Yamaha EX5 No sound

First of all congratulations to all Syntaur team for this forum and to all the synth community worldwide.
I am writing from Portugal to know if you can help me with a issue with a Yamaha EX5. The synth was give to me to repair after a friend of mine tried to replace one broken key and not complete the job with success and knowing that I could complete the job due to my knowledge mostly in vintage 70’s and 80’s synths repair.
Well, the synth was give to me with some missing screws, and most of the plugs inside disconnected.
I have replace the broken key, connected the plugs, replace the battery and restore the factory with disquette.
The ex5 powers up, but no sound on the LR outs and phones. Run all the service manual factory tests and all gives ok messages, although there’s no audio 1khz output in that specific test.
The only sound I can hear is a little ‘scratch’ sound when I turn up and down the slider volume.
I suspect something wrong with the jack board (JKDA). Any ideas for what possible could be wrong to start my debugging since these machines are normaly built like tanks ??

Thank you
Best regards,
Keep safe.

Is the individual out option installed?
If yes, do the individual outs work?

And is there a digital signal at pin 1 of IC21 and IC22?

Thank you so much for your reply. There’s no optinal individual out board, only the factory stock individual outs 1 and 2. But no sound on that outs either.
I’m not close to the ex5 right now because I’m on holidays but as soon I return I will test the Dac’s you mentioned. Your clue is very good and maybe is good place to start.
I’ll let you know if i and sucess.
Once again thank you for all your help
Best regards,
Miguel Reis

This is the output section left of IC23 is the DAC - lines up (yellow) go to/from volume pots.

So if you hear scratching from moving the volume pot then at least everything past the volume pot is working. You can try finding IC23 and IC5 and put your finder on TP3 and TP4 - with volume at max you should hear a hum - if so IC5 is good. If you poke with your finger around pin 2 and pin 6 on IC23 and hear a humm the IC23 is most probably OK too and so is the +/- 12V
The DAC runs +/-5V - check that next.