After spending some time on the repair thread for these scope power supplies, I did further research and also prepped a Murata supply which I had on hand, to replace the original Cherokee/GE CCH125 in the scope.
So today, I took the factory supply out of my pristine, dare I say virgin, DSOX3104T. The scope is super clean inside and out. Hadn't really looked at the supply much because ... it's a supply.
Also, haven't used the scope in weeks but it's plugged in a day at a time. First thing I noticed was that the ac mains cap was hot as hell. (thermo and IR camera in the days to come)
Back of the board looks like crap. Not blackened but clearly looked like it was sitting in an oven at high temp.
I'll cut this short with more to come later.
I've designed SMPS from low level to high level modulars into many medical products but do not profess to be an expert in topologies.
So, need some help here. The 420v mains cap is sitting at 397 volts dc
This is on a 120v mains.
This is why these scopes are cooking. Might this supply be using doubling up front? You just can't do it with these bleed resistors. They will be running red hot, and right below the cap with thermal coupling via the cap leads. With very little thermal decoupling, these 1W resistors are running at 0.4W, and the thermal rise is increased due to their close proximity.
The new Murata is sitting on the bench, most parts cool to touch and the voltage across the cap sitting at ..... drumroll ..... 168vdc. I could hear it saying "take me", "take me", "take me".
Ok little Murata, as soon as the connectors arrive.
It seems like these scopes are a ticking bomb, almost guaranteed to blow on European and other 230v mains, although I don't know what their design would do there.