MVS MV-4F Repair Logs

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Repairer: 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:

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


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.

Symptom: Z80 error and rapid audio click

Had a MV4F board that came in with a Z80 error and a rapid clicking sound in the speakers. Strange thing is that as the speakers clicked the picture on the monitor flickered.

(Little did I know this thing would kick my ass.)

Replaced the Z80 CPU to fix the Z80 error. For some reason SNK put a Z80 in there and the board is labeled for a Z80A (higher speed) CPU.

Now the system booted but flickered like mad. Audio ouptut was horribly distorted.

With the audio down the system worked fine except the audio output IC was getting very hot. Recapped the audio section. No help. Replaced the audio output IC. No help. Replaced the volume control. No help.

For some reason there was an oscillation happening in the audio circuit. This oscillation was causing the audio output IC to suck up large amounts of current. This was causing dropouts in the +12v output of my bench supply. These dropouts matched the screen flicker and the clicking of the speakers.

Checked every trace in the audio section - all were good. Replaced the 3 op-amps. No help. Checked all the film caps (the non-electrolytic caps)... all checked good.

All voltages looked OK but were a little lower than another MV4F board but they were within reason (+12v was a little low so the other voltages around the chip were correspondingly lower)... On a whim I removed both of the .1uf film caps on the speaker outputs. Audio was distorted but amplifier was stable.

Next I put them back in circuit one at a time. The one on the right speaker was the cause of the problem. Replaced the cap and problem was still there. It looks like there's an issue with the ground trace inside the board. Removed the cap and ran the board for an hour at 3/4 volume with no problems.

Troubleshooting reset lines

In working on a 4 slot board (MV4F) I encountered a Z80 error.

The board had other issues (documented elsewhere) and when those were fixed and the board not yet reassembled, it gave a Z80 error.

The reset line was held high.

The reset line goes through the 74AS245 IC at location E11. It goes through the latch on pins 2 and 18. If that IC is bad the reset line could be stuck high or low, causing the Z80 CPU to never boot.

That reset line must transistion from low to high to reset the Z80 CPU and make it start executing commands.

In this case the problem came because the IC at E11 was pulled from the board as a step in troubleshooting. Once the IC was placed back in the board the reset line cycled normally and the Z80 booted. (This was also tested by jumpering the reset line on pin 15 of IC A9 to pin 26 of the Z80 before replacing the IC)

Symptom: Stuck in Watchdog (Click of Death)

Have a 4 slot board in to repair that was giving the click of death - stuck in watchdog - problem.

This can be caused by many different problems, including:

Bad CPU Bad Work SRAM Bad Backup SRAM Missing control signals on Work or Backup SRAM Shorted 74AS245 ICs buffering program ROMs on top board to CPU on bottom Short on address bus Short on data bus Missing address or data signals to chips

First thing... Check for gouged traces. Found and fixed one. No effect. Next... Substitute BIOS. Nothing. Next... Check control signals on the Work RAM - Stuck high on pins 22 and 27. Next... Check control signals on the Backup RAM - Stuck high on pins 22 and 27. Next... Check control signals on the BIOS ROM - These were working.

So we had a problem somewhere in the system to where it was trying to read the BIOS but not able to initialize hardware.

Pulled the 74AS245 ICs buffering the data lines to the top board. Had no effect.

Checked for more broken traces. No more were found.

Checked the address and data lines going to the BIOS and SRAM ICs. All were good. Checked the data lines going to the NEO B1 chip. All were good. Checked the data lines. All were good.

Next checked the A22* and A23* alternate lines. Missing one one! Ran a jumper wire from Pin 55 of the NEO-E0 IC to pin 117 of the NEO-B1 IC and the board came up!

OOPS. Z80 Error. Reset line stuck high. On a Z80 since the reset line did not transition from low to high the Z80 never booted. Ran a jumper and the audio section worked. Replaced the pulled 74AS245 ICs and the board wouldnt' boot. Reset was stuck low.

Turns out the reset line for the Z80 CPU and chips on the left side of the board goes through the 74AS245 IC at E11. Removed the jumper for the reset line and board booted normally.

Plugged the top board back in, inserted some test carts and played a couple of games.

Symptom: Video RAM Error

This board had multiple problems due to liquid damage.

4F board came in with a Video RAM error. Replaced the upper Video RAM IC and error went away but video was glitchy and had bad colors.

Repaired a bad trace between the lower Video RAM IC and the LSPC2 IC.

Glitching was gone but the colors were bad. Test screen showed blank for red, blue for both blue and green, and white was a little off color. Put in a UniBIOS chip and noticed the green was off and the low end of red was missing AND a white block was in the place of the lowest red color. (SNK BIOS shows 4 color boxes, one for each color. The UniBIOS chip shows color spectrum bands for each color)

Checked the Color RAM and noticed that address line A2 was stuck low. Checked the NEO-B1 IC and noticed that A2 was stuck low there as well. Backtracked through the circuit and found that the TDI4 pin (pin 22 - signal name P16) was floating - no signal.

Checked the other ends of the signal - the 74LS245 IC buffering the L0 ROM and pin 134 of the LSPC2 IC. They had signals. The trace under the LSPC2 connecting to pin 134 was bad. Ran a jumper wire between the 74LS245 and the NEO-B1 chip to fix. (It was WAY easier to solder to the big pin on the 245 than the small one on the LSPC2!)

Audio was missing on one channel. Grabbed the oscilloscope and found that the volume control was bad. Replaced it with one off of a parts board.

Reassembled the game and tested. Slot 3 had vertical lines on the screen through graphics. Patched a corroded trace to fix. Played a few games to test.

Replaced the leaking battery then reassembled the board for one final test.

Symptom: Missing/Garbled sound problem.

This board has been a pain to track down. The board had been worked on before by another tech.

Fixed a broken trace on pin 15 of IC 4N (74AS245 - other tech damage - grrrrr!) and the board would play sounds but was distorted on the higher frequency digital music.

The sound ROMs are read through the YM2610 IC through data latches (74LS245) at board locations 2M and 2N. If the voice and lower frequency music notes are distorted then replace the IC at 2M. If the higher frequency music is distorted, replace the IC at 2N.

On this board the replacement of 2N did not fix it. Board had a damaged trace internal to the board and the ground pin on IC 2N was not connected. Ran a patch to fix the board.

Symptom: Lines in background AND foreground graphics. No audio but board would make popping sounds and the screen would flicker with the pops.

Board had strange graphics corruption on foreground and background graphics. This was different than what is normally seen as evenly spaced lines (1 bad line showing up out of every 8 bits across the screen) and instead was like every other line was missing.

Swapped top board out and narrowed problem down to the bottom board. Checked the 74xx245 buffers between the top and bottom board. No problems found. Put the board under the magnifying lamp and found shorted pins from pin 65 to pin 80 (65 through 76 are ground pins anyway) on the NEO-ZMC2 IC at location 11M. Resoldered the pins to fix the graphics problem. Installed an audio cap kit and reset the backup RAM to finish the repair.

It appeared to be leftover solder from when it was factory soldered that finally bridged the trace between pin 79 and 80. There were solder blobs between every pin down that side of the chip but only 79 and 80 appeared to be shorted.