I have a PROSCAN TV Model: PS27108 Chassis: CTC187BF2
The picture seems to grow when the TV is displaying a bright picture. It then shrinks when it is dim. I am assuming there is something wrong in the B+. Is this a known problem? Is the B+ actually fluctuating or is it too low? or High? I did replace most of the Electrolytics on the board with no changes. Any help would be appreciated.
Suspect problem with the tuner grounds.
Hi...
Wonder if it's not what we used to call in the olden days "blooming" ?
The higher the brightness the larger (and more out of focus) the picture becomes?
If so, this will be a B+ supply or regulation problem. (the higher the current the lower the voltage becomes)
So - if you're checking B+, watch it as you turn the brightness control up and down.
Take care.
Ken
This is usually due to 1 of two things: design flaw or poor voltage regulation. Has the set done this from new? if so, its probably the former. If it has begun to do this recently (how old is the set anyway?) you'll want to change the psu secondary caps espacially those on the b+ line (typically rated 160v or more). see if that brings an improvement.
-Ben
It is not just the B+, it is also the high-voltage. As the brightness increases, the CRT beam current increases. If the HV section cannot handle the higher current, the High-voltage decreases. This in turn reduces the velocity of the CRT beam current, and since the beam travels slower, the picture gets bigger, Hence the picture size appears to increase as the brightness increases. It can also be a problem that as the CRT tube ages and the phosphor efficiency decreases, more beam current is required to achieve a particular level of brightness, but the level of current is so high that the HV goes out of regulation and starts to bloom. So it can be also then not a B+ or HV problem, but just a CRT that is getting old.
H. R.(Bob) Hofmann
It could very possibly be a problem with the ground between the scan derived abl line and the signal side of the circuit board. This use to be a very common problem on the ctc177 higher end chassis when a certain ground would go bad in the tuner shield. Seems the T-chip (as Thomson called it), aka jungle IC, also uses the ABL feedback line to do some tweeking to the h-drive or pin drive signal to help improve the hv and scan regulation.
The Set is about 8 years old and has done this almost from day one.
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