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MV4F backup RAM error diagnostics.

Kujako

Kujako

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So, got drunk and ordered another MVS board. A MV-4F specifically. Overall it's in good physical shape, but it has a backup RAM error. Of course the UNIBIOS lets me bypass this, but it's a three day weekend so thought I'd take a look at the issue. Pulled the dead battery, and can see no damage in that area. But looking at the board, it's clear that someone's been in here before and replaced most of the RAM. You can see the first four are clearly not stock and are in a bit crooked in places (along with a repaired trace). Serial IO (NEC D4990AC) looks like it was replaced as well since that's not normally socketed.
IMG_1332.jpg

So my question is, from the memory address throwing the error; can I figure out which of the chips is having issues? The error seems consistent, but I have no idea which chip would be address 00D01036.
IMG_1330.jpg

Thanks,
 
tiff_lee said:
Not sure about which specific chip itself but this may help

https://wiki.neogeodev.org/index.php?title=Repair_help&t=20220529124245

See the backup ram section, it would seem the fault is with the 'lower ram'. There are some posts on the other forum about which pins connect to the cpu etc.
Click to expand...

Correct, and I know that of the six 256k RAM chips labeled 43256 there are the following...
2x video ram
2x work ram
1x backup ram (lower)
1x backup ram (higher)

But, I don't know which order the chips are in on this model board (may be in the order I listed, but I know there are different revisions). It would be interesting if it is in the order I listed (which is correct for some 4F boards), since that would mean the last person who replaced the RAM stopped just short of the one that I'm getting a failure from.

Edit: In theory I can trace back from the BIOS to the backup RAM, but that's assuming the trace is good (which it might not be). Also the backup RAM would draw power (VCC pin 28.) from the battery region rather than the JAMMA +5v (nope, all are hooked to the JAMMA 5v). So think I might be able to figure it out... but would love someone who knows more than I do to shove me in the right direction.
 
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I'd check a schematic (generic MVS) or whatever other sources are available (the other forum...) and see what those ram chips connect to and then just 'pin it out' using a multimeter in continuity/buzzer mode. So for example, test from the relevant CPU pin to each ram chip until you identify the correct one.
 
tiff_lee said:
I'd check a schematic (generic MVS) or whatever other sources are available (the other forum...) and see what those ram chips connect to and then just 'pin it out' using a multimeter in continuity/buzzer mode. So for example, test from the relevant CPU pin to each ram chip until you identify the correct one.
Click to expand...

Found it (and a great resource listing RAM locations for all models).
https://github.com/jwestfall69/neogeo-diag-bios/blob/master/docs/ram_locations/images/mv4_ram.jpg

It is indeed the first of the non-replaced chips. I'll check traces and dig through my parts bin for a valid replacement.
 
Oh nice, i'll be adding that to my bookmarks for a few of my boards that are in need of repair

Hopefully it's just a faulty chip and no trace repair required.
 
tiff_lee said:
Oh nice, i'll be adding that to my bookmarks for a few of my boards that are in need of repair

Hopefully it's just a faulty chip and no trace repair required.
Click to expand...

Honestly I kind of hope it's a simple bad trace, since I doubt I have a suitable replacement around and things being what they are I'd have to order some from China and hope I get what I ask for. But I'll poke around and see what I have on hand.
 
That GitHub is a goldmine and jwestfall is a top dude, one of his other projects is an updated diagnostic bios that gives better error messages.
 
Kujako said:
Honestly I kind of hope it's a simple bad trace, since I doubt I have a suitable replacement around and things being what they are I'd have to order some from China and hope I get what I ask for. But I'll poke around and see what I have on hand.
Click to expand...
I have a few mv1b parts boards if you don't find something.
 
ack said:
I have a few mv1b parts boards if you don't find something.
Click to expand...

I have a two slot board that I've already pulled the PCMCIA card reader off of, but I can't remember what was wrong with it... that's always the issue with parts boards, never being sure if the part you're pulling is good. I think what I need to find is a list of know working alternative RAM modules.
 
