AKAI VX600 MATRIX SYNTHESIZER
It takes about 15 minutes for the synth to stabilise before doing a VCO tuning calibration. I looked at this on a DVM and found the voltage sits at .22v then rises up to 2.5v over a period of 9 minutes. Why this happens is unclear. I have found that if the auto calibration doesn't complete, or takes an eternity you my need to check the DAC voltage. It should be 2.5v DC. Procedure is in the service manual. If your VX600 sounds bad this is possibly the reason.
If you cannot set the DAC voltage at 2.5v, check the SSM2300 multiplexer IC's. These are roughly equivalent to 4051's but differ in that they have S&H and internal capacitors I believe. This means you can't do a direct swap in the Akai. I discovered a bad SSM by removing them all, plus all the voice cards. I monitored the DAC voltage with a meter and inserted the SSM's one by one, turning the synth off of course whilst doing it. I notice one IC caused a voltage drop and this turned out to be a bad IC.
Some pictures of the insides:
Voice Cards x6. Voices play from bottom to top, when first powered on
Below voice cards. Opamps and multiplexers
If servicing, watch you don't bend the 2 regulators in the middle of the image too much as they can just break off as must have append previously on my synth. The heat sink is not supported
There was some UV damage to the side panels. I'm not a fan of nicotine yellow so I must try and restore these.
The synth can have 6 separate outputs. I believe back in the day the VW-X6 DIN/PHONO cable was available. It possible to make your own using a 13 pin cable similar to those found on old Atari ST monitors, Digikey part number CP-1013-ND. The pinout for the VOICE OUT connector is shown below.