PCB Repair: Battletoads
Problem
Game plays but displays only colourful vertical bars:
Diagnosis
Battletoads is one of three arcade games to use the TMS34020 Graphics System Processor (GSP), the other two being Revolution X and Metal Maniax. The GSP appears to handle the foreground and background layers, while an FPGA draw sprites into a framebuffer. Being a 90s game, there are of course, no schematics available.
The colour bars suggested to me that the VRAM horizontal scanout was broken. I decided to probe some of the VRAM to see how they were being addressed. I observed a bad signal on the #DT/#OE pin (Data Transfer/Output Enable) of U63. It seemed that this signal is common to all the VRAM:
I traced the signal back to the 47Ω resistor network R6 and saw that the signal on the opposite side was valid. I wondered if the signal on the VRAM side was shorted to another signal. Indeed, after testing #DT/#OE for continuity against other pins of U63, #DT/#OE had a resistance of 8Ω to SIO1:
A resistance of several ohms typically indicates an internal short. Desoldering U36 eliminated the short at the PCB, while it remained 8Ω when testing the RAM out of circuit. Replacing the RAM resolved the issue completely:
Fix
Replace HM53461ZP-10 at U63.
Very cool. Thx for sharing.