maxtherabbit wrote on 2021-06-08, 15:07:
Assuming you mean TR24 - the MJ2955 power transistor.
I do. I read the label "T2" at the base terminal to be the reference number, but T1, T2, T3 seem to be the pins of a connector where that transistor is mounted. The transistor is called TR24.
maxtherabbit wrote on 2021-06-08, 15:07:
I actually had that same transistor fail on my 5151, but the symptoms were quite different. The raster never collapsed fully like OP's monitor it would just get "shaky" after a bit of run time, and B+ didn't get reduced it actually jumped up a little over 16V.
Your symptoms sounds like that transistor is leaky, and passes more current than intended, especially if it gets warm. Because the transistor passes too much current on its own without being controllable, the B+ voltage rises out of regulation, so the 100/120Hz ripple that should be eliminated by the 7815/MJ2955 combination will (partly?) pass through the regulator. If you live in a 60Hz country, the 120Hz ripple can cause interesting interference with the 50Hz refresh rate, causing 20Hz jitter.
On the other hand, in the case of the OP, I assume the transistor doesn't pass too much, but too little current. It might just be corroded pins in the socket. Because the assumed failure is different, the expected symptoms are different, too.