Ok, this is the main thing to check:

On the back of the display board are 3 through-hole components - with the display towards you, there are 2 caps on the left and an inductor on the right. The leg of the righthand capacitor nearest the inductor is the display anode power (this is also connected to TP1, which is labelled "60V" on the PCB just below this leg).

With the display at maximum brightness, this leg/test point should be around 60v - test against ground, ie the case of the empeg - (ie, in the range 57-63v). If it's any less, the display will be dimmer than it should be. Some of the displays we had back were in the range 12-25v.

WARNING! 60V is nasty voltage, it can give you a snap! Don't run your empeg with the top off unless you really know what you're doing!

The cap in question is a 100nF one which stabilises the reference voltage for the 60v PSU switcher. It's under the display, connected to pin 5 of the MAX770 chip (near the rotary control). In all the cases we've had, this one fails for some reason and as such the display PSU goes very low; there is a patch to the PCB near here for another reason on empeg (not rio) units, which may explain why the cap has failed (too much heat when the assembler was soldering the other components on?).

Hugo