Troubleshooting 4 slot Top Boards

OK...

I have the top board from hell here. I'm going through it step by step to find all the problems and am learning a LOT about how it's wired up.

I've pinned out 3 of the 4 connectors on the 4 slot... and will post the pinouts when done.

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Problem: Cartridges do not work. Crosshatch pattern is all you get on the screen, no matter what slot. 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.

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Problem: 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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There's still a problem with graphics corruption on slot 1 to fix.

How did I find these? I used a logic probe and a databook. I looked up the chips to see what they did and checked for the right outputs.

Missing outputs? Then check the inputs. Duh! Oh, no inputs? Well, check the inputs from another slot. Those work? Well, find the broken trace.

If the inputs were there and the outputs were wrong then I would've swapped out the chip.

Hope that helps you keep your Neo Geo MVS alive!

This top board will be kept around the shop for a test board in case someone doesn't send in theirs with the bottom board when getting one repaired. Who knows. Maybe someday I'll repair the butchered traces on another odd bottom board I picked up in trade for fixing someone's system. (OMG! I wish people wouldn't try to solder/desolder on these boards for their first repair attempt)

Raymond

w00t!

Slot 1 fixed.

All the signals were there from the bottom board to the chips buffering the sockets... slot was cleaned... still garbled graphics. ARRRRGH.

What about the connections from the chip to the socket? Duh.

Damaged trace going from pin 5 of B4 a 74LS245 to pin 8B on the CTRG1 slot connector. That address line for the graphics chip wasn't getting there resulting in a block of memory read from the wrong portion of the chip on the cartridge. Patched the trace and the game looks great!

I found this with the logic probe too. Took the cart apart & plugged the boards in separately. By doing this I was able to find a pin on the cartridge that had no signal on it at all.

Enjoy!

RJ

That's some great info right there..Thanks..

Good work

First thing to check in a case like that is are any of the 244/245 getting hot.
Then the likelyhood of traces o/c.

Thanks guys,

Yeah, all the slots have the 244/245s connected together so if one shorts internally it pulls enough current to take out the others.

Seen that on another top board I fixed. I had to replace the same chip for each of the 4 slots.

RJ

When they get hot they tend to bubble, that is a good indication of the fault.

OK...

Another 4 slot top board from a user on the forum that needed repairs...

This time Slot 3 & 4 worked great. Slot 1 & 2 were dead.

No bent up pins, no corroded pins.

All signals looked like they were getting from the bottom board through the various chips and to the slots. I even went through an pinned out the slot to chip connections on the slots in troubleshooting it.

Everything was connected perfectly.

Went back to the 74LS138 - a 3 to 8 decoder at location A2 - which is used to choose the active slot. Pin 1, 2, and 3 are the inputs. Pin 2 was stuck high. The enable pins were fine. Traced back pin 2 to pin 5 of the 74F174 at location A1. The input to that logic gate was on pin 4 and it was tied to pin 13. There was no signal on those pins.

Fixed a broken trace from Pin 4/13 of A1 to pin 12 of C4 to repair the board.

Enjoy!

RJ