Lost all Audio - Tail Gunner II

i have not read everything above so what i am about to say may have already been covered.
often when testing analog sound boards a wet finger is a great test tool. not kidding
touch the input of the class b (final stage) listen for an ac hum.
did you hear a hum? yes? touch the input to the op amp before the final stage. yes? then your audio amp is fine. each individual sound is generated in a different section and the odds of all going bad at once are slim.
there is a latch on the input that triggers each sound circuit replace it or trigger its inputs manually.

verify 5v 25v-25v 15v -15v
 
And success. It's back working.

I pulled the ribbon cable between the CPU and the Audio Board. When I was pulling it, I noticed that the cables have black "smudges" like someone took a magic marker to them on the CPU side. The audio board connector was reversed - the black was on the audio board side. Weird.

I took it to my bench, and using the leads left over from the R74 2.2K mod, I checked each trace from end to end on the connector.

Everything checked out less than 0.4 ohms except 1 trace. I tried cleaning it, re-clamping it - nope, it was running near an ohm.

In any case, I flipped the cable around so it was black side towards the CRT board, tucked the wires in, and viola! The audio is back working, and is MUCH quieter.

@toledoflyer for the win!

So the R74 2.2K resistor mod works. And now my audio is back online. I ran up a score of 3464 and called it a night.

I had an aircraft master solenoid I had to do a post-failure investigation next.
 
i have not read everything above so what i am about to say may have already been covered.
often when testing analog sound boards a wet finger is a great test tool. not kidding
touch the input of the class b (final stage) listen for an ac hum.
did you hear a hum? yes? touch the input to the op amp before the final stage. yes? then your audio amp is fine. each individual sound is generated in a different section and the odds of all going bad at once are slim.
there is a latch on the input that triggers each sound circuit replace it or trigger its inputs manually.

verify 5v 25v-25v 15v -15v
Thanks! I should have "pinged" you earlier!

I wonder if we should designate some places to check voltage by installing a test lug or something? These early games are damn short of test points. It makes me appreciate what Asteroids has in terms of places to hook up and check things.
 
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