SNK Games (Neo Geo MVS / AES)

[channelmaniac]

Fixed: AES JAMMA system
Symptom: Would only play the supplied Puzzle De Pon cartridge.

This was a strangely modified home game system. It had a custom BIOS chip and an added on chip that was used to add in a test switch and game credits. It also had an external audio amplifier board and a JAMMA edge connector/wires soldered in to allow it to be played in an arcade.

Pulled the added-on 74LS245 IC, wiring, test switch, and custom BIOS chip. Erased and reprogrammed the custom BIOS chip with UniBIOS code. Resoldered cracked solder joints on the external audio amplifier. System will no longer take quarters or have an arcade style test switch with coin reporting,etc, but will now play all AES games as requested by the owner.

[channelmaniac]

Subject: Troubleshooting vertical lines on the MV6 6-slot system

Do you have vertical lines on the screen either on the moving foreground objects or background objects on your 6 slot board?

On the 6 slot board there are a bunch of 74LS251 8 to 1 data selector ICs. The ICs nearest slot 6 are for the foreground objects and the ICs nearest slot 1 are for the background objects.

The easiest way to check these chips for the bad one is to put a test cart in slot 5 and flip DIP switch 8 to pause the screen where you can see the problem images on screen.

Next take the logic probe and check pin 15 of each 74LS251. Look for the IC with no signals on that pin. If you find one without a signal then check the trace between the slot and that pin.

Not fixed? Next check pin 4 of each 74LS251 for data output. If one is missing then replace the IC.

Still not fixed? Next take the logic probe and put it on pin 15 of each 74LS251. Slide the tip over to where it momentarily shorts between pin 15 and 16. Look at the screen for lines. See lines pop up? That's not the chip you are looking for. Finally find one that doesn't pop up any extra lines when you short it across? OK. That's the data line that is the problem. Follow pin 4 of that chip and find where it connects to go to the bottom board.

Can't find where it goes to the connector? Check a neighboring 74LS251 IC and see where it goes. If you can find it then check for a bad trace between the suspect data line on pin 4 and the connector to the bottom board and patch it.

Still not it? Check the connection between the top and bottom board. If there is no signal then desolder the connector, disassemble it, pull out the offending connector piece, clean it, reassemble, and resolder it back in place. Test it.

Still no? Follow that trace to the chip it goes to and if the trace is good then replace the chip.

If that doesn't do it then call a priest and have the board exorcised.

Enjoy!

[channelmaniac]

Model: MVS MV-4 (4 slot)
Symptom: Missing background graphics

There's a 74LS138 on the top board. It's a 3 to 8 decoder. It's used to enable the slot wanted. Not part of the repair, but just wanted to doc it on the forum. Also, there are a LOT of 74LS244 buffers & 74LS245 latches on the board. Each one is hooked together with the neighboring chip that is doing the same function on each of the different slots. In other words, each cartridge slot has multiple buffers and latches. Each latch for specific connections (program data lines 0-8, for example) on the cartridge slot has their board side connections (not cartridge side connections) tied together to the connectors that take the signal to the bottom board.

Now, on to the repair:

Test Cart used: Shock Troopers, 2nd Squad.

This top board had a totally white screen with the exception of the "insert coin" words in the center and the "credit 0" at the bottom right and even that text was mispelled...

Used a logic probe to check each and every chip for input and output logic levels. Found a 74LS244 next to slot 4 that had incorrect levels for input vs. output. In tracing the pins on the chips I discovered that there is a chip doing the same thing for each of the slots and they are all tied together. That makes 4 video ROM buffers - one for each slot - all tied together when the connect to the bottom board.

Why does it matter that they are tied together? Well, when one shorts it tends to draw excessive current through the others and takes them out too.

Replaced the 74LS244 chips at locations N4, J4, E4, and A4 to fix.

