Oberheim Matrix 6 Strange Voice Issues

Hey all,

Thanks for having me on your forum.

I just acquired a Matrix 6 and am running into an issue that I have been unable to figure out.

If you are unfamiliar with the Matrix, it is based on Curtis CEM 3396 synth chips (datasheet here) designed to be controlled by a microprocessor. The various CVs are sent from a demuxer through 1 meg resistors.

The problem I’m having is that the pulse width is different for different voices.

Swapping voice chips makes no difference (the problem does not seem to follow the chips). I have run the calibration routine. All P.S. voltages measure ok. The ground seems intact.

I have attempted to measure the pulse width CV at each chip, but my (10 meg) meter loads it significantly.

My first question: Is the loading I’m seeing normal? The datasheet only specifies these inputs as “Hi Z”.

Anybody have any guesses on where to look in general?

The matric 6 service manual is around, but the example in the 3096 datasheet is a little easier to read and the implementation is largely the same.