[sdiy] Synare 2 repair notes

[Grant Richter]

Just thought I'd follow up with a few notes on the Synare 2 repair.

The problem was no sawtooth or PWM square.

The unit is quite strange, the VCO is actually a port pin from an 8752
microprocessor. The tune control is the RF clock for the microprocessor. the
analog section is powered from +/- 7.5 volts.

The drum pads are encoded by an 8752 microprocessor using 6 x 4051 as a
matrix encoder. The micro generates a square wave and controls a MC1408 DAC
through a PIA chip.

Here is the interesting part, the square wave is used to reset a capacitor
connected to a current source controlled by the DAC. So as the frequency
changes, the current source is changed to keep the saw amplitude constant.
There is a trimpot to set the initial amplitude, that was open circuit,
preventing the current source from working.

Once the saw started working, so did the PWM (741 used as comparator).

The DAC output scaling is 460 mv per octave, so with an amplifier you could
scale it to output 1 volt per octave. The gate signal is also a port pin on
the micro (5 volts).

The filter is an SSM2040 chip in a 4 pole lowpass mode, VCA is a 3080 and
the envelope gens are discrete transistor.

Does anyone know how to use the sequencer on this thing??