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Started by larsove, November 07, 2022, 10:50:54 AM
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Quote from: expelec on November 10, 2022, 07:28:24 AMAlways check the power supply voltages first. And if they haven't been replaced, replace the blue tantalum caps in the power supply.Ok, you see signal on the String Select LEDs, so input signals are there. It's unlikely that all the VCOs would be dead. The signals then get mixed and go to the VCF and the output Syn VCA. You can check pin 15 to see if anything is coming out of the VCF, but my first suspect would be the Syn VCA section. There's a bunch of stuff involved here but I'd probably look in this order: 1. Check IC14. pin3 to see if you have some voltage when playing. If not: Are you getting Syn Vol signal from guitar? Possibly problem with gating based on Total Envelope signal 2. If you have input and output (pin 1) of IC14, could be dead LED/LDR combo in the Syn VCA. If the LED dies, then you'd have no synth sound. 3. If you have output at IC15 pin 1 (post VCA synth signal), then could be the muting circuit (Q27 etc.) which follows.HTH
Quote from: expelec on November 15, 2022, 06:40:54 AMPin 3 seems to be correct for playing/not playing, but Pin 1 values do not seem correct. If you have positive voltage on Pin 3 the opamp will try to make Pin 2 equal to Pin 3 and it can only do that by turning transistor Q24 on with positive Pin 1. Pin 1 should be around voltage of Pin2 (same as Pin 3) + .7V. If Q24 were bad, open circuit, Pin 2 would stay at ground (via R176) and IC14:1 would saturate near the positive rail, but instead you have negative voltage at Pin 1. Makes me suspect IC14.
Quote from: expelec on November 17, 2022, 05:30:45 AMYeah, same as RC4558, MC4558, NJM4558 etc. Might be convenient to put an 8 pin socket in there and try any dual op amp you have: TL072, 082 etc.