We received this about a week ago, it has been dead for a long time and it was dead now.
But we didn’t even think of powering it up until the Backup Battery situation was checked out, it’s a NiCad and our caution was justified.
The keyboard looks a bit uneven because it is, the Pratt-Reed bushes are totally shot, they were so bad that all of the old bushings broke up while being removed, this requires all of the “legs” that the bushings reside on to be cleaned with some scrubbing and a lot of Isopropanol to ensure no bits of old bushes are left behind, this would make the action uneven and exacerbate the key height differences. Rebushing a 5 Octave keyboard is surprisingly time consuming, especially when the keys are filthy dirty, a recent online poster to a Forum said it took him 9 hours to do his synth and he thought that he was just bad at it… Not at all, we do this a lot and it takes pretty close to that most of the time, at least if you do it right.
This picture is of the IO board (it is called that in PPG Speak but we would call it the CPU board) the battery had leaked but seemed to with hold all of its nastiness under the battery itself, there looked to be some track damage but actually none of the tracks had been eaten away, and the 74HC138 chip at the bottom left of the picture seemed ok, which was surprising as it was immediately in the firing line of any leaky fluids, Another reason to be glad we didn’t power it up.
We put in a big Lithium Ion battery to replace the original, with a diode to replace the 100 Ohm charging resistor that the original had, as you would do with a Korg Polysix or 61.
We went to the main four boards that plug into the Motherboard, there is much folklore on the web that these have troublesome connectors, and there may be some truth to this. On this particular PPG two of the boards had Gold plated DIN connectors, but the other two did not, so we metal polished both of these back to shiny and were happy with these.
We were wrong about the Gold plating, but more on that later.
According to some information picked up from the Facebook- Synthesizer Spares or Repair group we found out that a PPG is a Brain Dead synth until it has it’s backup RAM filled with useful Data, and this certainly seemed to be the case. All the basic functionality of a synth barring sound was evident, we had a fully working keyboard with MIDI out, pitch bend and Modulation included, all of the pots could be altered, the display and all the switches were working, evrything but a noise. We concluded that the IO (CPU) board was good to go. Getting presets into the synth proved to be as problematic as a couple of the synths from last month, every attempt failed miserably. We Scoped the tape interface IC’s and basically got no sign of life from the OpAmp that takes the cassette input, the TL084 quad chip was obviously unwell and had to be replaced. Subsequent testing showed tape signals appearing right up to the IC that feeds the signal into the IO board.
We used the Wav file available from www.ppg.synth.net as this was surely going to be a good one, we played it and it seemed fine but a little clicky at the very end, but went ahead and tried it. At least the synth seemed to be taking an interest in the Data even if it displayed a number ‘9’ error at the end, but it was only at the very end, and unlike some synths, there was enough of a Data load to fill most of the preset banks.
We started to get sounds of this thing which did sound PPG’ish and very nice but they were very intermittent, as in perfect sometimes but degenerated into clicks and thumps for most of the time. When we had the voices working, we had eight of them with no bad artefacts, so all good there. Fiddling with the Proz board (PPG speak, we would call it the Digital Oscillator board, maybe Proz is easier!) in its sockets would make it work or not, which suggested poor contacts, which we thought were Gold plated but in fact were not, dammit we were caught out again with Fools Gold for the third time in the last two months.
Spending a couple of hours cleaning these “Gold” contacts back to shiny Silver took a while, but the results were no better, as the symptoms seemed to be a loss of the waveform data stream from the Proz to the Voice cards. So in our insanity, we pulled out, polished and replaced all 24 of the DRAM chips on the Proz board (that is 24 chips with 16 pins each and both sides of each pin polished). This did not improve things one bit, but no harm to do, as we did clean and reseat all of them, so that is another thing not to worry about. We suspect, as we have for days, that the Motherboard socket connector has been strained and needs to be replaced, the slight bending tension on the Proz board at the B connector does seem to keep the nice noises happening for longer, but there is no way that this is a useable synth, we will have to put this one on hold until some spare parts arrive from far flung places.
Later on in this month, and while working around all of the various pieces of equipment that flood into the workshop, we got back and did a bit more on the PPG. There are some weird anomalies in this synths behaviour, for example, program 2 is a lot more reliable than program 38 for example, as in it is playable for a longer time, and while it is is playable, changing the waveform in the Wavetable on both the main Osc and the Sub seem to work fine. It doesn’t seem to be the Motherboard connectors, we have had them all out, checked for weaknesses, and carefully put back in. There are a lot of socketed Octal latch chips on the Proz board, 74LS377’s and we have those, so a one by one replacement of those is the next step, although we would be very (pleasantly) surprised if it were as simple as that!