I have been working on one of these for a client, with the exact same symptoms and readings, and may have your solution if you are still trying to solve it.
The switchmode power supply for this unit is supposed to put out +5V, +15V and -15V. (Look for the label on the side of the big can capacitor near the input power connector.) The readings you indicated were +12V. This is too low.
The processor board has a circuit that compared the input voltage to the output of the 12V regulator. If the input voltage is low, it holds the CPU in a reset state, so there is no activity.
The problem I found on the one I have is that the filter caps in the switchmode power supply are going bad and not filtering the output voltage properly. I would recommend replacing all of them. The unit was probably built during the "capacitor plague" era. (Google it!) If you look closely at the tops of the capacitors, you may see that one or more is bulging slightly.
These capacitors can be ordered from Mouser Electronics (
www.mouser.com)
You will need:
Quan Mouser# MFR PN# Mfr name Description
2 - 661-EKYB160E102MJ16S EKYB160ELL102MJ16S United Chemi-Con (UCC) Aluminum Electrolytic Capacitors - Leaded 16V 1000uF 20% Tol.
1 - 661-EKZN100E222MJ25S EKZN100ELL222MJ25S United Chemi-Con (UCC) Aluminum Electrolytic Capacitors - Leaded 10V 2200uF 20% Tol.
2 - 661-EKYB250E471MJC5S EKYB250ELL471MJC5S United Chemi-Con (UCC) Aluminum Electrolytic Capacitors - Leaded 25V 470uF 20% Tol.
1 - 661-EKYB350E331MJC5S EKYB350ELL331MJC5S United Chemi-Con (UCC) Aluminum Electrolytic Capacitors - Leaded 35V 330uF 20% Tol.
1 - 647-UPM1H470MED1TA UPM1H470MED1TA Nichicon Aluminum Electrolytic Capacitors - Leaded 47UF 50V 105c
1 - 647-URZ2G680MRD URZ2G680MRD Nichicon Aluminum Electrolytic Capacitors - Leaded 400volts 68uF 20x25 20% 10LS
Carefully unsolder the leads from each capacitor using a 30 - 40 watt iron. Use a vacuum bulb to slurp up the excess solder, then use a knife or tool to straighten the leads and pull the cap out. Note the direction of the stripe on the side. The new cap will have to go back in the same way. Solder the leads to look like the original, then clip the excess lead. Be careful not to burn the board. Remove and replace one cap at a time. Mark the tops of the old ones so you can tell which ones to remove.
When you are done, the power supply should again be putting out + and - 15V on the end pins, and there is a very good chance the display will work. If it is not connected to its sensors, it will display "SIG FAIL" on both windows.