adventures in DIY music

Saturday, 6 May 2017

Roland Jupiter Eight

In the early nineties, on the look out for second-hand bargains, I would often trawl the music equipment stores of the Gold Coast. Once, I absentmindedly wandered past a big synth sitting unloved in the back of "Musician's Pro Shop" at Mermaid Beach. It was on consignment, with a price on it of around AUD$700 or so, as I recall. It looked a sorry sight, covered in scratches and scars, with rusted screws. Apparently it had belonged to a hard-touring aussie band during the eighties. It looked "heavily gigged".
Yet, as I looked at its rainbow of buttons and bright orange labels, a dim memory was surfacing. "A Jupiter 8.. hmmm, why do I know that name? Wasn't that once a big deal?". I had murky visions of nameless gigs at the Playroom, where some band's keyboard player, with the ubiquitous spikey haircut, stood behind a big synth sporting an orange pilot light, and the word "Roland" in white letters across the back.
Finally I remembered. When I had been falling in love with synth music, this synth had been the queen. Especially in 1982, its sound was everywhere.

"It's got MIDI" the Pro Shop salesman said, kind of optimistically, and he held up one end of a cable that definitely was NOT a MIDI cable. I found out later it was a DCB cable. One end was attached somewhere inside the synth at the back. Unfortunately there was no converter box to accompany the other end.
Anyway, it sounded incredible... and all those controls! It made those string pads I had always lusted after. There weren't too many problems with the electronics as it turned out - in fact, it just needed re-calibrating, and one oscillator was "sour". Darryl Watson, the Brisbane Roland tech, sorted that out for me, and after that she sounded as good as new. Shame about how she looked...

These days, I fix the Jupe and most of my other vintage gear myself, having picked up some synth electronic know-how and wisdom from textbooks and generous people around the internet (many thanks to Kevin Lightner, Doug Terrebonne, Juergen Haible, Steve Jones, and the Synth DIY mailing list folks).

I've noticed a couple of problems keep cropping up from time to time with the JP8. One is the "sour" VCO trouble, where the VCO just won't be tuned, although it is not out by a great margin, but enough to annoy. The other thing I've seen a few times is a problem with the selection of the waveform. A particular waveform will be absent or quiet, or just not sound the way it should.
The first problem is not on the VCOs PCB as you might first think, but further afield, over on the "Interface" board, that contains the DAC and the sample and hold circuits that control each VCOs pitch CV. A dual op-amp (TL082) buffers the hold capacitors for two adjacent channels, and somehow when this chip gets faulty the pitch CV going to one or both VCOs gets unstable. Often, only half the chip is faulty, so only one channel is affected. 

You can tell which op-amps I've replaced, cos they're in sockets. 

The same problem, but on a different PCB (the "module controller" PCB - there are two, controlling four voices each) will cause a fault with the control voltages for modulation and the envelope values. You should suspect this if 4 out of 8 voices have an unusual response to some parameters e.g. sustain level, decay time, etc.

The second problem I've found to lie with the waveform selector chip on the voice PCBs, a 4052 analogue switch. These chips can develop faults, sometimes only on one channel, causing a quiet voice, or wrongly selected waveform.

When they came from the factory, all but the earliest JP-8 s used a protocol called DCB to talk to the outside digital world. This was just before MIDI arrived in 1983. After that, Roland made a box that could convert midi to DCB messages (notes and patch change only). There were also several possible retrofits (i.e. an aftermarket modification to the synth itself) that gave the JP-8 midi capability. Either way, this connection gave you access to another feature that is not often mentioned: the Jupe can act as a midi/DCB-to-CV and gate converter, thanks to its CV and gate outputs round the back. This single channel of CV/gate comes from the highest note that is generated by the CPU, whether in response to the keyboard or via midi/DCB. The CV is 1 volt per octave, and the gate is a nominal 15 volts. So you could use this to drive your modular, or any one of a number of analogue monosynths. 

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