Galaxians Repair Logs
Repairer: Paul Swan
BOARD NO: 0131
(1) "BAD ROM" at power up.
(2) "BAD RAM 3" at power up.
(3) No stars, sound faulty.
(1) Preliminary checks of the Z80 address bus and buffers did not indicate anything obviously wrong. Removing the five program ROMS from the adaptor card and reading them in the programmer gave an inconsistent reading for ROM W. Taking two readings and checking them manually revealed that every fourth location was inconsistent. Replacing all the ROMS with a new set of EPROMS gave (2).
(2) The manual said that this error refers to IC4F and IC5F (2101 RAM). Going around each RAM with a logic probe indicated that both devices were totally dead. No outputs from either RAM were active, all the outputs were floating. Checking all the control inputs against the schematic confirmed that the outputs should at least always be driving on this game. Replacing both IC4F and IC5F started the game with (3).
(3) From the schematic, the star field was generated by a psuedo-random pattern generator formed by IC's A1 & B1 (74164). Checking the outputs of these counters with a logic probe revealed that all the count outputs were high. Any fault in the count chain can cause the whole lot to lock high or low and so a pulse injector was connected to the start of the count chain on pin 1 of IC A1. The injector pulses were then traced down the counter chain stage by stage. There was no output at pin 4 of IC A1 (or subsequent stages) and replacing IC A1 fixed the game.