[channelmaniac]

Model: MVS MV-4
Symptom: Cartridges do not work. Crosshatch pattern is all you get on the screen, no matter what slot. Unusual in that the problem follows the top board when swapping boards out.

Resolution: There is a 74F138, 3 to 8 decoder, at position A2 that determines which slot is active. Fixed broken trace on one of the 3 enable lines to allow the system to switch the slots and access the cartridges.

[channelmaniac]

Model: MVS MV-4
Symptom: Vertical lines in the graphics on slot 3.

Resolution: The graphics ROMs connect to the system board through a series of 74F253 ICs. These are dual 1 of 4 line selectors. Pin 21B on the bottom cartridge slot connector's data wasn't seen on the output (pin 9) of the 74F253 at position D1. Patched the trace to fix.

[channelmaniac]

Model: MVS MV-6
Symptom: Stuck in Watchdog (a.k.a. Click of Death)

6 slot board would constantly reset... Logic probe on pin 18 of the CPU confirmed that the reset line kept bouncing. This was the watchdog circuit trying to kickstart the CPU into booting.

Checked the data lines and enable lines on the work RAM - checked fine.

Checked the enable lines on the backup RAM and discovered that the WE* line on one of the RAMs was missing its signal and the OE* line on both RAM chips was missing its signal.

Repaired the bad traces to fix the bottom board.

It appears that something corroded the plate-thrus on the board. Seems to be a common occurance on all models of the MVS boards.

[channelmaniac]

Model: MVS MV-6
Symptom: Graphics corruption

Found and repaird 4 gouged traces on the top board. Traces were damaged due to insufficient padding when item was shipped. The top cover was bent and the traces were gouged by it.

[channelmaniac]

Board: MVS MV-4
Symptom: Crosshatch of death

Crosshatch of death is where the board shows the self test crosshatch screen. It is not recognizing any cartridges inserted into the board.

This is a common problem on the older, larger 4 slot board. The NiCD backup battery will leak and eat through traces that run under and next to it. Trace the lines with a multimeter from the connector next to the battery and run jumpers to patch any bad traces.

That connector is what the system uses to access the program ROMs on the cartridges. If it can't read and run the program then it cannot detect and use the cartridges.

[channelmaniac]

Model: MVS MV-4F (newer, smaller 4 slot)

Wow. This one was a bitch. Typical crosshatch problem, but the battery was not leaking and no traces were damaged near it. (Although the battery isn't in the same proximity to the traces on this board like on the older 4 slot board. Thus, this board doesn't suffer from the same Crosshatch of Death problem)

I spent a lot of time troubleshooting and pinning out the SMT chips on the top board before moving down to the bottom board.

The top board has the NEO-G0, NEO-244, and NEO-253 chips on it.

It appears from the troubleshooting that the NEO-G0 chip is made up of 4 74LS245 chips. I haven't pinned out the 244 or the 253 chips but assume that they are similar (4 x 74LS244 and ? x 74LS253 chips)

Enjoy the pinouts:

Code:

