My pal Vic has given me her Weltmeister Combo Bass keytar to reinvigorate. There’s not a great deal of info online so I’ll collate what I know here in the hopes that it will be helpful to someone else in the future.

Weltmeister Combo Bass

The Combo Bass was more commonly known as a Basset. These were in production from 1963-69: the ’6510′ serial number on this one makes me think it’s probably from 1965. It runs two and a half octaves from F to C (ish), has a sweet spring loaded volume swell button and two weird looking 1/4″ outputs (two different impedances by the looks).

Hammer assembly

Each key triggers a mechanical linkage that plucks a metal tine of a given pitch. If you wanted to tune it you’d have to fractionally adjust the weight of the tines, maybe with nail polish and sandpaper? I personally like the wobble so will leave it as is.

Insides

There’s a magnetic pick-up that runs the length of the keyboard and fed into an on-board 4.5V preamp. As an exercise in nerdiness I traced the circuit and discovered it is a pretty basic single-transistor preamp, besides the fact that it is positive ground. Back in the sixties transistors were pretty hit-and-miss; positive ground PNP transistor circuits were the most reliable at the time. (More info on positive ground here.) Positive ground confuses the crap out of me so I might have some polarities mixed up but as best as I can tell, the circuit looks like this:

Preamp circuit

The transformer is tricky to measure but I think it’s something like a standard 1:1000 audio. The transistor is an OC1044, but I can’t find out any more than that – anyone have a comprehensive transistor resource?

Circuit tracing sucks because you have to have X-ray vision to see both sides of the board at once. Unless you have photoshop. Take a photo of each side, vertical-flip the back side, and overlay it on the front. Distort and transform til the layers match up, then change the opacity so you can see both at once. See:

Front, distorted:
Front

Back, flipped:
Bottom

Both:
both

If you listen closely you can hear the sound of me being really proud of myself for thinking of this.

I’m going to experiment with replacing the preamp with a nice high-impedance FET buffer, then replace the weird old-school output jack with a standard 1/4″ one. Maybe if Vic lets me I’ll modify that volume pedal so you can switch it over to a wah control. Mmm… maybe I should just let this priceless piece of pre-synth history rest in peace.

Update: in the end even after I replaced the preamp and transformer the output was still awfully low – the magnet in the pickup must have lost its mojo. Consequently I had to disassemble the whole thing and rewire the pickup with 8 turns per tine in place of the original 1. Success!