[sdiy] Weird Yamaha CS40M problem

[Antti Pitkämäki]

Hi,

I’ve got a Yamaha CS40M with a strange problem, which I’m unfortunately
unable to solve by myself. I wonder if anybody could give any insight into
the following:

The synth works otherwise just fine, but certain settings result in a
strange situation. I can make it happen by setting both oscillators to 8’
square waves (50%), maximum volume for both in the mixer section, filter
resonance at maximum and filter cutoff around 50%. With these settings, the
resonance is mainly boosting the fundamental, and as the oscillators beat
against each other, slight clipping occurs at peak volumes. And then… It
sounds as if the filter breaks into total self oscillation (which the synth
can’t normally do) with extreme volume (wasn’t fun to find out the first
time when playing with headphones as the ”weird oscillation” volume is
much, much louder than the synth’s sound otherwise). The resultant sound
sounds like this screaming farty buzz, almost comical. No key needs to be
depressed for this sound to keep sustaining, it gives a rat’s ass about
anything the VCA is doing. The pitch of the strange oscillation changes
when changing filter cutoff frequency, and it ends if the filter cutoff
frequency is changed considerably. Also turning down the resonance ends the
weird oscillation, after which everything works normally. Decreasing the
volumes of the oscillators to zero from the mixer section does not end the
weird oscillation, but if I do so, I can hear the weird oscillation more
cleanly.

It seems that clipping caused at the filters creates the weird oscillation
situation, at least if resonance is set high, and only square waves from
two oscillators with filter resonance at maximum create loud enough a
signal to reach clipping - or at least that’s my theory. And yes, filters
in plural - Both channels of this two-voice synth are able to create the
weird oscillation, either together or either just one of them alone. This
rules out a defect filter IC, I believe (would have been nice and easy,
although costly…).

I only have a cheap old analog scope which is in a dire need of calibration
and likely recapping (even the calibration waveform looks slightly off),
but nonetheless I did some poking.

The schematics of the Yamaha can be found here:
https://manuals.fdiskc.com/flat/Yamaha%20CS-40M%20Service%20Manual%20(%20HI-RES%20).pdf

I checked the weird oscillation on the FA card (VCF/VCA) at pins A13 and
A23, where the signal enters the VCA and where it also leaves to the PB
card (where the two voices are mixed together before output). To clarify,
pins A13 and A23 send a pre-VCA signal to the final output amp board (both
pre- and post-VCA signals are being sent to the PB card). The weird
oscillation is about 15V peak-to-peak, and looks a bit like a clipped
triangle wave with a fuzzy component in the lower bottom part.

On pins A01 and A02 of the FA card the weird oscillation can only be seen
if a key is depressed - these pins carry the post-VCA signal.

Looking at FA’s +15V and -15V supplies, I see nothing suspicious either
during normal operation nor during the weird oscillation state.

I’m really puzzled with this. Both voices malfunction similarly, but what
causes it? Please let me know if you have any idea on what to do next, I
would appreciate any help a lot!

Regards,
Antti
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