NEO-G0 chip at location D1

1 - d3 to cart 4
2 - d0 - cn10-4a
3 - d1 - cn10-5a
4 - d2 - cn10-6a
5 - d3 - cn10-7a
6 - d0 to cart 3
7 - d1 to cart 3
8 - d2 to cart 3
9 - d3 to cart 3
10 - gnd
11 - d4 to cart 3
12 - d5 to cart 3
13 - d6 to cart 3
14 - d7 to cart 3
15 - d4 - cn10-8a
16 - d5 - cn10-9a
17 - d6 - cn10-10a
18 - d7 - cn10-11a
19 - N/C?
20 - N/C?
21 - d4 to cart 4 
22 - d5 to cart 4
23 - d6 to cart 4
24 - d7 to cart 4
25 - gnd
26 - vcc
27 - d8 to cart 3
28 - d9 to cart 3
29 - d10 to cart 3
30 - d11 to cart 3
31 - d8 - cn10-12a
32 - d9 - cn10-13a
33 - d10 - cn10-14a
34 - d11 - cn10-15a
35 - d8 to cart 4
36 - d9 to cart 4
37 - d10 to cart 4
38 - d11 to cart 4
39 - Connects to E1-39 & CN9-14
40 - Connects to 52 & to B1 52/40
41 - N/C?
42 - gnd
43 - d12 to cart 3
44 - d13 to cart 3
45 - d14 to cart 3
46 - d15 to cart 3
47 - d12 - cn10-16a
48 - d13 - cn10-17a
49 - d14 - cn10-18a
50 - d15 - cn10-19a
51 - CN9 - 13b & E1-51
52 - Connects to 40 & to B1 52/40
53 - d12 to cart 4
54 - d13 to cart 4
55 - d14 to cart 4
56 - d15 to cart 4
57 - gnd
58 - vcc
59 - N/C?
60 - gnd
61 - gnd
62 - d0 to cart 4
63 - d1 to cart 4
64 - d2 to cart 4

--------------

NEO-G0 chip at location B1

1 - d3 to cart 2
2 - d0 - cn10-4a
3 - d1 - cn10-5a
4 - d2 - cn10-6a
5 - d3 - cn10-7a 
6 - d0 to cart 1
7 - d1 to cart 1
8 - d2 to cart 1
9 - d3 to cart 1
10 - gnd
11 - d4 to cart 1
12 - d5 to cart 1
13 - d6 to cart 1
14 - d7 to cart 1
15 - d4 - cn10-8a
16 - d5 - cn10-9a
17 - d6 - cn10-10a
18 - d7 - cn10-11a
19 - N/C?
20 - N/C?
21 - d4 to cart 2
22 - d5 to cart 2
23 - d6 to cart 2
24 - d7 to cart 2
25 - gnd
26 - vcc
27 - d8 to cart 1
28 - d9 to cart 1
29 - d10 to cart 1
30 - d11 to cart 1
31 - d8 - cn10-12a
32 - d9 - cn10-13a
33 - d10 - cn10-14a
34 - d11 - cn10-15a
35 - d8 to cart 2
36 - d9 to cart 2
37 - d10 to cart 2
38 - d11 to cart 2
39 - Connects to C1-39 & CN9-13
40 - Connects to 52 & to D1 52/40
41 - N/C?
42 - gnd
43 - d12 to cart 1
44 - d13 to cart 1
45 - d14 to cart 1
46 - d15 to cart 1
47 - d12 - cn10-16a
48 - d13 - cn10-17a
49 - d14 - cn10-18a
50 - d15 - cn10-19a
51 - CN9 - 12b & C1-51
52 - Connects to 40 & to D1 52/40
53 - d12 to cart 2
54 - d13 to cart 2
55 - d14 to cart 2
56 - d15 to cart 2
57 - gnd
58 - vcc
59 - N/C?
60 - gnd
61 - gnd
62 - d0 to cart 2
63 - d1 to cart 2
64 - d2 to cart 2

OK...

After all of that I moved to the bottom board and checked the interface chips that connect the bottom board to the top board. WOW There were a lot of missing and incorrect signals coming from those chips. My logic probe got a workout on this system.

Replaced the following bad chips to fix it:

74LS245 @ N1
74AS245 @ M4
74F245 @ N9
74F245 @ L9
74AS245 @ F11
74AS245 @ D11
74AS245 @ C11

I'm thinking that someone MUST have disconnected the two boards with the system powered up. That's the only thing I could think of that would've taken out all those chips at once.

[channelmaniac]

Model: MVS MV-2
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:

Code:

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:

Code:

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.

[channelmaniac]

Model: MVS MV-4
Symptom: Stuck in Watchdog (Click of Death)

Board gave garbage on the screen with constant clicking sound. In other words, it was dead and the watchdog circuit was resetting the CPU.

