Discussion about the Korg PolySix synthesizer

Thank you, that was a valuable tip (even though it hasn't solved my problem
completely).
There certainly is something coming in over the VCO Mod line but it's only
slightly fuzzier than for example the VCF Mod line (and the self
oscillating filter is stable).
Comparing an original board and a clone in this area uncovered another
discrepancy: The parts list for the clone specifies R10 as 10k with 5%
tolerance while the schematic specifies 10k with 1% tolerance (which is
also used for the original KLM367). My R10 was actually measuring 9.88k
which would be slightly outside the 1%. Swapped it for a 10k resistor and I
believe the situation has slightly improved, at least for the sawtooth.
The PW is still somewhat shaky and I suspect some unwanted PW modulation
which of course could also result in such perceived instability.
Here's an interesting thing which naturally completely slipped by me when
doing the calibration: Is it normal that there is a huge AC signal on the
PW/PWM test point when PW is set to zero? It works quite well for non-zero
amounts but when going to zero there is a sine wave with 1.275MHz and
roughly 500mV peak-to-peak. This of course must somehow influence the PW.

Any ideas?

Thanks and best regards,

Malte




I've only heard symptoms like this after severe battery leak damage. The
VCO pitch modulation control voltage coming from the KLM-367 was picking up
some unwanted signals in damaged areas. With an oscilloscope I could see
that the pitch voltage looked strangely fuzzy compared to other DC signals.
However, this should not happen with a new processor board. It could be a
bad ground connection.
Don B.

From: <mailto:PolySix@yahoogroups.com>mailto:PolySix@yahoogroups.com
Sent: Tuesday, June 28, 2016 11:19 AM
To: <mailto:PolySix@yahoogroups.com>PolySix@yahoogroups.com
Subject: [PolySix] tuning instability?



Hello everyone;

I'm currently trying to fix two Polysix (including fittin a new processor
board) and both show similar behaviour. I am not sure if this is actually
normal (since the oscillators are true VCO's and not DCO's) or if there is
something wrong with those units.
All calibrations went very well and the unit is good in tune. However it
displays a very slight irregular "trembling" or "wavering" in the tuning -
sort of like a very small random modulation. It's a bit as if the pitch
jumps a little bit to the side and is immediately pulled back to the
regular pitch. When playing a chord or using the modulation effect it
becomes practically inaudible; the pitch deviation is perhaps 1 cent (when
it happens).
First I thought that this could come from some instability in the power
supply. However when making the filter self-oscillate the generated tone is
completely stable without the "trembling" phenomenom hence I don't think
this is a power supply issue.

Since it's been some time since I last worked on a Polysix: Is that perhaps
even normal and in the nature of the VCO's? Or is this a known problem?

Thanks,

Malte