“Warming up to the problem” from Don Borowski
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 immediately 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 immediately 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.