Replaced a shorted 74AS245 at location B7. It was shorted internally and was getting red hot. Board powered up with a Video RAM error at 8000. Replaced the lower bank IC at L11 to finish repairing the board.

[channelmaniac]

Model: MVS MV-2
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.

[channelmaniac]

Model: MVS MV-4
Symptom: Garbage on screen / Click of Death (Stuck in Watchdog)

This one had some physical damage that someone did a half-ass repair on. It's ugly as sin but the trace they patched is working.

The board would display nothing but garbage and would constantly do a click click click - watchdog circuit was trying to restart the system.

Replaced 2 74LS245 ICs at locations B7 and B8. Both were shorted and getting very hot.

Board was still in watchdog restart.

Patched a bad trace on data line D16 between the upper Backup RAM and the upper Data RAM. Now the board would constantly go through self test and restart with garbage on the screen. It was a different restart than with the watchdog circuit.

Examined the board under a magnifying glass and found one more gouged trace the previous tech missed between data line D1 on the CPU and pin 60 on the PRO-C0 chip. Patched the trace and the board is now running fine.

[channelmaniac]

Model: MVS MV-2
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.

[channelmaniac]

Model: MVS MV-2
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.

[channelmaniac]

Troubleshooting with the UniBIOS chip

The UniBIOS chip can be used to bypass Backup RAM, Calendar Error, and other problems to allow for a board to be further tested. If the system bypasses errors and runs then odds are it will be an easier repair. 

If it's crashing then there are other problems that are masked by the error message. Check the BIOS socket, look for gouged traces, and check for corroded plated thru holes.

[channelmaniac]

Model: MVS MV-2
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.

[channelmaniac]

Model: MVS MV-4
Symptom: Corrupted Graphics

The Neo Geo logo screen had the words broken into jumbled blocks and there were "0" (zeros) down the left and right sides of the screen.

Checked the board for broken traces. Did not find any. Swapped the top board with my test board. Same problem - it's on the bottom board. Replaced the 2 74LS244 buffer chips that were driving the address lines for the graphics ROMs. No change.

This was still an address line problem since the problems were in blocks and not in vertical lines on the screen.

Took the logic probe and checked each address line at the top board. Noticed one of the lines only had a slow pulse on it instead of activity. Shorted it to the next address line over and the zeros went away and there was just garbled graphics. Traced that line back and found where there was a very small area of board between connector CN8 and the edge of the board that had taken a hit.

Patched the broken trace to fix the board.

[channelmaniac]

Here's a pinout for the custom NEO-ZMC2 chip

The NEO-ZMC2 IC is documented incorrectly on a few websites. It does NOT multiplex sound data. Instead it takes the foreground and background graphics (16 bits each for a total of 32 bits) and interfaces with a single 8 bit (8 traces) connection to the Neo graphics subsystem.

On the older 2 slot the ZMC2 pins GAD0 through GAD3 and GBD0 through GBD3 connect to the NEO-C0 IC to get this multiplexed graphics data into the video subsystem.

Standard conventions apply - Vcc = +5v, Gnd = ground, a "*" means active low (when there's a bar over top of the pin's function), etc...

RJ

Code:

NEO-ZMC2
1 - GAD2
2 - GND
3 - GAD3
4 - DOTA
5 - LOAD
6 - EVEN
7 - H
8 - DOTB
9- NC
10- GBD0
11- GBD1
12- Gnd
13- GBD2
14- GBD3
15- C1
16- C1
17- C2
18- C3
19 - C4
20 - C5
21 - C6
22 - C7
23 - Gnd
24 - C8
25 - C9
26 - C10
27 - C11
28 - C12
29 - C13
30 - C14
31 - C15
32 - C16
33 - Vcc
34 - C17
35 - C18
36 - C19
37 - C20
38 - C21
39 - C22
40 - C23
41 - C24
42 - Gnd
43 - C25
44 - C26
45 - C27
46 - C28
47 - C29
48 - C30
49 - C31
50 - 12M
51 - NC
52 - Gnd
53 - MA11
54 - MA12
55 - MA13
56 - MA14
57 - MA15
58 - MA16
59 - MA17
60 - MA18
61 - MA19
62 - MA20
63 - Gnd
64 - MA21
65 - A8
66 - A9
67 - A10
68 - A11
69 - A12
70 - A13
71 - A14
72 - A15
73 - Vcc
74 - A0
75 - A1
76 - SORD0 (Can't quite make out the writing)
77 - CS*
78 - CSDOT*
79 - GAD0
80 - GAD1

