Of course, this is the reason fo rall that multiplex stuff. The
analog controlo voltages are digitised (via the D/A converter)
under software control and saved in the battery backed memory.
Otherwise the patches could not be saved and recalled. And yes,
(of course) this is all under software control by the 8048 CPU.
The 1408 DAC chip (it IS compatible with the DAC08) has, like
many DACs of this design a current output. You will notice that
there is no resistor in series to the inverting input of the
following opamp as it is normally the case for an inverting
amplifier. IC27/pin2 (which is the same point as DAC/pin4) is
the /virtual ground/ of the opamp; this means the opamp does all
it can (like all opamps with negative feedback) to make the
voltage difference between its two inputs zero. So we know from
your measurement 0V (few mV) at DAC/pin4 that opamp IC27 is
working.
Comparator IC6 works in conjunction with the software and the DAC
similar like a neg. feedbacked opamp: the voltage between its two
inputs is minimised. It is not perfect (by design) because it is
a digital feedback loop, which means it has discrete steps of
+5-(-5)V / 256 steps, which happens to be ca. 39mV.
So the reason of your problem may lie in IC6, the DAC, or associated
components or the software. Since we know that the software works
(what a rare case!) it only can be that the data bus is corrupted,
which is the most likely reason in the case of a battery leaked
Polysix.
Do you get the full span (-5 .. +5V) for other control voltages?
What is the voltage difference between pin2 and pin3 of IC6?
Are there good logic levels on CPU/pin1 and on all of the 8 data
lines of the DAC?
Is the reference voltage of the DAC (pin14; generated by 2nd half
of IC27) OK?
In any case, if there is a problem with the DAC or comparator or
data lines /all/ CVs are affected. If it is only certain CVs then
the problem lies in the multiplexer and demultiplexer circuits
(4051 chips on KLM369/370 and KLM367).
Johannes
Andrew Jury wrote:
> 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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