Problem

Bad colours and missing sprites:

Diagnosis

Deciding to tackle the missing sprites first, the first area to check was the sprite line buffer on the video board:

There was activity on both the DIN and ADDR lines but /WE was floating. Close inspection of the video PCB revealed a damaged trace. Repairing the trace led to this:

A problem with the ‘big sprite’ layer remained, presumably related to the addressing of the attribute RAM. Here’s the relevant area of the schematics:

After some probing, I found that the outputs of 4A were floating. This was due to a floating /G input, again caused by a bad trace.

Fix

Repair /MCLK trace connected to /WE of sprite RAMs (8J, 8K, 8L etc.).

Repair GND trace connected to /G of 74157 at 4A.

Philip Bennett