Dec 102015
 

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.

ace_si_1

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;

ace_si_3

Eventually located the problem to IC28 which is a 74LS83, pins 2, 6 & 9 were stuck LO.  Replacing this restored the display;

ace_si_2

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!