Problem

Extensive graphical corruption. All graphics ROMs reported bad during self-test.

Colour fault.

Diagnosis

The PCB was rather dirty and partially corroded in places and first required a damn good cleaning with isopropyl alcohol.

It seemed very unlikely that all of the graphics ROMs were bad. I removed one, cleaned the legs and then read it in my device programmer. It verified good against the corresponding ROM from the MAME set.

Probing the graphics ROM’s /CE lines revealed that they were floating. I traced them back to the Altera EPM7032 at U35:

A friend had a spare that he kindly gave to me. Installing that fixed all the graphical corruption and self-test errors. I was able to read the good U35 in my programmer but have not yet tried programming a replacement to verify if the data is actually valid.

As for the colour issue, I probed around and found that pin 3 of U82 was stuck high. This is one of the green bits. The inputs to that IC come from U38, so it appeared there was a broken track between the two.

Fix

Replaced EPM7032 at U35.

Fixed broken track between U38 (74LS374) pin 5 and U82 (74LS541) pin 3.

Philip Bennett