SNK Games (Neo Geo MVS / AES)
channelmaniac:
Model: MVS MV-4F
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.
channelmaniac:
Model: MVS MV6
Symptom: Vertical lines in graphics
Some background objects had vertical lines in them - in this case the "shock Troopers, 2nd Squad" floating logo had lines. It's the logo that is on the screen when it is playing in demo mode. I traced it to connector CN8 and started tracing it on the bottom board.
UGH. It goes straight to NEO-CT0
As one final step before replacing that surface mount chip I checked the continuity between the top board and bottom. Bingo! No continuity. Next step - pull the boards apart and look at the connectors. They had something sticky on 2 of the pins and up in the socket.
Desoldered and disassembled the socket then cleaned the pins on both connectors. Reassembled, resoldered into place, and tested. Board is fixed.
Moral: Don't discount those board connections! Look at them closely to see if there is a foreign substance in there causing the problem.
channelmaniac:
Model: MVS MV4F
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.
channelmaniac:
Model: MVS MV4F (newer smaller 4 slot)
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.
channelmaniac:
Fixed: AES
Symptom: Lines in overlay graphics (text, health, etc) and no sound
Fixed a corroded trace leading to NEO-B1, pin 131, from the cartridge slot to restore the video.
Looked at the YM3016 Sound DAC and there was no serial clock or data flowing from the YM2610 sound chip. Checked the Z80 CPU and it appeared good. Looked at the clock signal on the YM2610, pin 63, and it was dead. Traced it back to a 74AS04, pin 1 near location K4 and found a corroded plate thru hole on the trace. Patched the trace to finish the board repair.
Navigation
[0] Message Index
[#] Next page
[*] Previous page