[sdiy] Weird Yamaha CS40M problem

Antti Pitkämäki antti.s.pitkamaki at gmail.com
Mon Sep 20 14:05:27 CEST 2021


How critical is for the 4458 to be exactly the same (NJM4458DV)? I found a
source for those for like 13 euros per part + postage, but my friend has
generic 4485s that I could just fetch for free. The role for the 4458 is to
buffer the input of the VCF.


ma 20. syysk. 2021 klo 11.18 Antti Pitkämäki <antti.s.pitkamaki at gmail.com>

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