System 18 / 32 repairs: Anybody have schematics?

[Asure]

I've got my hot air wonder tool and am back to attempting to fix a S18 and S32 board.
I was about to pilfer them for the the i/o chip and fix a C2 but i cannot bring myself to that just yet.

S18
Fixed gfx issues with reflowing the custom under the ROM board.
Has IC35 (a ram) showing up as bad, but replacing it made no difference. All the other rams in that area are tested OK but i have partial broken sprites, and backgrounds are just colored blocks.
(Found a similar repair where a nearby 374 with wrong outputs causes allmost all rams to report bad..)

S32
The bank of 4 'dual port ram' in the bottom left under the rom board has issues with two rams (IC69 and friends).
At least 18 years ago i sent it off for a check and the guy could not fix it.
He did wire up one of the (OE? WE?) pins from one of the two and it had some flickering / teleporting sprites.
I have sourced these ZIP rams from a spare CPS1 donor board

Are there any schematics for these boards anywhere? Anybody fix similar RAM issues before on these boards?

 

[Apocalypse]

Unfortunately schematics for arcade boards are quite scarce.
I've stopped wasting my time hunting for them and just deal without them.

 

[Asure]

System32:

I think the board is screwed. At least the QFP below the rom board.
Things i did so far:

- Replace the 'dual port rams'. Previous owner had removed one. I removed the rest years ago but was able to clean up the mess.
(I do not think the rams are bad, but that diagnose was 10+ years ago. I kept the board just in case )
- Restore two broken tracks. This gave me at least the Spiderman logo and some score bits.


Pic from the board:

(A sample broken track is at the O in PORT, look up and follow the 4 connected holes. )

Anyway, bigger problem. The 3rd hole at the left, lining up with K in 256K (which goes off to the left) is dead. There is no signal on it.
If i follow it, it just goes off straight to the QFP custom.

It's the two lines under the row of C136 -> C139, nothing else is connected to this pin.
* Ignore the stupid holes at the top of that ram. I reflowed that after soldering, with some hot air and checked connections.

According to the data sheet for HM53461ZP (Yes, zip ram) this is SI/O4 or if i look at it the wrong way, it's A3.
Either way, I/O is needed to get data out/in and address is needed to utilize the full RAM size.
The line being routed on the pcb but being totally dead is also probably not a good sign.

What do you guys think?

 

[Apocalypse]

The square indicates pin 1 so it's SI/O4.
When you say no signal, you mean floating?

 

[Asure]

No signal: No leds light up on the probe. Not grounded, not high, not pulsing. It's like i'm touching air, like nothing is connected. Boohoo. which leads me to assume the custom is fried and the pin has become disconnected internally, inside the package on a level where i cannot repair it

 

[Apocalypse]

No signal: No leds light up on the probe. Not grounded, not high, not pulsing. It's like i'm touching air, like nothing is connected. Boohoo. which leads me to assume the custom is fried and the pin has become disconnected internally, inside the package on a level where i cannot repair it

Do you have an oscilloscope?
The pin is either connected to nothing or not reaching TTL levels (if one transistor is dead inside the chip).
But it's more likely it's disconnected.

 

[Asure]

It's reaching the custom, trace is OK. But there is no voltage there.
I checked on a working board, it's pulsing high. I guess i can safely steal the i/o custom from this board to fix my C2 board.