Fix neo geo 4 slot work ram error

version 1 4 slot has the below work ram error. I replaced the sony cxk58257m-12l wram with samsung KM62256ALG-10 which are compatible. I thought were comparable. But when I did all i got was the magic smoke releasing from it and a screwed up image. Im an idiot and put the replacement chip backwards and smoked it. Next one is fine. ive replaced both work ram chips with no change and contintuity seems to all be there.

Anyone help me out here? Thanks

 

Your screen has fingers on it. NOT ENOUGH RAM!

 

Try to find the specific ram or check with the neo geo community for a true compatible ram.

 

I figured out I placed the new chip backwards. The samsungs work fine. Im about to put this on ng as you suggest

 

Are you sure it's the work ram chip you have changed? I find it highly unlikely a smoked chip installed backwards would give the same error as a correctly installed one.

I suspect you have changed a different ram chip and you aren't working on the correct one based on your experience so far.

You can easily test its the right chip by shorting out 2 of the data lines on a chip and run the check. When you have the correct chip the first 2 numbers of the error will change.

 

The smoke one I didnt get any image at all when that happened.

When you say data lines do you mean the pins labled i/o in the data sheet?

 

Yes

The chips are 8 bit, so the value 4555 is split into 45 on one chip and 55 on the other.

By shorting the data pins on the chip, when the 45 value changes, that is certainly the correct chip to be looking at.

 

Doing this to any of the 6 just causes the screen to flash. am I even working with the right chips here

 

I followed d4 on the upper ram. It goes to the lower video ram d4pin. It then continues to a ls174. Is it possible this chip is bad and causing the error seen?

 

You can't short the Work RAM pins because if you do, you will crash the 68000 and that will cause an exception which will result in a RESET (SNK BIOS) or a error screen (Universe BIOS), which will in turn disrupt the testing you're trying to do. You can do that with the graphics related RAMs because they're not directly connected to any actual processor. Graphic chips on old systems are not processors but more like state machines. They won't be knocked out of whack in a non recoverable way if you short control lines on them. Sadly with actual programmable processors it's different.

 

The error message changing is what you are looking for to confirm its the correct chip. The current error is basically the same thing after all.

Or you can do the reverse - cause an error on all the other types of ram and by process of elimination the one left is work ram.

It's mainly for people who don't know which chip is which. I've got quite a lot of notes by now, but never worked on a 4 slot. Only 1s and 6s

 

Wiki states that lower byte will have the lower "ICxx" part number on the board solder mask labeling.

 

In the end, that trace is fine. I/o4 on the upper work ram has continuity to the z80. Does this mean the z80 is bad?

 

Possibly. If the chip is known good and so is the trace, all you can do it change parts on that signal path that could be pulling it up/down incorrectly.

 

Multislots are tough to diagnose, because the slots are insulated from the main bus through a horde of buffer ICs. On 4 slot units, you have only custom ICs as buffers, which is particularly annoying. On 6 slots all the buffers are discrete parts, which makes for annoying troubleshooting but nice "repairability" as all parts on the slots board are off shelf TTL ICs.

 

Do you think it might be the z80? Other than a different sl IC i cant see anything else causing the issie.

 

If you have a problem with the Z80 it won't pass the sound test function (noise tests) when you power the unit on without a cart.

If you have problems with communication between the M1 rom on the cart slots and the mainboard you will have Z80 error only when you try to use the affected slots.

The buffers act as a multiplexer so only the slot in use will be ever active.

 

Ill try without a game, but i can say every cart slot generates the same error as the first two pictures. and the diag bios didnt give me any errors besides whats seen in the photo

 

Consider how Z80 is tested:

68k sends "you alive?" command to Z80 through music control port.
Z80 replies "I'm alive".

68k proceeds to the next test step.

When no cartridge slot is enabled, the "SM1" rom on the mainboard is mapped into the Z80 BUS as it's ROM. The same happens with the SFIX ROM, as the system fix.

When a slot is active, the SFIX and SM1 roms are disabled and the S1/M1 roms of the cartridge are what the Neo Geo sees instead. If there's a problem with the connections and the Z80 cannot read from the cart M1 rom it won't boot. If it doesn't boot it won't be there to reply "I'm alive" to the 68k and that's why you see that "Z80 ERROR" message. The point of that "Z80 ERROR" message is let a operator know the game need servicing instead of allowing it to run sound-less.

 

Oops, I just realized that you were measuring the 68k RAMs. If you are measuring the backup memory it will be energized and will give false continuity readings to everywhere on the board. Please disconnect the battery before servicing so it doesn't confuse you. Put it back once you finish servicing.