Sequential Circuits Prophet 600

This was a recent repair job. When it was dropped off, the owner said it had a bunch of non-responsive keys and he wanted to see about getting the battery replaced. Testing it, however, showed an additional problem. The patches were all really, really wrong, and it would hang up randomly. I let the customer know; he just said “fix it’.

One of the things I absolutely love about older equipment is how easy a lot of it is to work on. Four screws, fold the control panel up, everything is there in front of you.  The date of manufacture – 10/19/83 – is clearly visible here. I was in high school when this rolled off the line.

I’ve got keyboard rebuilding down to a (tedious) science. As expected, it took about an hour. Also as expected, all keys worked.  I checked the battery; despite it being the original I was still getting the correct voltage. Since there’s no way to know how long that would continue, however, I went ahead and replaced it. Although it’s mounted from the top, I had a holder that was perfect for it.

Replacing the battery, of course, completely wiped out the memory. I was unable to get the factory preset WAV file to load, so I used MIDI-OX to push the SysEx files over. One bad thing about this unit is that there’s no indication that it’s receiving, so had to unplug it, schlepp it downstairs, and hook it back up to the amp to see if it worked. And it did! The memory must have been hosed from the start, because all the patches sounded fine and the locking up problem went away. But…

Two of the voices were really, really off; the oscillators were nowhere near each other in pitch. Since the unit tunes itself, that meant that the main voice board needed to be rescaled. Fortunately, SCI built in a relatively painless procedure for doing this. It took about twenty minutes, but after that it was all working perfectly.  I replaced a few missing screws, touched up the stain on the end panels, vacuumed the inside, cleaned the outside, and let the customer know it was done.

He got back a fully-functioning synth, and I got to mess around with a classic piece of vintage gear. And got paid for it,  too…everybody wins!

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Posted on August 12, 2012, in Repairs, Synthesizers and tagged , , . Bookmark the permalink. 2 Comments.

  1. Comment from Chris copied from “About” page.

    Hi,

    I really like your blog!

    I have a question – I have a Prophet 600 that, up until recently functioned perfectly. One day I turned it on and, although it lit up and everything seemed normal, it failed to make any sound when keys were pressed down. My guess is that, since it was sudden as opposed to gradual, the battery has died. Before I go ahead and replace the battery, I wanted to get your opinion. Also, I noticed on your Prophet 600 repair post you replaced the original battery with a holder/new battery combo – and I wanted to know which you used and where I can get them to throw into my P600.

    • Two possibilities. If the control panel is responding normally, the battery is most likely the culprit. There are any number of replacement lithium batteries that will work; I use a high-capacity 1/2AA size.

      If the control panel is locked up and pressing buttons doesn’t do anything, the culprit may be dirty pots. Since the microprocessor scans the front panel for changes in the position of any controls, a dirty pot can effectively force the system into a loop, freezing it. Cleaning the pots and switches at the same time the battery is changed is probably a good idea.

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