Irem M92 Screen RAM Fault

[xodaraP]

Hi guys,

I've got an Irem M92 Major Title 2 board that is throwing an unusual error when I fire it up. It goes through the usual tests and comes up with "SCREEN RAM CHECK NG" and then "ROM TEST OK"

I've looked in the MAME driver and had a look around Google to see if anyone has found a similar error and found nothing. I did have to replace a broken PAL at IC51 which I downloaded from JAMMArcade and burnt to a GAL16V8

I can also confirm after switching out the boards with a known working set, that the fault is on the ROM board, not the top board.

 

[ekorz]

Yuck, I don't see any schematics or anything online. Hopefully someone on here knows these boards. If not, other than probing around, you could piggyback those four SRAM and maybe get lucky that one is just bad. For pretty cheap you could buy 2x 62256's for ic25 and ic28, and 2x 6264's for ic15 and ic16. (positions may be different depending on your b board rev)

 

[xodaraP]

I have 6264 slimlines as spares and piggybacked them with no effect - playing around with the 62256s makes the writing on the screen change, go away or turns everything into a garbled mess - so I'm going with the 256s being the culprit here. They also seem to be connected directly to a nice big fat custom which would explain how the board can actually tell they're faulty.

 

[ekorz]

At least it's not too expensive to pick up a couple of those to test out. Otherwise cold joints on the custom?

 

[xodaraP]

Well there's good news and bad news:

The good news is, none of the RAMs are dead

The bad news is, none of the RAMs are dead

Output enable on both chips is handled by the custom, but write enable is handled by a pair of LS244s - so I guess I check those next. These tested fine.

 

[xodaraP]

I've run around the board with a logic probe and found the following ICs with floating pins:

IC19 - LS175 Pin 13
IC20 - LS32 Pin 2, 4, 5, 9, 10
IC23 - LS273 All Vcc side low (Channel 5, 6, 7,
IC29 - LS74 Pin 10, 11, 12, 13 (Channel 2)
IC46 - LS161 Pin 1, 3, 5, 7 * (/CLR, A, C, ENP)
IC50 - LS245 All GND side floating - All B channel disconnected

IC46 - Removed and Tested ok off board (I still feel like this IC is suspect, it has a very burnt smell about it)
IC50 - Removed and Tested ok off board (I suspect this IC is connected to the sound side of the board)

 

[xodaraP]

So this is weird. I've removed the 2 LS273s to check them off board, and 2 traces came off the board with it (argh) - while tracing those out to find out where they go, I decided to check all the other pins across both ICs and found that quite a few, particularly the very small connections to vias are not reading any continuity at all. Time to do some investigating and replacing I guess

 

[Maskman]

Hi,

I have a M107 (board similar to M92) conversion of Fire Barrel from Dream soccer 94 that worked perfectly, but after year of non play, I got the same errors on the screen !

could someone help me to fix this issue please ?
Maybe a way to reset the RAM…

RAM TEST NG
ROM OK

Thanks in advance.

 

[Maskman]

Nobody to help please ?

 

[ekorz]

do you have a logic probe?

 

[xodaraP]

I have this same fault on one of my M92 boards. I've socketed and replaced the RAM and tested it on other boards and it works fine.

I've tested all the traces and they all beep out ok, I'd be interested to know what the fix is here. I suspect on mine it's a bad connection on one of the interconnects but I haven't spent the time to beep out all of those connections yet.

 

[Maskman]

do you have a logic probe?

Nope, I don’t have tools for PCBs diagnostic.

 

[ekorz]

Nope, I don’t have tools for PCBs diagnostic.

I suggest you send it out for repair then; a RAM fault could be caused by a number of issues, and there aren't any better diagnostic roms for this board that I know of - beyond what the program code is already checking.