I have such an instrument. Unfortunately it does not make any sound, all functions seem to work. It responds to buttons. After pressing start, the diodes show the next bars. I looked at the motherboard and found a few 072CN and TL074CN amplifiers, and two TDA1545A digital converters. I wanted to listen with a headphone if there is sound on any of them, but unfortunately I couldn’t find any sound on either the converter or any amplifier. Does this mean the DAC is broken?
This is an 1980’s vintage keyboard and the electrolytic caps have all timed out. first you need to verify the power supply is withing spec, or fix that first. Then look for any signs of failed or damaged components on the rest of the board.
Tantalum caps either explode or internally fail showing no outside damage and are very hard to troubleshoot, but will test as a short with a multi-meter. If the power supply is too far out of spec it can damage the microchips and op amps.
I checked the capacitors, replaced them on the whole board, there aren’t many of them. Unfortunately, there’s still no sound. I checked with an oscilloscope that no audio data is being sent to the DAC. I don’t know how to check if the processor is broken or if any of the sample memory is broken. I checked all the connections and they are all good.