Space Invaders Deluxe PCB Repair Log

The game arrived in the Repair Station completely dead. A check of the monitor found it to be working. Pulling ROM 'H' did not produce the standard RAM bars. The CPU and crystal tested fine. There was output on pin 18 of the 18 pin connector (composite video signal) but still no picture. Checking the board with a logic probe revealed that pin 15 of the 74166 located at C4 was not pulsing. This was the reason the  sync signal was dead and working upstream from C4 the culprit turned out to be a shorted 7402 located at D6 on the mother board. Once the chip at D6 was replaced the RAM bars could now be seen.

After reinstalling ROM 'H' and powering up the game two problems were evident. There was no sound and there were two lines of dots on the screen which interfered with game play.  The dots blocked shots from the base as well as from the Invaders.  After some analysis of the PCB I determined I had two bad RAM chips.  RAM chip failure seems to be very common on the Midway 8080 boards.

I decided to tackle the sound issue first. Checking all the LM3900 op-amps revealed them to be fine. The 4006 chip at N5 also tested good. The previous checks pointed to the LM1877 amplifier being bad but in order to check that I ran my finger over the pins on the solder side of the board. Doing this should produce static or some audio output but in this case there was only silence. To confirm this finding I checked the power pin of the amp - pin 14 - with my digital multimeter and foud it was dead shorted to both output pins 2 and 13. I replaced the LM1877 and transferred the heat sink from the old chip to the new one.

Here is a picture with one of the RAM chips replaced and one removed with the holes cleaned up.  Normally I would socket a repair like this but at the time of the repair I found myself out of sockets!

This picture shows both of the ram chips replaced.  Notice MM5280 is compatible with UPD411.  Suitable other replacement chips are TMS4060, MM9060 and D2107C.

After all the repairs were done I reinstalled the PCB and turned the game on. No more dots and the Invaders were thumping away! All sounds checked good. Let the invasion begin!!!