[channelmaniac]

From Kyuusaku on the Neo-Geo.com forums:

Here's the ZMC2's early counterpart at least for home carts:

ALPHA-8921 / PRO-CT0

Code:

D1 odd.15 1 64 VCC
D3 odd.19 2 63 VCC
D5 odd.24 3 62 odd.26 D6
D7 odd.28 4 61 odd.22 D4
D8 odd.14 5 60 odd.17 D2
D10 odd.18 6 59 odd.13 D0
D12 odd.23 7 58 A8
D14 odd.27 8 57 A7 inputs?
D9 odd.16 9 56 A5
D11 odd.20 10 55 A9
D13 odd.25 11 54 test?
D15 odd.29 12 53 test?
D0 even.13 13 52 test?
D2 even.17 14 51 test?
D4 even.22 15 50 gnd
D6 even.26 16 49 gnd
D1 even.15 17 48 B39
D3 even.19 18 47 B38
D5 even.24 19 46 B37 mux outputs?
D7 even.28 20 45 B36
D8 even.14 21 44 gnd
D10 even.18 22 43 gnd
D12 even.23 23 42 B35
D14 even.27 24 41 B34
D9 even.16 25 40 B33 mux outputs?
D11 even.20 26 39 B32
D13 even.25 27 38 test?
D15 even.29 28 37 test?
B30 29 36 test?
select inputs? B31 30 35 test?
GND 31 34 test?
GND 32 33 test?

Notes:
test? = NC pins or pins with tracks that ONLY lead off the board for testing.
A## are not address lines but home cart pin numbers as are B##

Don't mind the bad formatting

[channelmaniac]

Here's the NEO-257 @ location J2 on the 2 slot board. (MV-2)

Code:

Pin - function/chip connection/slot connection (cartridge pin function)
1 - Slot 2 B26 (CA15)
2 - Gnd
3 - Gnd
4 - Slot 1 A19 (CA0)
5 - Slot 2 A19 (CA0)
6 - Slot 1 B19 (CA1)
7 - Slot 2 B19 (CA1)
8 - ZMC2 - 15 C0 (CA0)
9 - ZMC2 - 19 C4 (CA1)
10 - Gnd
11 - ZMC2 - 16 C1 (CA2)
12 - ZMC2 - 20 C5 (CA3)
13 - Slot 1 A20 (CA2)
14 - Slot 2 A20 (CA2)
15 - Slot 1 B20 (CA3)
16 - Slot 2 B20 (CA3)
17 - Input (Slot) Select 1/2
18 - Gnd
19 - Slot 1 A21 (CA4)
20 - Slot 2 A21 (CA4)
21 - Slot 1 B21 (CA5)
22 - Slot 2 B21 (CA5)
23 - ZMC2 - 17 C2 (CA4)
24 - ZMC2 - 21 C6 (CA5)
25 - Gnd
26 - Vcc
27 - ZMC2 - 18 C3 (CA6)
28 - ZMC2 - 22 C7 (CA7)
29 - Slot 1 A22 (CA6)
30 - Slot 2 A22 (CA6)
31 - Slot 2 B22 (CA7)
32 - Slot 2 B22 (CA7)
33 - Gnd
34 - Gnd
35 - Vcc
36 - Slot 1 A23 (CA8)
37 - Slot 2 A23 (CA8)
38 - Slot 1 B23 (CA9)
39 - Slot 2 B23 (CA9)
40 - ZMC2 - 24 C8 (CA8)
41 - ZMC2 - 28 C12 (CA9)
42 - Gnd
43 - ZMC2 - 25 C9 (CA10)
44 - ZMC2 - 29 C13 (CA11)
45 - Slot 1 A24 (CA10)
46 - Slot 2 A24 (CA10)
47 - Slot 1 B24 (CA11)
48 - Slot 2 B24 (CA11)
49 - NC
50 - NC
51 - Slot 1 A25 (CA12)
52 - Slot 2 A25 (CA12)
53 - Slot 1 B25 (CA13)
54 - Slot 2 B25 (CA13)
55 - ZMC2 - 26 C10 (CA12)
56 - ZMC2 - 30 C14 (CA13)
57 - Gnd
58 - Vcc
59 - ZMC2 - 27 C11 (CA14)
60 - ZMC2 - 31 C15 (CA15)
61 - NC
62 - Slot 1 A26 (CA14)
63 - Slot 2 A26 (CA14)
64 - Slot 1 B26 (CA15)

