Backup Ram Error

NeoGeoFanatic · Feb 3, 2016

Hi Guys,

I have had this 4 slot board in storage for a while now since I never had the patience to fix it until now. I am getting this error:

Anyone has any idea what ram chip could be causing this?

I have searched around but I have not found anyone with the same exact error.

Thanks,

mikejmoffitt · Feb 4, 2016

I don't know for this board off the top of my head, but the RAM configuration is two ICs representing the upper and lower 8 bits to form a 16 bit word. This looks like bit 4 (out of 0-7) is stuck low, likely a bad trace, on the backup RAM that is for the lower 8 bits. It's possible the RAM is actually bad, but this probably a trace issue given that it's in the backup RAM.

xsq · Feb 4, 2016

I'd check the continuity on all the traces of the ram and also look for corrosion.

MKL said:
When you get a READ 55xx (where x=any number), then the problem is related to the "lower" ram, that is the ram whose data lines (D0-D7) are connected to D0-D7 of the CPU. When you get a READ xx55, the problem is the "higher" ram, with data lines connected to D8-D15 of the CPU. Check Nightmare Tony's many and very useful posts on the subject.

Apocalypse · Feb 6, 2016

It could also be a bad IC controlling the read/write mode of the RAMs (74HC32). I had that issue on my MV-1FZ board, IC17 was faulty.

BilboBaggins · Apr 6, 2023

NeoGeoFanatic said:
Hi Guys,

I have had this 4 slot board in storage for a while now since I never had the patience to fix it until now. I am getting this error:

Anyone has any idea what ram chip could be causing this?

I have searched around but I have not found anyone with the same exact error.

Thanks,

I'm having a similar (same?) issue. 4 slot as well:

Did some (poor) soldering on my battery and am wondering if I wrecked some traces:

Any thoughts? Thanks!!

Neo Alec · Apr 6, 2023

Yeah, looks not great. Get a multimeter and start testing those traces that got burnt around the battery for continuity. You'll need to patch all the bad traces with wires.

BilboBaggins · Apr 9, 2023

Neo Alec said:
Yeah, looks not great. Get a multimeter and start testing those traces that got burnt around the battery for continuity. You'll need to patch all the bad traces with wires.

Thanks - I tried testing for continuity but those traces are tiny. Do I need smaller leads for the multimeter to give me that precision?

What kind of wire would I use for fixing the traces? Again everything's so small here. Thanks!

Neo Alec · Apr 9, 2023

Look up some videos. You test at solder connection points, not the traces. You will want to use a very thin gauge wire.

BIG BEAR · Apr 9, 2023

You want to use 30 awg kynar wire for patching.
BB

BilboBaggins · Apr 10, 2023

Neo Alec said:
Look up some videos. You test at solder connection points, not the traces. You will want to use a very thin gauge wire.

Cool so I need to test between battery and each of the backup ram chips? Just hit any leg on the chips or some other point?

Which chips are the ram chips? Thanks for your help!

BilboBaggins · Apr 10, 2023

Think I found it - https://github.com/jwestfall69/neogeo-diag-bios/blob/master/docs/ram_locations/images/mv4_ram.jpg.

Do I only need to check those two backup ram chips or all of the ram chips? I also don't have sound - which I think may be related here.

channelmaniac · Apr 10, 2023

Just the 2 Backup RAMs. They have their address and data lines connected directly to the CPU, like the Work RAM does, but they have their +5v (pin 28) connected to the battery backup area and don't have continuity to the +5 on the board edge connector. (Work RAM does have continuity from pin 28 to the +5 on the edge connector.)

Your error is in the low bits so it would be on the RAM that connects to D0 - D7 on the CPU. However, I think you'll find that the problem is with the control lines on the RAM and not with the bits of it. Check signals to pins 20, 22, and 28 (/CE, /OE, and R/W) as you'll find one is probably missing or not properly going active.

channelmaniac · Apr 10, 2023

BilboBaggins said:
Cool so I need to test between battery and each of the backup ram chips? Just hit any leg on the chips or some other point?

No. Test between the 74HC32 and pins 20, 22, and 28. IIRC on the 4-slot one of those (20 or22) may be connected to ground. That one can be ignored. Signals go through the 7405 too so look for dead traces there as well.

If you're stuck on a cross hatch screen after fixing the Backup RAM error, check the traces between the battery and CN10 for damage. Those carry the Address and Data buses to/from the CPU and the Program ROMs on the cartridges.

channelmaniac · Apr 10, 2023

Of, if you don't want to eff with it, I fix boards for a living.

BilboBaggins · Apr 11, 2023

channelmaniac said:
No. Test between the 74HC32 and pins 20, 22, and 28. IIRC on the 4-slot one of those (20 or22) may be connected to ground. That one can be ignored. Signals go through the 7405 too so look for dead traces there as well.

If you're stuck on a cross hatch screen after fixing the Backup RAM error, check the traces between the battery and CN10 for damage. Those carry the Address and Data buses to/from the CPU and the Program ROMs on the cartridges.

Cool I see an "HD74HC32" chip - is that my guy?

If so am I just testing continuity between that and the lower backup RAM chip - specifically its 20, 22, and 28 pins? Are the pins on that backup RAM chip numbered sequentially top to bottom, left to right? Can I hit any leg on the 74HC32 chip?

Thanks!!

channelmaniac · Apr 11, 2023

Look for the indent and you'll find pin 1.

Take a look at the data sheet:

Semiconductor & System Solutions - Infineon Technologies

Infineon semiconductor solutions - MCUs, sensors, automotive & power management ICs, memories, USB, Bluetooth, WiFi, LED drivers, radiation hardened devices.

www.infineon.com

BilboBaggins · Apr 12, 2023

Thanks but I'm still confused.

I don't know what an indent refers to. That link also isn't taking me anywhere.

I was able to find this diagram from a YouTube video. This was derived from a 1 slot though so I'm not sure it applies. But it looks like pin 20 on the backup ram connects to pin 2 on 74HC32? Is that right? Do I count all pins on all chips bottom to top and left to right?

BilboBaggins · Apr 12, 2023

channelmaniac said:
Of, if you don't want to eff with it, I fix boards for a living.

I'd like to finish this one myself but I might have other work for you.

channelmaniac · Apr 13, 2023

BilboBaggins said:
But it looks like pin 20 on the backup ram connects to pin 2 on 74HC32? Is that right?

that's right. Now, you'll need to also verify the other connections on that HC32. A dead trace will cause problems with the logic that controls those connections to the Backup RAM.

BilboBaggins · Apr 14, 2023

channelmaniac said:
that's right. Now, you'll need to also verify the other connections on that HC32. A dead trace will cause problems with the logic that controls those connections to the Backup RAM.

Yeah

but what I'm saying is I don't know which pins on the HC32 I need to check against those on the backup RAM.

1) How do I know which pins are which on each chip? Are they numbered bottom to top / left to right? So the bottom left pin on each is 1 and so on?
2) We've established that I'll check continuity between pin 20 on backup ram and pin 2 on HC32. What about 22 and 28 on backup ram - which pins respectively on HC32 do I check those against?

I've spent time trying to find documentation on this but so far haven't gotten anything definitive.

Thanks again for everyone's help!!

Backup Ram Error

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