Hi smallfreak,
You could do a couple of measurements with just a DMM before you dig into it.
The schematics is on pg. 208, the brief description of the circuit on pg. 55 of the manual you referred to.
Do check the rectified/filtered mains voltage on C10, C11.
If you have 220VAC mains voltage you should see over 300VDC voltage on those two series connected caps.
Moving on, the basic switching power supply is controlled by IC1, an Iwatsu hybrid IC.
The IC is controlled (feed back) from the +5VDC output stage, that is the bottom most circuit on the right side of the switcher transformer T1, fed from the winding with terminals marked 1 and 2.
At the output of that circuit you should see +5VDC, for the digital circuits.
You can measure this voltage at the W6 connector, going to the CPU board (top right corner on the page), at pins 1, 2, GND, and pins 3, 4, the +5V.
You should have this +5V before anything else, then you can check the other supply voltages.
There is quite a long thread talking about repairing such a PS.
Here is the thread, though it is in Hungarian:
http://hun.lists.elektro.narkive.com/EcUwcO6N/flyback-tap-melegedesI guess you can run it through some translator...
The PS was basically working for the guy, but he felt it was overheating.
He experimented with replacing that Iwatsu hybrid IC, B365N, with a more widely available switching regulator IC.
PS: I would rather get another older analogue scope instead of that DSO112A, unless you need the small size!
Good luck, Peter