To measure voltages on the I/O board with the back panel removed, you will need to bypass the back panel safety interlock switch.
Whenever the power cord is plugged into the Lisa (and into a power outlet), the linear supply powers the COP421. If your Lisa doesn't do anything when you turn it on, test the voltage on this chip.
The COP421 is CMOS and especially sensitive to ESD and brief short circuits. BE CAREFUL NOT TO SHORT THIS CHIP Due to it's fragile nature, you may want to investigate other causes before probing the COP421.
To avoid accidentally shorting adjacent pins on the COP421 while probing, disconnect the power cord (and turn off the I/O board battery if present), wait a few minutes for any stored power to discharge, secure your voltmeter leads, then connect the power cord to make your reading.
Measure the voltage on the COP421 chip (U9F on the I/O board). Pin 9 should read approximately 5V when the standby power is operating (Lisa plugged in and interlock switches engaged).
If pin 9 of the COP421 shows about 5V, and you still cannot turn on your Lisa, monitor pin 12 of U7F while you depress the power switch. If this signal changes state but the Lisa does not turn on, the switching power supply circuitry may be bad, or if you get no activity at all when you press the power switch, the COP421 or U7F may be bad.
The chassis is connected to digital ground inside the power supply, but there are a couple of problems with using it to measure voltages accurately, most notably that it is not necessarily at exactly the same ground potential as the circuitry.
To make precise measurements, pick a ground on the circuit board itself. One near the source of power (the card edge) would be the best choice, as farther away may read at a slightly different potential.
A suitable location is the negative side of capacitor C47 or C50 on the I/O board.
You can access almost all of the voltages via the I/O board, which is relatively safe to do (no need to be near the high voltage in the power supply or video circuitry).
In particular, on the I/O board you can find:
From the I/O board, you cannot access the +33 volts used by the analog video card, or the high voltages generated by it, but you can usually tell if the video card is working by the presence of something on the screen (even if it is random).
If you have an internal hard drive that is not working properly, checking the voltages at the hard drive would be appropriate as it may see lower voltages compared to those appearing at the I/O board. (Due to the higher current drawn by the hard drive, and the different wiring path.)
Source James MacPhail / LisaList 2006.04.25
If you have a Lisa 2/10, there is a power connector for the Widget hard disk in the drive cage area, which is a convenient place to check the +5, +12, and - 12 voltages:
|Green/Yellow||Power Good (open collector)|
Suggested by: Jason Perkins / LisaList 2006.04.25[LisaFAQ] > [Hardware] > [Power Supply and Power issues] (Comment on this answer)