Zenith pict comes on bright then goes dark
 

Hi
I was wondering if any of you have had any experience with Zenith
H0958x or H09c59x 9" TVs. These TVs were also modified by a company
called PDI for hospital TV usage (PDIZ9TV-B)but doesnt seem to be a
problem in the modifications, I could be wrong though.

When first powered on the TV will come on full brightness then quickly
fade to a very dim picture. The audio is ok as is the on screen
displays but as I said the video get very dark, almost completly black
on some. And on some of the TVs the brightness will return to normal
after being on for about 3 or 4 min while others dont seem to ever come
back up. But turning off tv then back on will continue to give the
normal brightness good pict then fade away results until TV has "warmed
up"

Also the voltage on the high voltage anode is still present. It seems
to be something in the video chip and associated components since the
on screen display and menu appear normally but I havent had any luck
tracng the problem down.

I have recently inherited about 7 or 8 of those TVs so any help would
be apreciated

Do those have the microprocessor PDI board in it? If so they have to
be unlocked to allow video display via some computer code coming in
through the cable.

These sets are only able to be activated by infared remotes and dont
have the over the cable remote activation features. Once activated only
chan vol and cc funtions can be accessed.
Also I have a number of the same model purchased at the same time as
the others that are working fine.
And these TVs worked fine for a couple years until they developed the
problems
Now I am not ruling out a chip problem since the problem will either go
away after being on for a few min or do the opposite and fade away
after a few min. But the most common problem seems to be the come on
for a second full brightness then dim down to a either very dark
picture or completly black.

I contacted PDI and they are pointing me to a capacitor but after
changing it out on 1 of the TVs (only had 1 cap lol) the problem didnt
change.

Oh well back to proding and tracing to try to track it down with my
poor schematic lol

Thank you very much for your responce, I appreciate any thoughts.