Yep, I'd start with checking the power supply.
With the lid off, you'll see an aluminum rail going front to rear onto which voltage regulators are mounted.
Refer
http://www.ti.com/lit/ds/symlink/lm7805c.pdf for the pinout on those.
It's easy: With the regulator facing you, mount/heatsink up, legs down, the output pin is number three - the one to your right.
The regulator referred above is a +12 volt regulator, but all other voltages types, both positive and negative, have the same pin-out.
Using a voltmeter, initially set to 250 volt scale, you should measure:
+5 volt - for cpu and other digital
+15 and -15 volt - for most of the analog circuitry
+24 and -24 volt - for more analog (discreet preeamp, parts of compressor and wah) and negative bias for tubes
+12.6 volt - for tube heaters
There's also a +200 volt line somewhere (actually more tha 200, just don't remember how much) volts - for tube plates
Don't be scared about this, just be careful about not touching anything else that what's required, and only with insulated measurement pins.
Start by checking these voltages - and
be aware of the 200+ volt line zapp zapp
Clamp one lead from the meter to chassis, and pin the other one to pin three on each voltage regulator in turn.
And don't bother with the light coming up in different colors; this is just semi-random behavior until other things hs been fixed.