Hi. My synth had two issue: 1st weird sound on VCO/VCF on module E. I replaced the IR3R05 VCA/VCF chip and it solved it. 2nd issue was the vibrato LED on the front panel that was not working. I checked it with a 9 volt battery and it was working. So I replaced the IC1 74LS174 flip-flop but now, poly and unisson aftertouch LEDs are no longer working.
I checked the IC on a breadboard and all entries work perfectly. I also make sure that the 7 pin femal connector is at the right position in the 9 pin male connector CN2 on the main board.
When testing the LED with the 9V battery did you connect the battery directly across the LED or did you include a resistor in series? 9 V directly across an LED will damage it.
The vibrato LED is normally powered from 5 V with a 330 ohm resistor in series. It’s possible the LED was no longer producing much light at 5 V but at 9 V it does. I would use a voltmeter or oscilloscope to confirm the cathode of the LED is properly being switched low. You should see a drop in voltage there when the vibrato switch is enabled. If the LEDs are burnt out then there will be a voltage drop but no light. If there’s no voltage drop then the problem is elsewhere. It might be too difficult or impossible to measure the voltage drop directly at the LED if it’s behind the mainboard so you could instead make sure the voltages are changing on connector C3.