Looking at your Youtube, something is obviously failing with heat so freeze spray would be helpful (upturned canned air is fine). My initial thought was U760 as the timebase is collapsing but I've not seen one fail such that the power led would flicker. Unplug the timebase, see if it still fails. (You are looking for the power LED to flicker, nice symptom, unusual!). Surprised the scope isn't tripping completely. I'd suggest monitoring the power rails to see if its just one rail failing or all rails. Complicated with switching supplies because all will drop on overload as the switcher detects a fault but not impossible to ascertain which rail is overloading. I would start by disconnecting the feed to the multiplier (so nothing on screen) and just observing the power led (or monitor the supplies) for any failings. You can also remove power to the attenuator board, an unlikely culprit but I have had the opamps go short. You could just pull all the power supply links and monitor the rails but without a load the fault might not show. What is the Y amp doing when it fails? Amplitude still good? Your replacement parts look ok. Hopefully you have checked the obvious, I've received these with loose screws jammed in unlikely places, solder bridges due to careless repairs. JC
