Sequential Prophet VS

This instrument arrived from one of our regular clients last week so this weekend, we took the time to pop it open and see what’s wrong.

First off, the backup battery is old and could use replacing. Next up, we were able to download and restore the factory patches and waveforms over sysex. This brought the synth up to usable level, so we left it to come up to temperature.

Sadly, as the machine stabilised, the beautiful glassy sounds that this instrument produces had turned to harsh digital noise. We noted that one of the two gigantic CEM 5530 analog sample-and-hold multiplexers is running very hot to the touch. Cycling the power, letting it cool, and restarting, we could see that it exhibited typical thermal failure characteristics. Scoping the outputs while playing a sine patch tells us all we need to know.

Of course, these ICs haven’t been made in about 30 years and are thus as rare as hen’s teeth. Fortunately, we have two potential sources – one French synth tech built a small batch of replacements in 2014, and Tom Virostek from Straylight also produced a run of boards. In fact, he’s just cooked up another batch, so we just may be in luck!

The plan going forward is to replace both CEM 5530s with modern clones – these are considered ticking timebombs by many, so there’s no point in keeping the 30-year-old functioning one in pace. Next up, swap out the memory battery, refresh the power supply and replace the notorious buzzing mains transformer with a modern toroid. Oh, and maybe swap out the now-fading backlit LCD display for an OLED type.