Sequential Prophet VS
Tactile Switches
This Prophet VS needs new tactile switches ... they work but ... not as good as they should. So I decided to change all of them. Unfortunately there are not enough switches available with the correct color but the switch-caps of the old switches could be demounted and reuse with the new switches.
It is a little bit tricky to demount the caps, but after a while and some broken pieces it will work :o)
Recap and Voltage Regulators
Recap of old electrolytic capacitors is always a good idea ... you want a quiet, non noisy instrument ... so look carefully after the voltage supply ! I changed all voltage regulators and all capacitors for voltage smoothing against new high quality electrolytics with low ESR.
Force sensor
This is the second Prophet VS on my repair desk with a defective Aftertouch-Sensor. They are not very sustainable and will break after some years of usage. I tried to get a better force response with a long sensor element instead of the two single point sensors in the original design. But ... you need a stiffer case for this ... the original case delivers no satisfying results. It's better to look for replacement of the single sensor elements.
OLED Display
Changing the display of your Prophet VS against a OLED Display gives you 2 real benefits. First of all, it is so much brighter and clearer in comparison to the standard display that you can read
it very easily and under all viewing angels. And second, it needs no longer the little, mostly hissing, HV-Generator for supplying high voltage to the backlight-foil. So your VS is even
quieter than before.
But be sure to purchase a OLED that is specified especially for the Prophet VS. Don't try anything else, because the timing of the control-signals is very important for a proper function. I tried
one standard OLED first, and I ended after some days with only the first line visible in the display :o(
Changing needs some soldering work. You have to remove the old display, clean the holes and solder a new pinheader and the OLED in place.
Chorus Board Issue
Some day I realized that there was a slightly distortion on the right Chorus channel mostly if you choosing sounds with high resonance and low cutoff frequency. It comes definitely from the
chorus ... if you switch the right chorus line off the distortion was gone. It was not really loud, but it was noticeable so I decided to take a look at the schematics and try to fix this.
My first thaught was that the 4007 CMOS Analog Switch (U918) at the chorus output stage are defective. But after changing it, the problem was still existing. What a unsatisfying result ... Then I
tried, armed with my old Tektronix Scope, to measure in which stage this distortion comes up. But as I told initially the distortion wasn't loud ... so this try fails again ... everything looks
like normal waveforms and there was no visible difference in comparison to the left channel. There was no spectrum-analyser or distortion measurement bridge in reach so there was only one way
left. Go on with changing ...
Next I tried the bucket brigade memory, the MN3209 chip (U911). I ordered a spare but the problem was still there :o( Then I changed all active components, mostly OpAmps (U902,U901,U914,U915) in
the right chorus channel path. No positive result :o(( After that, the compressor/expander chips NE571 (U903, U916) ... No positive result :o((( After that, all electrolytic capacitors in the
signal path (C987,C966,C954,C945,C909,C925,C924) ... No positive result :o(((( Then, shortly before I was about to give up, I changed all supporting electrolytic caps
(C908,C922,C920,C953,C975,C939) ... and this works ;o)
I can't tell you exactly which of the last 6 capacitors the problem caused ... but after this ordeal, I don't even want to know !