Bit One Mission Successful

I’ve had a broken Crumar Bit One synth sitting around for about a year now. This last week I finally got it fixed up and it’s already on its way out the door (courtesy of eBay).

Crumar Bit One

I bought this from a guy in Chicago last summer. I was heading to Chicago for an old roommate’s wedding and I noticed the synth on Craigslist. I emailed the guy and we started trying to figure out the logistics of meeting up. Apparently I mentioned that I was in Michigan and he immediately grew very cautious and said that the synth likely wasn’t worth driving down to Chicago solely to see and that it had a few problems. I told him I was heading that way anyway and left a little early to make a slight synth detour.

Everything was fine, but occasionally a note would sound sort of weird. The seller wasn’t a keyboard player and wasn’t really sure what was going on. I noticed that this was only happening to every sixth note, and it sounded like the filter simply wasn’t being applied. The Crumar uses Curtis chips for its filters, so I hoped that maybe one of the chips was just bad. I figured out which Curtis chip was tied to the faulty voice and tried swapping in another chip but the problem remained. At that point I got busy with other stuff and the synth sat around all winter. Resolving to finish it up, last week I got it back out and opened it up.

Bit One - Workbench

I don’t have an electronics background and I feel like I’m over my head whenever I start poking around at the circuits. But I’ve watched my friends Kris and Jon (both gentlemen who have a much stronger grasp on electronics theory) troubleshoot circuits and I’ve gleaned that sometimes it’s better to just try to follow your gut, and swap parts out for a true empirical test. I think I’ve erred on wanting to conclusively prove that a resistor or diode was bad before I replaced it. Sometimes it’s better to just have a hunch and then test out that hunch. So in this case, I started measuring voltages around the Curtis chips and found that the voltages coming out of a couple op-amps were far different than all the others. I’ve seen old 70s vintage op-amps fail a couple times in other synths, so these seemed like worthy suspects. A quick trip to Radio Shack and I had two new op-amps installed.

Sure enough, that fixed the filter issue. Apparently one or both of these guys were bad:

The Culprits

I cleaned up a few more little things on the synth and then listed in on eBay. It’s a cool synth and I like it. But these days I’m trying to focus on using my Matrix 12. I’ve trying to narrow my focus a little, so I need to thin out the herd a little. Hopefully this will find a new happy home somewhere!