panasonic UG2 fluctuates between too hi / too low

Update - on old Panasonic (TX-28ps1)... ... on first switching on ... picture shows very bright

-sometimes almost white... if it gets too white it seems to cause the TV to switch off into standby... if not then after 10 minutes it settles down to a more normal (perhaps slightly dark, overly contrasted?) picture... but eventually end up too dark

From the service mode1 screen...

The one value that looks most odd is UG2 test

Ug2 Test shows "Ug2 255 high" (rather than manual indicative value off "Ug2 130 O.K.")

service manual says Select Ug2 adjustment and adjust the screen VR until the display shows "O.K."

This I have done ... (though the VR adjuster screw was very sensistive)... but everytime I power it TV on ... UG2 starts of too hi then fluctuates but usually ends up too low...

any ideas on how to fix ... thanks AJ

Hi AJ,

it sounds like the SCREEN adjust potentiometer is defective from what you say it is 'sensitive'. I've come across this problem in the past both on PC monitors and televisions. Usually the SCREEN adjust pot is located on the LOPT or high voltage flyback transformer adjacent to the FOCUS pot. I've had some success by pushing the pot inwards when the optimum adjust point is determined and quick-glueing it in place as a quick-fix. I don't think switch cleaner will resolve the problem or is advisable in this location.

The real cure is a new LOPT if the SCREEN VR is located thereon. Of course I have had defective CRTs that give a similar fault description but your mention of the sensitivity of the potentiometer would lead me to believe that this is where the trouble lies.

Regards PFJ