Display Problems

It wasn't long before the problem originally described by this PET's previous owner made itself visible. Now that the screen was actually displaying something, a slight shaking of the image was visible shortly after powering up. As time went on, the shaking got worse and the image itself started to distort.

A minute after powering on...

...and a couple of minutes after that.

After 15 minutes, the distortion is significant.

Above picture links to a movie of the image dancing.

Thankfully, there are schematics available of the video board inside the monitor, complete with a comprehensive list of test points and illustrations of what the waveforms at each of these points should look like. With this documentation and an oscilloscope at hand, I didn't think it'd take too long to diagnose the problem.

Sitting down for a trouble-shooting session, I started working my way through the test points. When I got to the third, I heard something start to sizzle and saw a big plume of exceedingly aromatic smoke coming from the back of the oscilloscope - so now I had two pieces of equipment in need of repair! Thankfully it was only a blown capacitor in the 'scope's power supply, which was easily replaced after I could finally get at it.

Working through the rest of the test points, I confirmed that at least the circuit for the video signal was OK. The other two (the vertical and horizontal circuits for the CRT) looks a little strange, but not any stranger than just after power up, when the screen looked fine. Looks like this wasn't going to be as simple as I thought...

Next: Fixing The Flickering >>