the synth from its usual place in my bat cave, this hadn't shown up. As soon as I started messing with it, it became obvious. Replacing the broken crimp spring connector fixed that.

While I had the synth apart I gave it a full tuning, which I hadn't done for a good while. The tiny holes on the front panel don't line up that well with the trimmers on the board underneath and it's much harder to see what you're doing, so I prefer to tune it when I have the board out. It now sounds better than it has in ages, and I can change octaves again without having to fiddle with the oscillator Frequency knobs.

If there's any moral to the story it's that a working synth can have serious faults you're not aware of, and that wildlife and synths don't mix.


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