Cosmic Monsters - 2 Faults

[yorkshire_spam]

I've got my Universal Cosmic Monsters on the bench trying to map the circuit and figure out a fault with the shifter.
Then it stopped working, it drew the top and bottom of the intro screen on start up and then stopped (at the point it would usually draw the invaders and scores part)

What an absolute f**ker of a fault to find on a PCB without schematics.

Almost by chance I noticed that the NOR gate input on pin 9 of the 74.02 at H10 is shorted to ground.

Inspecting the board it's obvious that it's not supposed to be grounded (as it routes to 2 other ICs on the board through all sorts of vias etc.)

I pulled the 74.02, but that tests fine out of circuit and the short is still there between the pads on the PCB without the IC in circuit.

I traced the tracks to the 74174N at L5 and the 74s04 N8.

Pulling the IC at L5 out made no difference.

Pulling the IC at N8 cured the fault - an internal short between pin 1 and GND

It was bound to be the last one of the three! Bstd!

Anyway - board runs again, but still has shifter issues. I'll post more when I track down the shifter fault.

yorkshire_spam2021-01-10 21:22:19

 

[yorkshire_spam]

When I got my Cosmic Monsters table it was clear it had some sort of graphics fault.
The bottom half of the "invaders" are fine, but the top half was "random"

I was convinced it wasn't a ROM or RAM fault - the game plays 100% ok and the "invaders" look fine on the intro screen that shows the scores.

Because there's no schematic available for CM I started to figure out the shifter circuit and the daughter board that holds it (in place of the 4 original custom chips at N1,N2,N3,N4... I assume this worked out cheaper than the custom shifters?)

The fault turned out to be:

1) Daughter board connected wrong, causing the 2 enable signals from a flip-flop on the main board to have Q and _Q shorted

2) Faulty 74.174 at M6

3) Bad contacts on the socket that the daughter board links to @ N4

Fr reference the 7814-E shifter board should be connected as follows:

N1 -> CN1

N2 -> CN2

N3 -> N/C

N4 -> CN3