Peter,
Here are some comments regarding the 576, 577, and TDS644A.
Yes, the brown HV transformer is the bad kind, so it will likely crap out soon. The symptoms are about right.
The 576 v. 577 discussion will likely never end, because they each have their pluses and minuses, and either is a fine CT for most uses. There is no consensus, except that it's nice to have both.
The TDS series scopes, as far as I can tell, all use a similar PS topology, but unfortunately not identical in the details. I had to fix the standby PS in a TDS544A a while back. As I recall, the main supply/preregulator is a power factor correction (PFC) type. In its common form, it is a boost converter, so it is set up to make preregulated DC somewhat above the highest expected peak voltage of the power line - typically 350-375 V for 240 V operation, or doubled 120 V in a dual-range configuration. In a full-range, continuous coverage design, the input line can be anything between the lowest and highest line possible, in any country. I think the ones I have (TDS 544A, 754A, and 820) are full-range, but there may be some that use dual-range. Regardless of this detail, the highest expected rectified raw DC should be somewhat less than 400 V, so somewhere around there is what the PFC converter is designed to put out, and still provide regulation. All lower voltages are simply boosted up to this level, giving very wide coverage.
It's been a while since I studied it, but I think that in standby, the PFC is actually running all the time too, and the standby supply runs from the PFC output. It isn't necessary for the PFC to be running all the time, since when it's off, the rectified DC from the line passes through, but would not be boosted or regulated. This would be OK too, since the standby supply should be able to operate over a wide input range too. Either arrangement can work, but controlling the on and off states would be different.
The standby supply keeps it ready to go, and supplies power for scope-side circuits, including the one-bit memory (I think it's a latching relay) of its last power state. When the power button is pushed, the state is switched to the opposite of whatever it was. In the case of turning the scope on, the PFC converter is activated - if it's not always on anyway - then the main output chopper fires up, converting the 400 VDC to the low voltage secondaries.
Anyway, in standby, you may see the input voltage to the standby converter be something similar to the peak of the input AC, or fixed near 400 V, depending on the implementation details. In the scope on state, this voltage should be around 400 V.
Ed
