Archive of the former Yahoo!Groups mailing list: Discussion about the Korg PolySix synthesizer

Hi,

Seem to be getting nowhere fast with this one. I made the
observations with the oscilloscope you mentioned and seemed to be
able to reproduce the waveforms you described, if a little on the low
voltage side.

However, careful study of the schematic reveals that the tansmitting
and receiving mux are not directly connected. Which adds to the
confusion! The data from the pin 3 of the 14051 on the control panel
terminates on the non-inverting input of comparator IC6. The voltage
being compared is that fed back from from the output of the D/A
converter IC33. My guess is that when you turn one of the pots on the
multiplexed bus the actually control is provided by the logic of the
CPU programmed via pin T0. This would mean that the actually
multiplexed voltages are actually controlled by the D/A converter
rather than receiving the raw voltages directly from the transmit end
(if that makes sense). Now here is the bombshell... The DC voltage at
pin 4 of the D/A converter is always 0v (strictly 0.00v, like it is
grounded, which it isn't!) The output of op-amp IC27 varies only a
matter of millivolts when the controls are turned, which would
explain the weird MG control and lack of release on the VCA. So I am
guessing that the data bus d0-d7 is low/missing? I noticed on this
board the 1408 has been replaced in the past with a DAC08. Looks pin
compatible, but maybe not.

Oh, and you'll like this! I managed to purloin a working KLM-367 from
a friend and transplanted it into my P6. It works fine showing all
the peripheral circuitry is working ok. Back to the continuity
testing...

Cheers,
Andy