#elekton

Probing the circuit I found that there were irregular voltages coming from the wiper side of the contrast adjustment pot.  The proper voltage  coming from that pot looked to be about -4.9V for a good contrast balance.  Whenever the screen would freak out the voltage could vary as much as +/-1V relative to the proper amount.

What I did is not a fix for the underlying problem but a workaround.  The contrast adjustment pot gets -10V on one side and +5V on the other.  Proper contrast seems to be ~4.9V or thereabouts.  So what I did to keep away the voltage fluctuations is to install a -5V regulator (7905) in place of the contrast voltage divider.

The input of the regulator comes from the -10V supply, and the output goes straight to R805.  I changed the value of the 1K R805 to about 1.3K and that gives a good contrast balance.  The two 1uf capacitors are on the input/output lines.