No background in electronics. Learned everything by reading pdf books and expecially Video game logic Vol 1 by Atari and in general early Atari and Williams arcade manuals
I got this pcb in December 2014 from Smitdogg of Mameworld as not working:
It was in good condition but it had already some TTLs replaced (socketed)
After firing it up I got a static coloured screen.
The Z80 was socketed so I first tried to replace it and to my surprise the game started apparently without any major faults with correct colours.
It was too good to be true and infact when the demo play started I got a very peculiar fault:
The background colours changed alternating between correct and not only during horizontal scrolling. During vertical scrolling the colour of the background was fixed in the last state (being good or bad).
I started the test of the pcb and everything looked good except the color check.
The “Back” had some strange colours which didn’t look correct
I decided to start Mame and see the same screen on the emulated game and I got this:
it seemed that there were some fixed bits on the background colours but that didn’t explain the alternating colours.
I looked for Nova 2001 schematics which luckily were available.
This game has plenty of 4116 drams which are prone to fault normally but they weren’t related to the fault I had.
I probed also the 3x 6116 SMD rams responsible for the background colors and the related 74LS273 buffers and they were all good.
At this point I had no clue where to probe, the schematics were really bad scanned and difficult to read.
I asked an advice to Charles MacDonald which pointed me to look on the AY8910 sound chips whose I/O are responsible for the horizontal and vertical scrolling.
The first 3 bits BH0, BH1 and BH2 of AY8910@1,2A are negated and sent to a series of 74LS151 @12U, @11U, @10U and @9U.
All the signals going to these 74LS151 were good.
I probed the outputs of all the 74LS151 and noticed that the TTL @12U was oscillating in sync with the alternating background colors.
After checking the inputs of this chip, I noticed 3 stuck signals which were back traced to the 74LS164@12T whose inputs CLK and CLR were correct.
Changing this TTL restored the correct colour of the background during scrolling.
I bought this (very) expensive pcb from ebay in October 2014.
I played the game a couple of times and then after about one week, all the sprites disappeared while I was playing. Only background and text were present:
I couldn’t believe, at first I began to press everywhere hoping it was a loose or oxydized connector but soon I realized something got faulty.
This game hasn’t any schematics available so I knew from the beginning it would have been very tough to fix it.
I connected my trusty logic probe and began to short some pins on the pcb to see what changed on the screen so that I could focus on the right part of the circuit.
After about 30 mins turned on, the sprites started to reapper but missing some lines:
I then remembered that 1942 hardware is very similar to Son Son and the schematics are available.
I was right, the way that TTL chips are connected on 1942 is equal on Son Son. Only the positions of the ICs are different but the logic is really similar.
I probed the 2148 rams @F2 and @F4 on Son Son pcb which on 1942 schematics are described as OBJ rams and I found a stuck /WR signal @F4 on Son Son (J4 on 1942):
I followed back the signal always taking a look on 1942 schematics, and I found finally the source of the problems on the missing signal on pin 11 of an 74LS00 @F11 (on 1942 I circled the equivalent IC @K11).
On 1942 schematics available on internet, someone circled the 74LS00@N8 writing “no plane”, probably he had the same fault. 😉
Replacing the 74LS00@F11 gave me back all the sprites
