Zaccaria The Invaders Maintenance 2019


Since it's initial restoration in 2012 the game had run without issue. In the 2019 show I'd noticed the UFO sound wasn't working properly and the picture was a little dim and bleeding. I also didn't have a spare game PCB ready to go for it.

24/05/2020 - CRT rejuvenation adaptor

The Invaders with BK 467 adaptors 5 & 6 The Invaders with BK 467 adaptors The Invaders A50-120W 20BM1 CRT

Adjusting the brightness on the monitor caused increased bleeding and not much actual improvement in brightness so the first step was to setup to test the CRT. Looking through the adaptors I found two, 5 and 6, that appeared to have a matching neck socket for this CRT (an Italian FIVRE A50-120W 20BM1). However, the centre of the adaptor socket was slightly too small to fit into the neck plug.

25/05/2020 - Spare game PCB repair, part 1

The Invaders game PCB The Invaders game The Invaders game PCB on the bench

On the bench player 1 start wasn't not working. Both of the player start inputs went to IC5 (45038) pins 12 & 14 and I noticed that pin 12 had solder residue on it, hinting someone may have worked on this fault before. There was also some graphics issues around the base. The game was playing but collision detection didn't appear to be working. Checking the 'PORT 7' (collision reset) signal and collision detect output at IC24 (LS00) pin 3 found them both pulsing during the game.The next step was to add The Invaders to the Arduino ICT to further investigate.

01/06/2020 - Spare PCB repair, part 2

The Invaders with Arduino ICT The Invaders game

With The Invaders support added to the Arduino ICT the tester reported IC10 (2102) bad. It was at this point I noticed that IC10 served databus bit D0 that was also the same bit used for start 1 and collision detect. Using the scope to compare the data bus bits in CPU idle state found D1-D7 either floating or slightly above GND but D0 was high. Similarly comparing during an active cycle found D1-D7 consistent but D0 looked different with a much harder high. I suspected that something was high overloading data bit D0, not a dead short to +5V because that would cause the game to not boot, but high enough to overload weaker bus drivers :(

The attempt to find the culprit was based on an idle CPU with D0 tied to GND and measurement of the D0 voltage around the board. The theory was that if enough current was drawn by the overload there would be a voltage gradient towards it:-

IC5pin 114.8mV
IC5pin 54.6mV
IC23pin 34.5mV
IC10pin 114.1mV
ROM 1Hpin 93.3mV
ROM 6Lpin 93.5mV
2636pin 103.4mV

None of the target IC's were especially hot with the short to GND and there was no consistent effect on any IC with freezer spray :( With no other options seemingly viable, I cut IC5 pin 11 that resulted in no change. Cutting IC 5 pin 5 caused the voltage at IC23 pin 3 to drop to 1.5mV! Removing the D0 tie to GND confirmed that D0 was now idle floating and I suspected IC5 (45038 == 40097) was bad. Replacing it fixed the game.

The Invaders PCB sound pots

A couple of the sound pots and audio amplifier had broken off the spare PCB. The values of the pots were not noted in the schematics and instead confirmed from the PCB in the cabinet. After replacing the sound pots and installing an audio amp the sound was working OK.

14/06/2020 - CRT rejuvenation

CRT rejuvenation with BK 467 BK 467 measurements The Invaders picture

A Dremel with a sanding cylinder attachment was used to slighly increase the centre hole size of adaptor #6 so that it fit on the CRT neck plug. The initial TEST reading was just above POOR and the LIFE reading fell back into the BAD range after a few seconds. A single round of RESTORE with CLEAN-BALANCE brought the TEST reading firmly into GOOD and a GOOD steady LIFE reading. Back on the chassis even the unadjusted picture was brighter with no bleeding :)

21/06/2020 - Picture adjustment

The Invaders picture The Invaders picture The Invaders picture

Picture geometry was left mostly untouched to keep it aligned with the coloured overlay. Some minor tweaks to the focus and brightness was enough to get a good picture.

26/06/2020 - PCB repair

The Invaders game PCB The Invaders game PCB on the bench

The Invaders PCB from the cabinet worked without issue on the bench - the UFO sound was working OK. I cleaned the power pins and exercised the sound pots. On re-test in the cabinet there were no sound issues.

The Invaders Maintenance 2021