Taito L-Shaped PCB Repair Log

[Kaizen]

I had a Taito L-Shaped Space Invaders PCB on the bench for repairs.
When powered up there was garbage on screen, it was strapped for and still had the original 9316 proms.

https://youtu.be/Z9jFLJmDbsQ

The motherboard was clean and had no previous repairs, the sound board had four chips replaced and the chips were then coated in a resin on both sides of the board that made it near impossible to get a probe to make good contact.

The boards were checked over for damaged tracks and any other signs of damage, all okay.
It had the original 40 year old white single wipe ROM sockets so they were replaced with new dual wipe sockets and the CPU was removed, socketed and tested (okay).

I usually start with checking the simpler logic gate chips first before going to the schematics, I found the 7408 at 3N had a stuck output at pin 3.



It was replaced and a small difference on screen but its output now had an active signal.



The board was restrapped for 2716 and a Test ROM was inserted, it hung at the second RAM test screen so I started looking at the RAM Address BUS and found a faulty 74157 at 5L.



After replacing it the Test ROM showed that RAM 1 at 8N was bad (image below is for a Midway board but same same).





The dodgy RAM was replaced and the board was powered back up, RAM test passed and the Test ROM went into the sound check mode.



A good set of Taito SI ROMs were fitted and the board showed no improvement.



The Test ROM doesen't use the 74154 when going through the test and when I touched pin 1 (goes to the first ROMs chip enable pin) I could see the game trying to boot so it was swapped out and I was greeted with nice Spacies screen...



I moved onto the sound board which had no output whatsoever.

The first thing I did was touch the LM377 pins and got a humming noise so I guessed that it's okay, working backwards I started to look at the LM3900 at 27, this is the final 'preamp' before the LM377.



I planned to resocket all the LM3900's anyway so once that was done and a new LM3900 fitted there was still no sound output.
The Opamp is turned on via the SX5 signal which was stuck high, following this back I found that the schematic has an error, the gate on the 7417 is on chip 16 and not 11 as the schematic indicates.




Link to schematic...
https://drive.google.com/open?id=14z...T0gohRgHLQw9HB

The outputs on the chip were stuck high but the input went to low when the game starts (most likely had no output and the pullup meant that it would stay high), it was replaced and this brought some of the sounds back. The board now had all new LM3900 but a few sounds were still missing so I checked the other 7417 at 11 and was also faulty.
After changing it the sounds were all present and the board is now fully working.

https://youtu.be/ppaoErSMMy8

The board was restrapped back to 9316 and the original PROMs fitted, at least one was faulty so a new set of 2716's will get burnt and the board will be strapped for 2716's.


Side Note...



These sixteen lines (one is cut off at the top of the screen) are caused by the one bad RAM, if you see this there is a good chance you have a bad RAM chip.

[channelmaniac]

If I see horizontal lines on the screen (vertical if you have the monitor oriented correctly for the game) then I start troubleshooting address lines. When you have address line issues you'll the the lines doing that on the screen.

Sometimes when you touch the 74LS08s with your logic probe tip the game will boot and start running. Replaced the chip you just touched.