PCMCIA slot is not required for the board to function, so you could just power it on and see whats wrong. The most common fault on 2 slots is battery leakage which can lead to sprite related issues.
 
ack said:
PCMCIA slot is not required for the board to function, so you could just power it on and see whats wrong. The most common fault on 2 slots is battery leakage which can lead to sprite related issues.
Click to expand...

Yeah I tried that, just reports "calendar error". Will poke at it a bit when I can, but I'd rather get new or ideally old stock to replace the chip on the MV4. That or figure out a tester... but I think I just need to determine what RAM modules I can source that will work. I think it's just a SOP28 256kb SRAM at 150ns or faster. But I'm likely missing something. I can get what claim to be Sony CXK58256MM-12L or IDT 1256S45SO chips, but I want to hunt around a bit to see if I can find something more trustworthy than aliexpress.

According to the neo-dev wiki I should be able to use any of the following...

Work, Backup, Video​

  • CXK58256MM-12
  • KM62256CLG-7
  • W24258S-70LE
  • CXK58257AM-12LX
  • any "43256", "62256", CXK58257
 
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I would imagine on the stock bios the calendar test is going to be after its already done its ram tests. You could also use the diag bios to do ram tests.

For out of board testing I use both of these with an adapter

NeoLock Inquisitor IC Testing System with the SRAM module. I think the guy that made it sold the designs to pinball related company at http://lockwhenlit.com/products.htm. I bought my kit from NeoLock a long time ago so I can't vouch for that url.

http://www.tauntek.com/LogICTester-low-cost-logic-chip-tester.htm This one is pretty bare bones and its ram testing isn't very good, but the cool thing about it is will tell you the exact voltages of each pin. This can give you insight into if a chip is having trouble driving a pin. Other testers can miss stuff like this since they are relying on an MCU to determine if something is a 0 or 1.

Both also are good soldering projects as they can come as kits.

For random replacements, if it not a part number seen on a neo geo board you will want to find the datasheet and verify its low power or it might drain your battery much quicker. That is often what the "L" is referring to in the part numbers.
 
ack said:
I would imagine on the stock bios the calendar test is going to be after its already done its ram tests. You could also use the diag bios to do ram tests.

For out of board testing I use both of these with an adapter

NeoLock Inquisitor IC Testing System with the SRAM module. I think the guy that made it sold the designs to pinball related company at http://lockwhenlit.com/products.htm. I bought my kit from NeoLock a long time ago so I can't vouch for that url.

http://www.tauntek.com/LogICTester-low-cost-logic-chip-tester.htm This one is pretty bare bones and its ram testing isn't very good, but the cool thing about it is will tell you the exact voltages of each pin. This can give you insight into if a chip is having trouble driving a pin. Other testers can miss stuff like this since they are relying on an MCU to determine if something is a 0 or 1.

Both also are good soldering projects as they can come as kits.

For random replacements, if it not a part number seen on a neo geo board you will want to find the datasheet and verify its low power or it might drain your battery much quicker. That is often what the "L" is referring to in the part numbers.
Click to expand...
Familiar with those tools, there’s also the Ultimate Retro Chip Tester, but all are too expensive for the limited use I’d currently get out of them.

Found some SONY CXK58257AM-12LB chips fairly local for cheap so going to give them a shot. Everything on the spec sheet would seem to be correct.

The two slot was always going to be for parts, but I may take a stab at reviving it when time allows.

Thanks all for the input. Will report back when I’ve tried out the new ram (found no bad traces and ram does fail).
 
Tested everything, including all slots and can find no other issues. So, guess I'll be replacing the SRAM.
IMG_1333.jpg
(I have the RF shield, but not bothering to put it back on while working on the board)
 
Well that was an ordeal... seems that whomever replaced the other chips used a flux that does not play nice with a hot air rework station. So much smoke... but the results aren't half bad. Everything boots without error now. Will run a more exhaustive test with the diagnostics BIOS and cart later, but for now it seems to run just fine. Also need to add in a battery holder, removed the old one but can't find where I have the coin cell holders right now...

[after cleaning the flux and pads]
IMG_1334.jpg

[can see here the amount of sticky flux under the chip I removed, what'a mess]
1669142065245.png


[and the replacement SONY CXK58257AM-12LB installed]
1669142100202.png
 
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