Just wanted to share my first Vectrex Repair video with ya'll! The symptoms of this one are a shrinking screen and bad noises coming from the high voltage transformer after the unit warms up a bit.
Something I forgot to mention in the video was if you don't have an AC power supply to vary the voltage a little, you can simulate a lower input AC voltage by running the Vectrex on an extension cord which might lower it just enough to temporarily fix and surface such an issue. If not low enough you can plug a heat gun or hair dryer into the extension cord as well and turn it on. That will definitely lower it. This is actually one of the things I was doing when trying to figure out why my unit was behaving the way it was. I was using the heat gun to heat up sections of the circuit board, and freeze spray to cool them down. Then I noticed just turning the heat gun on and not even pointing it at the board was having an effect. This led me to just using my AC power supply that the Vectrex was plugged into the entire time After considering everything about the LM555 circuit, I figured it must be the chip itself and that theory turned out to be correct.