Are the 6821 chips older AMI brand? If so those are known to be questionable. Other chips by Motorola and Hitachi should be much better and less likely to fail.

Probably time to recap that sound board if it has the original caps.

Have you checked the bridge rectifier? I don't think bridges just go bad as often as people think but I have had to replace one before on one of these sound boards. If a diode in it opened up then it will only end up being a half wave by bridge and you will probably get more hum.

Did you just add solder when you reflowed the header pins for the connectors or did you clean out the old solder first?

Whenever I rebuild those boards I remove those connectors and clean the oxidation off the connectors where they are soldered. They can get heavily oxidized when the solder joints are cracked. If you just try to reflow it it tough to get a proper connection.

You may still have bad connections for the grounds throughout the game as well.

If the sound card works and speech is ok I don't think the ribbon cable would be bad. Maybe I've just been lucky but I haven't had to replace one yet and I've fixed a bunch of these sound/speech boards.

Sorry, I should have been more clear. Meant the larger .156 connectors where everything attaches to the board at the top.

Is there someone in the area with a Blackout game you can try running your boards in? That would help to confirm the issues are really on your boards or not.

I've learned never to assume anything doing repairs. I'll have to take a look at the schematic again. If you have access to a scope it would be interesting to see if that can help pinpoint the issue.