Vintage Synth Explorer Forums

Solderman wrote:+1 well done on finding the problem. I've got one of my own with my CS-15 currently. Noisy VCA's, even after changing all the electrolytics. Guess it never ends.

On several occasions I have found the Yamaha VCA's to shift their gain oddly for some reason, I guess it is a failure mode. The result is that to get it to function normally it needs to be trimmed up, which results in a poor S/N ratio. I have a noticed this issue with the VCA's on CS5's and CS10's in particular. What sort of noise are you hearing? Try swapping one of the other VCA's in the synth with the VCA in the final gain stage and see if it is different, if so replace the VCA.

From a "so called" synth tech

Hey Steve...We didn't mean you...You know what your doing and give great advice..

Steve Jones wrote:On several occasions I have found the Yamaha VCA's to shift their gain oddly for some reason, I guess it is a failure mode. The result is that to get it to function normally it needs to be trimmed up, which results in a poor S/N ratio. I have a noticed this issue with the VCA's on CS5's and CS10's in particular. What sort of noise are you hearing? Try swapping one of the other VCA's in the synth with the VCA in the final gain stage and see if it is different, if so replace the VCA.

I would have to say this is what has occured, based on what I am hearing, because I'm certain the noise is coming from the final output stage of each VCA. Drop out Envelope amount to zero(with both VCA's using the same envelope) on one VCA of the two and the noise level is cut in half. Noise level stays the same with nothing coming in or out of the VCF.

Now the agony part: Where do I get the parts? Already failed at this once, and that was just trying to find a replacement for the 10k rotary pots. The pots are not the standard surface mount mini-pots. They have PCB mount pins on the bottom, pot body in the middle, which is under a spacer under surface mount threads under the pot shaft.

As steve pointed out....What sort of noise ..?

Ie...Hiss or buzz...

Just to be sure of what's going on, do the right thing and make sure that the power supply is in spec with no ripple, and particularly that the + & - 15 Volt lines are correct so that the power is symmetrical to the IG00151's. Do the calibration from the service manual (adjust VR4/VR8) to spec. If there is still a disparity and you have good desoldering skills, swap the two final VCA's (IC2 and IC4) and see if the fault moves, if not then the issue is something other than the VCA. (Don't forget to recalibrate after you move the chips).

For this kind of fault finding, it is really handy to build a cheap audio probe. It is a multimeter probe with a non-polarised series cap around 0.1uf and a 1/4 inch jack socket which allows you to listen around the audio circuitry without putting DC into your amp and speaker. This way if there is noise on a signal you can trace it back along the circuit using your ears to see where the noise originates.

Here is a good article on how to build a signal probe, it is an extremely useful tool for servicing audio gear. Sometimes even an expensive oscilloscope is no where near as useful as your ears when looking for low noises within a signal.

http://www.diyfactory.com/data/mbsignaltracing.htm

Sam, the noise is hiss. Thanks Steve, I'll look into all points specified in the near future. I'm actually more interested in where to get replacements parts at this point. I just finished the Don Solaris mod too, and let me just say, serializing the channels for double the resonance, at the exact same lowpass cutoff freq. for each(with no self-oscillation) is FAT!! Like 70's fatness.