Discussion about the Korg PolySix synthesizer

Hi,

If you have been following the sorry tale of my P6 repairs then a 
small word of warning for you. If you ever need to change the d/a 
converter (IC33) or come across a non-working one where it has been 
changed beware! My KLM-367 had a DAC08 fitted. This is supposed to be 
pin compatible with the HA17408P which it replaces. IT ISN'T!

Pin 1 on the original chip is not connected, however, on the 
replacement it needs to be grounded! A small blob of solder 
connecting pins 1 and 2 and the KLM-367 is now back in full working 
order.

One for the field notebook I fear...

Cheers,
Andy

P.S. The 'Alldatasheet.com' website is a lifesaver!

--- In PolySix@yahoogroups.com, "Andrew Jury" <andy@...> wrote:
>
> Hi,
> 
> This one is really turning into a pig! 
> 
> Here's what I have done and still I have a sweep voltage on each 
> demux output from -4.58 to 0.16v. Just imagine what this P6 sounds 
> like with only partial control over it's functions!
> 
> 1. An agressive continuity check on data bus lines DO0-7. All OK
> 2. The following ICs replaced IC6, IC33, IC27, IC18, IC19
> 3. The P6 works fine when a replacement KLM-367 is fitted from a 
> working unit.
> 
> I must admit it sounds like one of the positive supply rails is 
> missing. However, all the silicon seems to be supplied with the 
right 
> voltages. All I can assume is that the inbound muxed voltage lines 
at 
> X (pin 3) are wrong and this implies that the d/a converter is not 
> playing back the correct voltage steps when the various different 
> bitmaps are applied to the data bus. 
> 
> Why this one sided representation of the input voltage? Bah! Will 
my 
> hair ever grow back!?
> 
> Cheers,
> Andy
> 
> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@> 
> wrote:
> >
> > 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
> > > 
> > > 
> > > ------------------------------------
> > > 
> > > PolySix "Digiest" Page: 
http://www.acc.umu.se/~amber/Poly6Yahoo! 
> Groups Links
> > > 
> > > 
> > >
> >
>