Gah this one is going to be difficult then.
Next step is a visual inspection of all capacitors on all the boards. Look for cracked or burned tantalum capacitors that look the same as that one. They may be present on any of the four boards in the scope. The timebase one is difficult to see but there's one on that you can observe if you look top down. I use a plastic ruler and move it across in inch steps then scan from left and right to find them all and inspect.
If you can't find anything obvious then the next steps are:
1. Check for an AC voltage on the TPs listed before. You can probably get away with a DMM on AC volts for this but another oscilloscope is better. This will identify any problems in the left half of the power supply (rectifiers, smoothing caps, voltage regulators). If that looks ok, i.e no AC voltage registered, proceed to...
2. Finding all tantalum capacitors on the 15V rail and testing them.
I have had to do a wholesale replacement process on the entire scope before. Takes a while but is worth it.
Once this is reading 15V, diagnostics may proceed. The problem as always with these is that everything depends on this being spot on.