Moon Patrol by channelmaniac
Symptom: Garbage & RAM NG message on the screen
RAM NG is RAM No Good. Error on screen was: RAM NG 8000 55 95
Looked at the schematic. The only RAM location labeled was E000-E7FF for the main program RAM. Checked the wire harnesses between the boards and found that the harness for CN2 on each board had 3 wires cut out. (WHY?) Two of the wires went to nothing. The third went to CN2, pin 17A, which connected to pin 11 of the 74LS157 at location 8L. This was 1/2 of the multiplexed address line A6/H256B signal to address line A6 for the 2114 SRAM ICs at location 7K and 7L on the character board.
Patched the wire on the harness and tested the board. Reassembled the board stack and replaced 5 broken standoffs.
Symptom: Horizontal lines of garbage or solid colors on screen
The video data is written to a set of Intel 2125 SRAM ICs (or equivalent) before being sent out to the monitor. If one of these ICs is bad then you will get thin horizontal lines of junk on the screen.