[channelmaniac]

If you look at a 74257 data sheet you will see that it is a 2 to 1 data multiplexer. The NEO-257 is simply 4 of these chips combined into one package to save some board space.

The hard part is that I cannot tell which pins are for "output enable" like on the 74257 since the only function that isn't set to ground or +5v is the one for input select. It appears these chips are set to always be enabled.

Enjoy!

Code:

NEO-257 pinouts

1 - Input 16B
2 - Gnd
3 - Gnd
4 - Input 1A
5 - Input 1B
6 - Input 2A
7 - Input 2B
8 - Output 1
9 - Output 2
10 - Gnd
11 - Output 3
12 - Output 4
13 - Input 3A
14 - Input 3B
15 - Input 4A
16 - Input 4B
17 - Input Select A/B
18 - Gnd
19 - Input 5A
20 - Input 5B
21 - Input 6A
22 - Input 6B
23 - Output 5
24 - Output 6
25 - Gnd
26 - Vcc
27 - Output 7
28 - Output 8
29 - Input 7A
30 - Input 7B
31 - Input 8A
32 - Input 8B
33 - Gnd
34 - Gnd
35 - Vcc
36 - Input 9A
37 - Input 9B
38 - Input 10A
39 - Input 10B
40 - Output 9
41 - Output 10
42 - Gnd
43 - Output 11
44 - Output 12
45 - Input 11A
46 - Input 11B
47 - Input 12A
48 - Input 12B
49 - NC
50 - NC
51 - Input 13A
52 - Input 13B
53 - Input 14A
54 - Input 14B
55 - Output 13
56 - Output 14
57 - Gnd
58 - Vcc
59 - Output 15
60 - Output 16
61 - NC
62 - Input 15A
63 - Input 15B
64 - Input 16A

[channelmaniac]

Troubleshooting Color and Sync problems on MVS boards:

This post is from the perspective of repairing a MVS 2F board - the newer version of the 2 slot - but will work for most other MVS boards.

The output of the video subsection will go through the NEO-G0 chip then on to the Palette RAM (8k x 8bit SRAM) and the 74LS273 latches. From there it goes to a resistor ladder per color with a couple of the lines being buffered/amplified (higher current) by a 74LS05 IC before going out to the JAMMA connector.

When troubleshooting MVS color issues take a moment to divide the problem into sections.

1. Palette, or Color RAM error
2. No Sync (either horizontal or vertical)
3. Missing or incorrect colors

Part 1: Palette, or Color RAM error

If there are Palette RAM problems this can be caused by a problem in any of the ICs connected to that data bus: NEO-G0 chip, Palette SRAM, or the 74LS273 latches. The easiest thing to do is to power OFF the board and check the data lines on the SRAM ICs for shorts to +5v or ground connections. If there are none then replace the SRAM first.

