ça te ferme une porte de plus comme ça
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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.