Oberheim Matrix 6 Strange Voice Issues

[plumsolly]

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 attached) 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?

I'll attach the service manual in the next post, but the example in the 3096 datasheet is a little easier to read and the implementation is largely the same.

Thanks.

 

[plumsolly]

Service Manual.

 

[plumsolly]

Anyone?

Just to restate the question: Should a 10 meg meter load down a high impedance input fed from a demux through a 1 meg resistor?

I just want to know if this loading is normal or if it is related to the problem.

Thanks,

Ben

 

[Rob Flinn]

I would suggest you join the Vintage Synth repair & mods group on Facebook.  The people there will know how to help you with this.

 

[plumsolly]

Thanks, Rob!- I'll give it a shot.