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!

One thought on “Bit One Mission Successful

  1. Hi! Interesting. These Crumars are really kind of mysterious and rare instruments to my eye. Desirable, at some moment. Another day I wanted one. But it is quite difficult to obtain synths in good condition, properly mended. And then, there are other issues: careful transportation, price. I ended up with old Polivox from garage of one man from my city. It was not good at all and I sold it.

    Thank you for posting!


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