Assessment, Clean up and Power supply update:

In February, an SCI Prophet 5 Rev 3.3 came my way from Marin County. It had seen better days and had been sitting in a barn for some time. I acquired it sight unseen and only knew that it was in rough shape cosmetically and did not power on.

SCI Prophet 5, first arrival

After getting it to the studio, I gave it a once over clean up as it the keys were covered in dirt, grime and mold from years of sitting and neglect. Once the initial filth was removed, I realized that the synth was missing its fuse and fuse cap. Easy fix! I had a spare fuse on hand though I needed to order a replacement fuse cap. Once I got these in place, it powered right up! However…

From looking at the front panel, several modifications had been made to the control panel- several pots had been added for what looked like modifications to the modwheel controls as well as the polymod section. Additionally, a second row of destination LEDs had been added below the modwheel destinations and some alternate labels had been added (WIND?).

Upon opening the synth to get a better look at what was going on under the hood, I encountered a spaghetti network of wiring and veroboard circuits, totaling 5 additional boards and their related wiring and control pots.


Though the synth powered on at this point, it was making sound but would only make sound in unison mode, the oscillators were not functioning properly (though noise was) and the preset patches were all garbled noise.

Though I was eager to pull out all of the modifications and get this synth back to its original configuration, I knew it would be best to first rebuild the power supply and replace the aging components there, which would have been stressed due to a less-than ideal circuit design especially for the 5V rail. I replaced all of the components on the power supply board and also replaced the power transformer (which hummed quite nicely) with a new cool and quiet toroid transformer. After a quick calibration, I had my +15V, -15V and +5.6V dialed in on the spot.

Original PS board (PCB 5)

With the power supply updated, it was time to move on to the next big task- getting rid of all of the mod mess…

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes:

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>