Contrast

[VA561]

I have a 1950 philco tv , model # 50T-1404. The tv has been recapped and all weak or shorted tubes replaced.The problem is this: the brightness is pretty good on the tv but with the contrast turned all the way up I still get a somewhat washed out picture. I try to get a balance between the brightness and contrast controls but there just isnt enough contrast. what causes the lack of contrast? Is it the picture tube or some other area. Any input will be appreciated.

[miniman82]

Drifted resistor in the video amp section? That's where the bright/cont controls are usually wired in.

[DaveWM]

crt checked for cut off ok?

AGC ok? (if it has it)

I alway start by injecting a composite signal at the grid of the video amp tube.

This would bypass all the AGC/Tuner/IF and see if the problem is before or after the video detector.

[VA561]

I also wanted to mention that the crt has a picture tube brightener attached to it. Would the fact that the picture tube may be weak also play a part in the lack of contrast?

[DaveWM]

I would guess so, but to trouble shoot, I would still recommend the composite video at the grid fo the video amp tube. Just make sure there is no voltage before hooking up to the dvd. I couple thru a big cap (.1 should do) to avoid any DC just in case there is any.

[DaveWM]

I have had some pretty bad testing CRTs (b&w) that still make an ok pic, not great but ok. by passing the tuner/IF allows you to quickly know where you are, plus is a good way to see how good your tuner/IF is. I compare the picture (RF at tuner to composite at grid) to determine how well the alignment is setup.

[ChrisW6ATV]

Originally Posted by VA561

I also wanted to mention that the crt has a picture tube brightener attached to it. Would the fact that the picture tube may be weak also play a part in the lack of contrast?

Lack of contrast is exactly what a weak CRT has. Here is a simple test: Make the room with the TV pretty dark, then turn the contrast and brightness down low. Do you have good focus, can you see the horizontal scan lines easily when you look at the CRT close up, particularly on the darker parts of the picture? Adjust the focus if necessary and possible. Now, turn up the contrast medium to high and look at the bright areas. If the brighter areas of the picture are a lot less in focus now than they were with a dim picture, then the CRT is weak indeed. If they stay sharp but you just cannot get enough overall brightness/contrast, it is a different problem.

Also, try the test with and without the brightener. It will most likely fail the test without it, but that will give you a comparison.

[VA561]

Thanks for all of the advice. I am trying my best to learn about my new hobby.
So much too learn!

[DaveWM]

great fun learning eh! I have been having a ball learning new stuff.