If there is a short, remove the 74LS273 ICs first as they will be the most likely culprit. Check the data lines again. If the short is gone, replace the latches. If not, move on to the next chips, the SRAM. Repeat tests and replace the NEO-G0 last.

Part 2: No Sync (either Horizontal or Vertical)

The sync line goes from the NEO-I0 chip through a 100 ohm resistor to the JAMMA connector. It's also tied high to +5v through a 470 ohm resistor. Check both resistors. Replace if bad. If there is still no sync, replace the NEO-I0 chip.

Part 3: Missing or incorrect colors

Missing colors can be caused by any of the parts in the output section. Unless there's an esoteric problem with the Color RAM the digital part can be mostly taken out of the equation.

The output is made up of the 74LS273 latches, a series of resistors forming a ladder network, and 74LS05 Hex Buffer/Inverter IC.

With the power OFF, check the inputs and outputs of the 74LS05 IC for shorts to either +5v or ground connections. Replace the IC if any shorts are found. If there are shorts but the 74LS05 IC isn't it then replace the 74LS273 ICs. Next check the resistors for any that are burned.

If the RGB outputs are shorted the most likely culprits will the the 220 ohm and 150 ohm resistors as these are the lower value resistors. (The lower the value the higher the current that will flow through during a short.) check these resistors with an ohmmeter and replace any that are not reading correctly.

The next step is to check the outputs of the 74LS273 ICs with a logic probe. On the datasheet there are D and Q pins listed. The D is the input, the Q is the output. Check for signals on both sides. Replace the ICs that have inputs but no outputs between their D and Q pins.

Hook everything back up after replacing the parts and give it a test!

I had a board in the shop with missing red colors. There were no other symptoms on this board such as lines through images, etc... just screwy looking colors.

I replaced the NEO-G0 IC before stepping back and taking a better look at the circuit. After analyzing the circuit I replaced a shorted 74LS05 IC and two bad resistors to fix the board.

The 150 ohm and 220 ohm resistors were bad and the 74LS05 gate driving the 150 ohm resistor was shorted to ground.

[channelmaniac]

Model: MVS MV-4
Dignosing bad RAM ICs

Board is a NEO-MVH MV4 (4 slot)

Code:

Backup RAM Error:

Address    Write  Read
00D01406   AAAA   AA2A

The Read code is broken up into 2 parts... AA and 2A

The first 2 characters are the upper bits and are handled by the IC at D9
The second 2 characters are the lower bits and are handled by the IC at F9

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Code:

Video RAM Error:

Address    Write  Read
00000607   5555   5455

Same thing here... the Read code is broken up into 2 parts... 54 and 55

The first 2 characters are handled by the IC at L9
The second 2 characters are handled by the IC at K9

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Work RAM is laid out backwards from Backup RAM and Video RAM on the NEO-MVH MV4

Code:

Work RAM Error:

Address    Write  Read
xxxxxxxx   AAAA   AABA

(Yeah, I was too lazy to write down the exact address of the error...)

The first 2 characters are the upper bits and are handled by the IC at H9
The second 2 characters are the lower bits and are handled by the IC at G9

[channelmaniac]

Model: MVS MV-6
Diagnosing bad RAM ICs

Here's the info on the 6 slot board:

Upper Backup RAM: E9
Lower Backup RAM: F9

Upper Video RAM: L9
Lower Video RAM: K9

Upper Work RAM: J9
Lower Work RAM: H9

Just keep in mind that if you have an error at the bottom boundary then you need to check traces.

For example a video error of:

Code:

Video RAM Error:

Address    Write  Read
00000000   5555   5455

Shows up on the zero boundary. You need to check to see if the traces (wiring that is on the circuit board) are all good. A bad trace or a bad chip can each cause this type of error.