Rolling Thunder repair log #2

While I was playing this game from my collection  ,I discovered it was missing all the samples (gun shots, walk and so on).

The game is a Namco system86 with an additional daughter board used for the samples

Before beginning the troubleshooting I searched on Internet for a similar problem and found a repair log

where all the caps on the daughter board weere changed , restoring the samples.

I am not a fan of changing caps randomly so thanks to Atari I checked the schematics and

found that the on the daughterboard there is a little circuit which produce 24V out of the 5V for the op amp.

Around that circuit there was a hissing noise therfore I checked if the 24 volts were present which was confirmed.

I proceeded with my sound probe to see if from the op amp I could hear the samples but when I turned on the game,

it continued to reset itself….the watchdog was active.

At this point I was a little puzzled and tried to disconnect the daughterboard which granted me with the game that booted

but with missing background and crashed as soon as the attract mode began.

I took some customs from various Namco games and checked if the game booted but no luck.

Also I hadn’t another Rolling Thunder to check the two customs on the daughter board therefore I put the pcb apart for sometime.

After some weeks i resumed the pcb and without much hope I checked various TTLs until I found a 74LS244@A1 on the daughterboard

which had pin 12 not oscillating properly while the input was healthy (Fujistu part….)

 

I piggybacked it with a good one and the game booted briefly correctly.

After changing it, I returned to the original state with the gameplay without samples.

I tested the output of the op amp TL074 with the sound probe and I could hear the samples.

Ttherefore there could only be one cause which was the capacitor @C1 placed between the output and the daughterboard connector, which infact was tested as open with an ESR meter

Changing it restored all the samples.