[sdiy] Weird Yamaha CS40M problem

Antti Pitkämäki antti.s.pitkamaki at gmail.com
Mon Sep 20 10:18:03 CEST 2021

Now that I think again, the weird oscillation can appear on either of the
filters of the two voices, independently or together. The resonance control
voltage is common for both voices, so I don't see how it could cause one
filter to self oscillate while the other one operates normally. So the 4458
is still the best candidate for replacing, I think.


ma 20. syysk. 2021 klo 11.08 Antti Pitkämäki <antti.s.pitkamaki at gmail.com>

> Thanks, Michael! When I disconnected the VCF/VCA board from the final
> amplification board, where the 4016 is located, I could still see the 15V
> p-p "weird oscillation" in the VCF-section. So the 4016 is not connected to
> the problem, but it explained why the weird oscillation reaches the output
> even when the VCA is closed. I'll try to do some further measurements
> today. I want to see what happens to the filter resonance control voltage
> when the weird oscillation appears. The weird oscillation really seems like
> extreme feedback in the filter.
> Antti
> ma 20. syysk. 2021 klo 10.09 Michael E Caloroso <mec.forumreader at gmail.com>
> kirjoitti:
>> It's a safe bet that the 4016 has been damaged from the excessive voltage
>> (max is +/-7.5vdc).  Those CMOS switches can be downright unpredictable
>> after an excessive voltage at input and/or output.  I had a CMOS failure
>> mode that caused feedthrough through the other switch sections.
>> I'm not sure about the TC makes of the 4016, but first generation RCA
>> CMOS switches did not have overvoltage protection at the I/O pins and are
>> often found defective on vintage gear.
>> You could be chasing a ghost.  More than once I traced a failure to a
>> component, only to uncover the root cause was another component connected
>> to it or the supply rail.
>> MC
>> On Sun, Sep 19, 2021 at 5:15 PM Antti Pitkämäki via Synth-diy <
>> synth-diy at synth-diy.org> wrote:
>>> Hi,
>>> A small update: I think I solved why the "weird oscillation" reaches the
>>> output even when the VCA is off. This synth has a feature labeled POA,
>>> "Pass Over Amplifier", which sends a signal from the VCF directly to the
>>> output amplifier, bypassing the VCA. It's controlled by a semiconductor
>>> switch (TC4016BP) on the output amplifier card. I was able to detect the
>>> weird oscillation at the input pin of the switch and also at the output pin
>>> of the switch (in a distorted form) even when the switch was set off.
>>> Apparently the weird oscillation (15V peak to peak) exceeds the voltage
>>> limits of the switch and peaks get through. Also removing the VCF/VCA
>>> connector from the output amplifier card stopped the weird oscillation from
>>> reaching the output.
>>> But what causes the weird oscillation? Well, I noticed the filters of
>>> both voices share an IC, which is a dual opamp (NJM4558DV), each filter
>>> utilizing one of the opamps, which are configured as simple buffers.
>>> Incoming signal (from the oscillator mixer) goes through this buffer. The
>>> buffer's output is connected to the input of the VCF through a resistor.
>>> Also connected to the VCF's input is the pin of the VCF labeled "FB",
>>> meaning feedback. Now could the shared dual opamp be the culprit, causing a
>>> similar symptom for both filters? It's really the only shared component
>>> between the filters. Well, there's another similar shared component, but it
>>> buffers the VCF's envelope generator control voltage, so I don't think
>>> that's the problem.
>>> So I guess it would be a good idea to try changing the mentioned dual
>>> opamp buffer at the VCFs' inputs (being the only common component for both
>>> filters, which both malfunction in the same way)? Could such a component,
>>> in theory, cause out-of-control feedback at the filter? It's weird that it
>>> only occurs in very specific conditions. Somehow I feel a defect filter IC
>>> would have made more sense, but it would be quite strange for two separate
>>> filter ICs to malfunction in exactly the same way.
>>> Regards,
>>> Antti
>>> la 18. syysk. 2021 klo 0.34 Antti Pitkämäki <antti.s.pitkamaki at gmail.com>
>>> kirjoitti:
>>>> 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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