I wanted a small black and white TV for my workshop in order to work on video
equipment. I spotted one at a garage sale for $1. It had a horizontal line
burnt into the phosphor on the CRT. I immediatly recognized this as a sign
of a faulty vertical deflection circuit. The burnt phosphor would permanently
affect the picture, but this didn't matter for my use. I figured it would
be easy to fix, and the price was right.
I got it home. I didn't have a schematic for it, but identified the vertical
deflection circuitry in short order. I began testing some key parts. Finding
nothing amiss, I turned on the TV. There was still no vertical deflection.
I then grabbed my voltmeter and made some measurement. The voltages looked
"strange" around one of the output transistors in the vertical circuit. I
turned the TV off, and made the standard junction voltage drop measurements
on the transistor. This measurement showed the transistor to be fine. Turning
on the TV again, I saw no vertical deflection, and the voltages again looked
strange.
After several cycles of this, I grabbed a can of cold spray, and sprayed
the suspect transistor. The vertical deflection circuit immediatly started
working, and a good picture was visible on the screen.
After replacing the transistor, I did some experiments. It turned out that
the transistor would fail open as soon as it warmed up just slightly over
room temperature, but would return to normal operation when it cooled. Turning
on the TV caused the transistor to heat up. In the time it took for the CRT
to warm up so that I could see if there was vertical deflection, the transistor
was already warm enough to go open. Turning off the TV allowed the transistor
to cool. By the time I got the probes on the transistor leads to make the
junction test, the transistor was functional again.