to AP !
Common problem on the often problematic ST TNG. Before responding with a possible technical fix, you should notify the seller, as most reputable sellers should offer a shop warranty. A "fully working" machine should not have these issues.
The problem will most likely be on the driver board (Q15 or Q16). You should also tie the 50 V line with an additional wire as per Clay's Guide (3/4 of the way down this page);
http://www.pinrepair.com/wpc/index3.htm#oddYou can use a 1/2 W resistor in the place of the 1/4 W resistor. You cannot substitute a 1/2 W with a 1/4 Watt.
