Jun 282017
 

Got this game cheap for my collection because the seller stated that it has very noisy sound

When powered, the game had a constant white noise while music and the other sound effects were working correctly.

The sound problem was not due to the amplifier or bad caps but it was produced by the logic of the game.

To make a long story short, this game has a similar circuit to what Space Invaders use to produce analog sounds.

It uses a SN76477, which is a programmable sound chip which has a white noise function.

On Space Invaders and in this game, the chip continuosly output the white noise which is switched OFF and ON using operational amplifiers and it is modulated with capacitors and resistors to produce different sounds.

I downloaded the schematics from the US version ( called Zarzon ) where it is shown how the chip is connected to the LM3900 OP AMPS

 

 

Instead of trying to understand which of the two chips are broken, I piggybacked a good one on both chips until

the white noise disappeared.

Problem was fixed by changing the LM3900 Nr.47 on the schematics