Sequential Synth Repairs

Pro 1

The Pro 1 is another very popular synth, we think it sounds great, definitely up there among the best monosynths ever made. Despite comments on the web stating that it is plasticky and not very well made, we would have to disagree. The plastic chassis is very tough, never seen even a crack on one yet. The other early complaint was that a transformer mounted on the main circuit board used to break the main circuit board.

All of the Pro 1’s we have seen have had the transformer mounted on the metal chassis (perhaps all European version Pro 1’s were like this).The most common serious problem with a Pro 1 is a dead LFO (which also does sequencer clock and arpeggio rate). The interesting thing about this is that the chip that handles all this is a Curtis CEM3340 with a white dot on it, ( the white dot denoting good low frequency performance), The dead White Dot CEM3340 also rears its ugly head in failures in the Roland SH101 and MC202 both of which used them and both of which die as synthesisers due to this chips demise.


Perhaps the feature of the CEM3340 that made them suitable in a low frequency way made them unreliable in the long term. Obtaining white dot 3340’s has become increasingly difficult, but replacing them with standard 3340’s has worked out fine, no obvious tuning issues are apparent in either the Pro 1 or the aforementioned Roland synths.

Prophet 5 Rev2

Surprisingly enough, we have had 2 of these in the last 6 months. This relatively rare version of the Prophet 5 used SSM chips instead of the later Curtis CEM IC’s. The internet conventional wisdom is that these were the best sounding Prophets and having serviced and played them, they do sound special. In our own humble opinion we think the Rev2 sounds like a polyphonic ARP Odyssey more than the brassy Van Halen Jump sound of the Rev3’s.

The obvious downside of owning this jewel of a synth is that the SSM chips are even rarer than Curtis chips, and the general consensus is that their long term reliability is inferior to the Curtis IC’s.

We do have some of the SSM’s in stock but it’s first come first served on these. An optional cassette interface was available and most examples of this instrument seem to have it fitted.

dsc00183Rev 2 Prophet 5

We have one of the two back again, as a year has now passed. It has a couple of broken j-wire keyboard contacts and an issue with the tuning. It does Autotune OK but after a few seconds it is so out of tune on several voices that it becomes unuseable.

Suspicions obviously lead to either a sample and hold chip or some of the buffer chips after the sample and holds, another possibility is a fault in the DAC circuit. Time will tell, and the results may end up in the monthly blog section.

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