After checking the power rails it was obvious that the 5v regulator wasn’t working, so first action was to replace it.
With the regulator replaced I hopefully tried the power again. This time all the power rails looked good, but still no picture. After checking with my multi-meter and oscilloscope everything looked good. Clock signals fine, memory address and data lines ok , ULA video output ok….. I was at a bit of a loss.
While thinking the problem over I decided to apply the composite video mod in the hope that when it did work at least it would look better.
With that completed, I plugged in once again to started diagnosing the problem, and surprise …. I had a picture. So the problem must be with the analog video out.
I wasn’t out of the woods yet, I had the typical memory fault colour bars on boot. Using my new diagnostics cartridge quickly identified the two upper DRAM chips that were to blame.
The upper DRAM was already socket-ed making replacing very easy.
With the fix complete it was time to play some games 🙂
Still some cosmetic fixes, but that’s a later post.