MVS MV-2 Repair Logs

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Repairer: channelmaniac

Symptom: Multiple - Video RAM Error $8000 and Calendar Error

Had an MVS 2 slot sent to me for repair that had a couple of issues. The first was the typical Video SRAM issue at $8000 - The CXK5814 chips were both bad.

The second issue was the game was stuck at the green screen. It would boot to the Backup RAM test if you turned all the DIPs on and would work fine with a UniBIOS chip. It was a Calendar Error problem. This problem does not always show up as the text "Calendar Error" on the screen. It may manifest itself as a "stuck on green screen" problem.

Armed with my trusty logic probe the calendar IC (D4990) checked like this:

Pin 1 - H
Pin 2 - H
Pin 3 - H
Pin 4 - L
Pin 5 - H
Pin 6 - L
Pin 7 - L

Pin 8 - L
Pin 9 - L
Pin 10 - H
Pin 11 - H
Pin 12 - Nothing
Pin 13 - H
Pin 14 - H

Hmmm... Crystal looks to be defective.

Grabbed a working board and the signals look like this:

Pin 1 - H
Pin 2 - H
Pin 3 - H
Pin 4 - L
Pin 5 - H
Pin 6 - L
Pin 7 - L

Pin 8 - L
Pin 9 - L
Pin 10 - Pulsing H/L
Pin 11 - H
Pin 12 - H
Pin 13 - H
Pin 14 - H

OK - Really looks like a bad crystal now. Swapped one off a parts board and POOF. System is now working.

Hope the logic readings help you troubleshoot your system if you ever need it.


Symptom: Will not boot. Resets.

This one is a 2 slot that was shipped to me with the description of "stuck on green screen with garbage"

When it arrived I hooked it up and it would get to the green screen as normal but would then reboot. This cycle repeated indefinitely. Battery had leaked and there was corrosion on some of the board traces. It was not a Calendar Error issue as it rebooted. Calendar error would've manifested itself by freezing on the green screen and not rebooting.

Pin 20 of the backup RAM was missing its signal. Removed the battery, cleaned the board, replaced the corroded 74HC32 at location J2, and patched a corroded trace between J2 Pin 2 and transistor C1815 to fix. Replaced the battery with a new one and tested the board.


Symptom: Vertical bars through graphics

This 2 slot had some bad battery damage and vertical bars through the Neo Geo logo and certain sprites.

Checked the ROM Data Lines on the cart slot pins A19/B19 through A26/B26 with the logic probe. Cart was working fine. Followed the traces to the NEO-257 IC at J2 with the logic probe. Traces were good. Followed the outputs of the NEO-257 IC to the NEO-ZMC2 IC. Two of the data lines (D0 and D1) were missing from the NEO-257.

Replaced the surface mount NEO-257 IC at J2 with one pulled from a parts board to fix.

Cleaned the board & replaced the battery as well.


Symptom: Random Backup RAM errors

The control signals were missing from the Backup RAM chips. Installed a UniBIOS to further check the board and it would constantly crash. A close inspection of the board revealed corrosion damage from either liquid or rodents. Cleaned the board and found that the trace for address line A13 was damaged coming off of the CPU. Patched the trace and tested the board.


Symptom: Backup RAM error

Board was giving a Backup RAM error (written 5555 read 5500.)

The "00" typically means the chip is missing control signals.

Checked the board and it had 2 bad traces going to the control lines of the 2nd Backup RAM IC. Patched the traces and tested the board.


Backup RAM locations

When diagnosing the Backup RAM problems on a NEO-MVH MV-2 2-slot board:

Upper Backup RAM location: H3 Lower Backup RAM location: H4


Symptom: Lines in foreground (player) AND background objects

On the 2 slot Neo Geo boards the NEO-257 ICs are the 2 to 1 multiplexer ICs that choose which slot to get the graphics data from. There are 3 of them on the board. If one of these is bad then there will be lines in the graphics elements they are selecting. Top left is background graphics, bottom left is player graphics, and the one on the right side is for text and other overlaid objects.

Replaced both of the left side NEO-257 ICs and the corruption was still there. Logic probe showed that the data was coming in and exiting the NEO-257 ICs correctly but I thought that maybe they were garbling the data. Nope. Problem still there.

Did some further tracing down to where the data goes to the NEO-C0 chip. It flows from the 257s through the ZMC2 IC and then on to the C0 chip.

Replaced the NEO-ZMC2 IC and tested the board.


Symptom: Backup RAM Error

Board would get to the green screen then reboot. This was a constant repeating cycle. Flipped all the dip switches to run the system through a Backup RAM test. It gave a Backup RAM error with the message Written: 5555 Read: B240

(The B240 error is stuff of legend over on the Neo-Geo.com forums.)

Checked the Backup RAM with a logic probe and found pin 20 (CS*) was dead on both. Traced it back to the 74HC32 and back to the battery backup circuit. Traces were good. The resistors and diodes in that section were good. Checked the C1815 transistor with an ohmmeter and it was OPEN from collector to emitter. Replaced the transistor and the board booted right up.

I have to say this is the first time I've seen an open transistor that wasn't blown in pieces. I think the corrosion from the battery must've traveled up the legs and into the body of the transistor and caused it.


Symptom: Calendar Error

Inspection of the board turned up gouged traces in several different spots. Removed 2 shorts and patched 9 broken traces. Tested game.


Symptom: Graphic corruption

Board had rodent damage on one of the NEO-257 ICs. Cleaned the board and slots then replaced a leaking battery. Tested.


Symptom: vertical lines in game

Board had a leaking battery that caused trace damage. Jumpered 2 bad traces between the NE0-ZMC2 and the NE0-257 IC, replaced the bad battery, and tested.