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Williams Corvette Pinball - DMD Blotchy



Game: Williams Corvette Pinball
Symptoms: DMD display characters was a little blotchy
Troubleshooting Steps

Looking at the display, I could see that it didn't look like, it almost looks like the characters were blotchy and distorted. You could see dots around the characters, it was super visible in person as well.

The first thing that I did was checked was the voltages that the DMD was getting. One thing that I noticed was that the -125V line was rather out of spec, I was measuring -140V, where as other DMDs at the shop were measuring closer to -116V to -120V This difference can cause some biasing issues, which could cause display issues. With that in mind, I had to troubleshoot how the -125V rail was being generated.


The -125V rail is ultimately generated by Q6, a MJE15031. Checking everything else in the circuit, everything seemed like it was working just fine. I did notice that the voltage Q6 was generating was pretty close to BR2, which implies that maybe it was shorted. So I went ahead and replaced it. After doing that, it resolved the issue with the blotched characters went away.


Solution: Bad Q6 (MJE15031) on the display driver board


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