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Yes! Yes I would like to buy a MV4-FT2 for $20 - that sounds like a perfectly sensible idea.

ic3b4ll said:
Check the Neo Geo development wiki. There's pinouts there and a rough board map.
No booting usually means a (CPU) data or address line is bad. If the associated custom buffer is shot address lines tend to get stuck high. You can check this quickly with a logic probe, no cart needed.

I commend your patience. Good luck!
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Thanks! This one is definately a learning project.

I probed and traced the pinout for every single pin on slot 4 today (all 240 of them!) no bad lines - so Im suspecting that either
one of the custom buffer chips is half dead
or one of the lines from the inter-board connectors to the custom buffer chips is bad

Either way it's time to use the logic probe exactly as you suggested - thankyou!
 
In my experience the custom failing is unlikely and bad traces from the interconnects through to the custom are very common.

Track down which interconnect is associated with the slot you are having issues with and beep out from there
 
Oh man this was so cool.

ack is the human working to update and document the diagnostics bios. also an absolute genius and taught me to read mempry maps for fault finding.

To trace the remaining fault in slot 4 I looked at the memory map from slot 3 and slot 4 and compared.

The 4th bit is wrong (stuck high) on slot 4. That 4th bit is addressed by d3 which is pin A8 on the prog board

IMG_0329.jpegIMG_0328.jpeg

Working backwards I traced address line D3 which is pin A8 on the prog board for each slot
Slot 4 PROG A8 to NeoG0@D1 pin1 = continuity
Slot 3 PROG A8 to NeoG0@D1 pin9 = continuity
Slot 2 PROG A8 to NeoG0@B1 pin21 = continuity
Slot 1 PROG A8 to NeoG0@B1 pin1 = continuity
So all the D3 lines are making it from their NEO-G0 to the slot OK

For slot 3 and 4 it looks like the NEOG0 @D1 splits out the signal it gets on pin 5 and sends it to pin 1 (for slot4) and pin 9 (slot 3)

NeoG0@D1 pin 5 goes to CN10 pin A7 on the top board = continuity
So the D3 line is making is making it from the top board connector to the NEO-GO@D1 OK

CN10A7 on the top board goes to CN10A7 on the bottom board = continuity
So the D3 line is making it from the bottom board to the top board OK

CN10A7 on the bottom board goes to pin 5 of the AS245 at C11 on the bottom board
So the D3 line is making it from the AS245 at C11 to the bottom board connector OK

All of the input pins on the AS245@C11 (13-20) are wired directly to the bios pins 12-19 = continuity
So the D3 line is making it from the BIOS to the AS245@C11 OK

All the traces are good - really that leaves me with a possible bad AS245@C11 or a possible bad NEO-G0@D1

Changed the AS245 = no improvement (I didnt really expect that there would be, but it was much easier than changing the NEO-G0)
So onto the NEO-G0 at D1

Looking inside the NEO-G0@D1 it looks like it is made of 4 x AS245’s all mashed to together on one chip
https://wiki.neogeodev.org/index.php?title=File:Neo-g0_internal.png

All of the problems seem to be associated with the address lines for slot 4, all of which are switched in what used to be U3 an AS245, before they all got bundled together onto the custom.

The AS245 is an SNK custom chip - furrtek on tindle makes replacements, but I was able to borrow one of the MV4F with the z80 problem
IMG_0334.jpeg

And now, finally, working perfectly. Flawless Victory!
IMG_0338.jpeg

I'll write up my notes properly shortly - before I did that I wanted to say a huge thanks to everyone who has posted in this thread. This project has taught me more about electronics repair than I ever thought possible - thanks team!
 
in future, as well as looking for a broken trace for stuck bits,
meter the trace to ground and 5v - to check it's not shorted somewhere.
 
stj said:
can your programmer test sram?
Click to expand...
I’ve been looking for a low cost sram tester. Would have helped with my recent repairs. But the common TL866 series of eprom programmers doesn’t seem to. Or at least I couldn’t find a way in the software. There are a number of different open and closed source projects for memory and other chip testers, but even the kits tend to run in the hundreds of dollars. Always been to expensive for a tool I’d use maybe once a year.
 
Kujako said:
Yes, but not the chips used in the MVS. At least not as far as I can tell.
Click to expand...
It does work, use the compatible generic JEDEC types, its just a simple test so it won't verify the timings of the ICs, but the general function will be tested.
 
maki said:
It does work, use the compatible generic JEDEC types, its just a simple test so it won't verify the timings of the ICs, but the general function will be tested.
Click to expand...

43256 wasn't listed under the generic types last I checked...
 
maki said:
It does work, use the compatible generic JEDEC types, its just a simple test so it won't verify the timings of the ICs, but the general function will be tested.
Click to expand...
The retro chip tester doesn't test the timings neither. I wonder if any programmer actually does (seems to be harder than I thought, with lots of false positives).
 
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