I got that dam'd TDS520 to repair. Powered up, it works perfectly.
Then, the screen gradually expands and then darkens in about 20 mins. The process initially is very inappreciable, and gradually accelerates over time.
I dismantled the display board and its associated CRT to make it work out of the oscilloscope (it's connected to the main chassis with a flat cable only).
Searching for some component in thermal runaway, using coolant spray give me no result. Mumble...
I made some voltage measurement. The A20 board uses only the +24 V supply from the main PSU and uses an LM317 to step down to +21 V.
When the screen starts to expand visibly (and darkens) the +21 V decreases rapidly down to +15/16 V and the LM317 becomes damn hot! Surely and excessive current is drawn from it.
I've changed the C308 speed-up capacitor (base of Q205) with no results.
I'll check the Q205 drive signal to see if it changes over time but I'm strongly oriented toward a failure in the flyback transformer T225.
I know already know that this transformer model is practically unobtainable (120-1841-00).
I wonder that an equivalent or similar transformer cannot be found that can be adapted. I don't think it's such a so critical component!
What is your opinion?