Your set probably uses an STR regulator . Lets use an STR 30130
as example. The 130 means it is a 130 volt regulator. ( the also come
in 30123, 30125, 30135. etc).
Near the STR there is a BIG resistor. One side is apx 150 VDC the other is 130 VDC. That voltage should stay the sam from no raster to full brightness.
Now if you have a Variac hang you meter on the 130 V end of resistor
& run the set from 100 VAC to 135 VAC It should be a constant 130 V.
If either tests fail you MAY have a power supply problem.
Nature of the beast is also possable. Low price sets have simpler designs,
are a generation or 2 behind, & overall quality is lower than the
higher end sets. Thats why some sets can get NTSC stations on
rabbit ears up to an easy 150 miles & others wont see it at all.
73 Zeno
LFOD !