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
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Re: Anyone know how it really works?
2008-09-04 by Andrew Jury