If you have another scope, set it to AC coupling and adjust the V/div setting so that you can see it (nominally 120 Hz on a standard USA P/S). Alternatively, use your multimeter on the AC setting. Typically, the ripple should be small, like 10-50 mV, but that is hugely variable on the P/S supply design. Having said that, you have another problem having nothing to do with ripple - namely, your 15V measures 0.5V. At least on the service manual I looked at (from ko4bb.com), the schematic is annotated with lots of voltage measurements. Work your way back from the test point until you find a spot where the schematic's indicated voltage is good going into the spot and bad coming out of it. Could be something simple like a shorted output capacitor or a fried transistor. Or it could be much more complicated if the P/S is
actually OK and is going into current-limit shutdown mode because something in the scope circuitry is loading it down. For the latter, you need to disconnect the P/S from the rest of the circuitry to see if the P/S returns to normal voltage.
Also, since you say you are a newbie, be careful when working on this. There are some nasty voltages lurking around. Also, don't go anywhere near the CRT tube since there are even nastier voltages there.