I was recently given a Taito 3-Layer Space Invaders boardset by a friend to look over. The problem was that it was not running, completely dead with a clicking sound coming from speaker (sounds like watchdog barking to me).
I powered up the PCB stack on the bench and checked the status of the 8080 CPU. I could see right away from probing pin 19 (SYNC) that the clock circuit was not running.
On the ROM board, I could see that the crystal was intact, so I checked the 74S04 (IC1) which I was sure was drives the clock next to the crystal. I found that most of the pins were floating. Replaced it and I now have a clock (note to myself, 74HC04 will not work), watchdog no longer barking but still no display.
On further probing, found a bad 74161 counter at location IC6 with pins 11 to 15 stuck HI. Replaced this and I now have a display, but it appears to be doubled up;
Eventually located the problem to IC28 which is a 74LS83, pins 2, 6 & 9 were stuck LO. Replacing this restored the display;
Note, I am using the composite input on my Plasma TV, as the PCB is composite only and not RGB.
Space Invaders live to invade another day!
Sorry, the comment form is closed at this time.