Discussion about the Korg PolySix synthesizer

I replaced the battery and now have this issue, VCO modulates no matter the setting. Even when level is down all the way!

I disconnected the Mod/ pitch wheel and the problem is the same so that's not the issue.

I tried reseating the 367 connectors etc, it's very frustrating because I fixed all other issues with the synth!

any help would be awesome.

Thanks

Do you have an oscilloscope? If so, we could point you to places in the circuit to see if the modulation signal is showing up in places where it is not supposed to be...

Unrelated to the oscilloscope, does the Speed knob for the MG affect the VCO modulation that you're speaking of? If you switch the "MG" target from VCO and set it to VCF or to VCA, does the modulation of the VCO go away and move instead to the VCF/VCA?

Chip

--- In PolySix@yahoogroups.com, "a440dreamin" <superiorvideo@...> wrote:
>
> I replaced the battery and now have this issue, VCO modulates no matter the setting. Even when level is down all the way!
>
> I disconnected the Mod/ pitch wheel and the problem is the same so that's not the issue.
>
> I tried reseating the 367 connectors etc, it's very frustrating because I fixed all other issues with the synth!
>
> any help would be awesome.
>
> Thanks
>

I did recently get an Oscilloscope, yet to hook it up, but could easily do so!

Yes, the speed knob affects the VCO modulation.

When I switch to VCA it then affects the VCO and VCA, if I set it to VCF it affects the VCF and VCO. Any way I adjust it I get modulation on the VCO.

If I set it to VCF and turn the level all the way down then no modulation occurs on the VCF, but modulation still occurs on the VCO.

thanks!

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Do you have an oscilloscope? If so, we could point you to places in the circuit to see if the modulation signal is showing up in places where it is not supposed to be...
>
> Unrelated to the oscilloscope, does the Speed knob for the MG affect the VCO modulation that you're speaking of? If you switch the "MG" target from VCO and set it to VCF or to VCA, does the modulation of the VCO go away and move instead to the VCF/VCA?
>
> Chip
>
> --- In PolySix@yahoogroups.com, "a440dreamin" <superiorvideo@> wrote:
> >
> > I replaced the battery and now have this issue, VCO modulates no matter the setting. Even when level is down all the way!
> >
> > I disconnected the Mod/ pitch wheel and the problem is the same so that's not the issue.
> >
> > I tried reseating the 367 connectors etc, it's very frustrating because I fixed all other issues with the synth!
> >
> > any help would be awesome.
> >
> > Thanks
> >
>

I would use your oscilloscope and start tracing backwards from the voice board to (1) confirm that the VCO is being modulated by the MG signal (which seems obvious) and (2) to see where the unexpected MG signal is present and where it is not present.

For example, on KLM-366, you could go to one of the voices and probe the voltage on R152, which is receiving the pitch CV for that voice. One side of R152 should show zero volts. The other side should show a DC value (related to the note you're playing) along with the MG modulation. Is it there? If so, you've just confirmed that the problem is indeed present on this line. Good.

Now, start moving backwards...

On KLM-396, probe the voltage at R3 (either side should be fine). Do you see the modulation signal there? Again, in a properly-working synth, you should not see it. On yours, you probably do. So, you're still on the right trail.

Back up some more...On KLM-367, probe pin 7 on IC1 (it's on the right side of the schematic, labeled "VCO Mod". Is your modulation signal there? If it was on KLM-396, it also ought to be at pin 7.

Now, backing up again, you've got two places to check: (1) check R9 and (2) check R11. One side of R9 should be zero and one side might have your MG signal. The same is true for R11...one side should be zero and one side might have your MG signal. Your MG signal is probably not on both. On which resistor does it appear?

If it is on R11, it's probably the switching IC that is just upstream of R11 that is broken (is it IC1? its name is right on the fold of the paper). When your MG is switched away from "VCO", that IC should prevent the MG signal from going to the VCO. If it were broken in a way that let the MG signal through all the time, you'd get the effect that you're reporting.

Alternatively, if your MG signal is not on R11 but is on R9, you'll have to keep chasing it backwards through the schematic.

Chip

--- In PolySix@yahoogroups.com, "a440dreamin" <superiorvideo@...> wrote:
>
> I did recently get an Oscilloscope, yet to hook it up, but could easily do so!
>
> Yes, the speed knob affects the VCO modulation.
>
> When I switch to VCA it then affects the VCO and VCA, if I set it to VCF it affects the VCF and VCO. Any way I adjust it I get modulation on the VCO.
>
> If I set it to VCF and turn the level all the way down then no modulation occurs on the VCF, but modulation still occurs on the VCO.
>
> thanks!
>