Discussion about the Korg PolySix synthesizer

I've been wondering why some models have the known LFO bleed problem and
some don't. Is there an accepted theory for this?

If not I would like to offer another possibility.

I recently picked up another "partly defective" Polysix. The description
was a bit ambivalent - some keys don't work, the battery has leaked, it
sounds funny - but all the program buttons and LED's work fine.

A first test basically confirms the description: contact problems from the
keyboard, the programmer section works basically, most of the knobs seem to
perform their function at least partly; and it sounds funny (like there is
some kind of very quick amplitude modulation on the voices; almost like a
modulation by mains frequency).

And there is very noticeable LFO bleed.

Close examination reveals that the battery has started to leak but not
much. In fact all the traces on the top layer are still intact. Still, IC31
and 30 get removed because I don't want to take any chances and can clean
this part of the PCB thoroughly.

Next stop: Key contacts (and key tops). Those were very dirty; soaking them
in water removed an awful lot of stuff.

While the keyboard was out of the Polysix I looked at the plastic guides
that fix the PCB's under the keyboard. Those guides were yellow-grey.
Rubbing them with alcohol turned them nearly white again.

This got me thinking: Doesn't that mean that a similar amount of
contamination and residue will exist on the PCB's as well? So the PCB's get
a thorough cleaning from both sides, too. No, it did not look nice. There
was really bad contamination on the component sides of the boards.

The interesting part: After this cleaning both the LFO bleed and the
strange high frequency modulation have completely disappeared.
From other synths I have learned that some sections are very sensitive to
contamination. For example, on certain ARP synths the buffering of the
keyboard CV can be somewhat erratic if the PCB isn't properly cleaned.
Could it be that the LFO section of the Polysix is similarly affected?

We all know that the Polysix was one of the "affordable" synths back then -
which of course means that many of those machines will have been gigged
continuously. So they were exposed to really unhealthy amounts of cigarette
smoke (and possibly other things).

Any opinions?

On the Polysix I'm currently working on there was some gunk under the CN06 header which seemed to be leaking the LFO signal. I just pulled the crimp pin out of the connector instead of trying to desolder the header to get underneath to clean. So, yes it probably is a contamination problem. Deoxit and a toothbrush work wonders a lot of the time, but I think there are some chips where you will never get the gunk from underneath without removing the chip, and I've seen these bleed the LFO into VCA, VCF and VCO. Moving a wire may be preferable to cutting out and replacing a few chips.

Don B.

--- In PolySix@yahoogroups.com, Malte Rogacki <gacki@...> wrote:

>
> I've been wondering why some models have the known LFO bleed problem and
> some don't. Is there an accepted theory for this?
> 
> If not I would like to offer another possibility.
> 
> I recently picked up another "partly defective" Polysix. The description
> was a bit ambivalent - some keys don't work, the battery has leaked, it
> sounds funny - but all the program buttons and LED's work fine.
> 
> A first test basically confirms the description: contact problems from the
> keyboard, the programmer section works basically, most of the knobs seem to
> perform their function at least partly; and it sounds funny (like there is
> some kind of very quick amplitude modulation on the voices; almost like a
> modulation by mains frequency).
> 
> And there is very noticeable LFO bleed.
> 
> Close examination reveals that the battery has started to leak but not
> much. In fact all the traces on the top layer are still intact. Still, IC31
> and 30 get removed because I don't want to take any chances and can clean
> this part of the PCB thoroughly.
> 
> Next stop: Key contacts (and key tops). Those were very dirty; soaking them
> in water removed an awful lot of stuff.
> 
> While the keyboard was out of the Polysix I looked at the plastic guides
> that fix the PCB's under the keyboard. Those guides were yellow-grey.
> Rubbing them with alcohol turned them nearly white again.
> 
> This got me thinking: Doesn't that mean that a similar amount of
> contamination and residue will exist on the PCB's as well? So the PCB's get
> a thorough cleaning from both sides, too. No, it did not look nice. There
> was really bad contamination on the component sides of the boards.
> 
> The interesting part: After this cleaning both the LFO bleed and the
> strange high frequency modulation have completely disappeared.
> From other synths I have learned that some sections are very sensitive to
> contamination. For example, on certain ARP synths the buffering of the
> keyboard CV can be somewhat erratic if the PCB isn't properly cleaned.
> Could it be that the LFO section of the Polysix is similarly affected?
> 
> We all know that the Polysix was one of the "affordable" synths back then -
> which of course means that many of those machines will have been gigged
> continuously. So they were exposed to really unhealthy amounts of cigarette
> smoke (and possibly other things).
> 
> Any opinions?
>