Got trouble with WPC display boards. Over the years, we've built up quite a collection of boards. When one didn't work in the past, they'd just order a new one and toss the old ones in a pile. In the past two years, I've rebuilt the HV parts of them all, and got most of them working. I still have a nice stack of boards with a problem I can't seem to figure out. They all blow fuse 602. They've had the HV rebuilt (Q2, 3, 4, 5, 6, 7 and 10 - D3, 4, 5, and 6 - R8, 9, and 11), the big caps replaced, and the bridges checked, but they keep on blowin' the fuse. Some of them even have killed or scrambled DMDs, and one had q5 and r5 explode. Obviously, something is pulling a huge load on the highest voltage parts, but what? I guess my question is: can any of the IC chips ever been responsible for blowing the fuses? Looking at the schematics, my guess is no, but I'm not too familair with the DMDs themselves. Also, any suggestions beyond rebuilding the HV again?

Alright, I admit my ignorance. How do you test zener diodes?