There is a capacitor on the mother board that can cause this problem. The capacitor is designated C2 and the long and short of it is that if your display driver assembly is covered in epoxy the you use a 470 pf cap and if it is covered with a plastic cover you use a 0.01 uf cap. Check that you have the right cap and that it is good.
The following is from the service manual.
There are differences in appearance as well as in the automatic
(10 minute) time-out featUre. This is primarily due to a change 1n ~
capacitor of the logic board from .01 uf to 470 pf .
Revision "G" uses 470 pf/D160-057l
Revision "F" uses .01 uf/0160-3914
The difference in appearance can be seen by looking at the covering
over the display hybrids.
Revision "G" is covered by epoxy .
Revision "Fit is covered by a plastic cap .
If the value of capacitor is interchanged with revi sion of the display driver assembly the following symptoms occur:
1. Revision "Gil with 470 pf
Frequency = 8.3 KHZ
Timeout = 9 mi nutes
Display = OK
2. Revision "G" with . 01 uf
Frequency = 435 KHZ
Timeout - , •
Display = fli ckering
3. Revision "F" with . 01 uf
Frequency = 6.3 KHZ
Timeout - 12 minutes
Di spl ay = OK
4. Revision "F" with 470 pf
Frequency - 110 KHZ
Timeout - 40 seconds
Display - OK
