The issue solved partially.

The reason of the Pacman like sound on all VCOs was a broken line on "resistance" circuit upon VCO4 (I am not sure what is the right word) which has to do with the temperature of the VCO. We fixed it.

Now the instrument works ok but a new problem arises:

If you keep nay key pressed for about 2 seconds, a slow lfo comes into all VCOs or each one too, in every octave setting, in every mode (MONO/POLY/XMOD,XSYNC etc), even if filter is off (cutoff 100% resonance 0%) and VCF envelope is 0, even if all effects are off, appregiator is off too.

We have replaced the filter chip 2004 with a working one but the issue remains the same. We have also tuned completely the Synthesizer, VCO, filter, KBD TRACK, according the service manual.

Just to refer that all capacitors are new, including the power supply capacitors. Finally, we have sprayed with a freezer spray almost all chips/gates etc, as the Synthesizer is working for more than 30 minutes, the issue remains the same. Power supply capacitors (filters) give the appropriate Volts to the instrument.

I was wondering if there's a hidden/secret (on service manual) that we don't know or there is an idea what this could be.

Thank you.

--- In korg_mono-poly@yahoogroups.com , "dir_marillion" <dir_marillion@...> wrote:
>
> Thank you Laszlo, we will check your suggestions and will let you know. It is more likely we have a defective component which died due to capacitors replacement as now circuits run with the right voltage.
>
> Will come back next days with more results.
>
> --- In korg_mono-poly@yahoogroups.com , Laszlo Kovari <friendomatic@> wrote:
> >
> > Good work, recapping and socketing always useful.
> > Here are my thoughts:
> >
> > 1. Set resonance to max for self-oscillation. Now you can control the pitch
> > with the cutoff. Set kbd track to max and play on the keyboard.
> > The kbd track cv is generated from the same DAC as the oscillator CVs.
> > If the noise comes from the DAC you can hear an instable pitch.
> > This way you can be assured the noise is not from the oscillators.
> >
> > 2. The DAC ICs are IC 10 and IC11. Type is CMOS 4069UBP, UBP means
> > 'unbuffered', buffered does not work here.
> >
> > 3. You mentioned it was good before recapping. Strange but cap type
> > sometimes makes a difference.
> > Once I replaced a 1 micro electrolytic to a foil cap with the same value
> > and it made the circuit instable.
> > Stay on the safe side and closely match the new components.
> >
> > Laszlo
> >
> > On Thu, Mar 28, 2013 at 8:20 PM, dir_marillion <dir_marillion@>wrote:
> >
> > > **
> > >
> > >
> > > Hi,
> > > We have the following results:
> > >
> > > Turning on the instrument. All knobs working great.
> > >
> > > When I press any key on the keyboard, on any octave and any pitch, with
> > > filter or not (cutoff 100% - resonance 0%), with LFO(MG) or not, I listen
> > > this note which comes together with a strange noise and it ends with a
> > > pacman like sound.
> > >
> > > It has nothing to do with noise knob, which is also working, but it is
> > > different than the "noise" I describe. It seems that this "noise" is a not
> > > a ground like sound. Whatever is the waveform we select on any oscillator,
> > > it seems like a sound that passes through the VCOs (all VCOs), so Monopoly
> > > exports this strange "noise" sound on any key press.
> > >
> > > Technician has already replaced the following parts:
> > >
> > > 0. all capacitors on boards (including the power supply capacitors)
> > >
> > > 1. ssm2044 with a new one (which is tested and working on a polysix)
> > > 2. tl072cn (put on socket)
> > > 3. hd14051bp (DAC)
> > > 4. all the chips on sockets (on cpu board) with exact the same chips
> > > versions
> > > 5. cpu D8049C-217 (which is tested and working on a polysix)
> > >
> > > has already checked the VCO chips and they have the appropriate
> > > temperature while Mono/Poly is working.
> > >
> > > has already checked all board connectors/cables and they are ok
> > >
> > > capacitors have been checked and they are all with correct polarity
> > > installed
> > >
> > > volts on cpu board are just
> > >
> > > no cut lines can be seen as all circuits are in very good condition
> > >
> > > when portamento is at full, it seems that the "noise" comes smoother with
> > > lower volume.
> > >
> > > Thank you for helping me.
> > >
> > >
> > > --- In korg_mono-poly@yahoogroups.com , "dir_marillion" <dir_marillion@>
> > > wrote:
> > > >
> > > > Hi Laszlo,
> > > > Thank you for your message.
> > > >
> > > > I have already forwarded your steps to the Technician. He will probably
> > > check them during next week and I will let you know the results of course.
> > > >
> > > > --- In korg_mono-poly@yahoogroups.com , Laszlo Kovari <friendomatic@>
> > > wrote:
> > > > >
> > > > > 1. Set osc volumes to min and check the noise issue with open filter
> > > and
> > > > > vca. This way you can detect whether problem relates to vcf+vca or
> > > > > oscillators.
> > > > > 2. Check board to board connectors. A loose connector situation can
> > > cause
> > > > > instability.
> > > > > 3. Probably the DAC makes the noise, "pacman" like sound -> instabile
> > > CV to
> > > > > oscillators. Check if all 4 oscillators involved in the issue or just
> > > one.
> > > > > 4. Maybe a bad cap-polarity Double check your recapping work.
> > > > > 5. Recapping makes sense in my opinion. I recapped the power supply
> > > and the
> > > > > vcf+vca board in my MP. Electrolytic capacitors have a 2000-3000 hour
> > > > > lifetime before they go out of specification. Out of spec means they do
> > > > > their jobs less and less effectively, sound becomes degraded and noisy
> > > over
> > > > > years.
> > > > >
> > > > > Laszlo
> > > > >
> > > >
> > >
> > >
> > >
> >
>