When I initially checked the voltage, I just used a DC multimeter to measure the voltage between ground and the respective lead coming off the power supply board. Looking at the schematic it does appear as though the processor board is powered off the +5V. I think I can understand where you're coming from with the logic level shifts: If the voltage is varying and drops sufficiently below a specific threshold then the logic gates are going to respond apppropriately - either on or off.

I do have a scope though my abilities to make full use of it are progressing. I hadn't thought about using it. Thanks for the tip.

On the flip side, there is still something else in the +5V circuit needing replacement. While replacing the 7805s and the 7812 did seem to improve the stability of the circuit, I have had it power on once or twice with the same random character problem in the display after cycling on and off the power switch.

--- In oberheim@yahoogroups.com , jim finch <jahg88@...> wrote:
>
> You didn't mention how you were reading the powersupply voltages-but I didn't read the whole thread.If you use a O-scope you can see what is actually there,not just the D.C component.  Most likely there is garbage riding on the +5V rail making the digital components see it as logic level shifts.
>