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.
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