metal tube sheilds

[timmy]

hi all,just how important is the metal tube sheilds that go over a tube. reason i ask is my ctc15 silvertone has had an on and off problem with colors turning into blotches after warm up so i narrowed it down to the chroma ref ocillator tube that has a shield and when the blotches start they stay until cooled down. this time the blotches were there and warmed up and i took the shield off the colors were fine then put it back on or just move the sheild near the tube and the color blotches.so i left it off and so far its ok, anyone ever heard of this befor. the socket is fine as i jiggle the tube and nothing happens but like a magnet just putting the sheild near the tube distorts the color.

[Gunslinger]

you need them. If you don't have them, make them out of sheetmetal and solder them at the seams.

[bob91343]

Make sure the shield is grounded. They cost money so the factory wouldn't use them without reason.

However, many of these old TV tube circuits were, to be kind, not optimum. So if what you do makes it better, do it.

[timmy]

i just dont understand why the color goes to blotches with it on or near it, so i guess ill leave it off. i had made sure the pin and contact area of the sheild was clean but no good.

[bandersen]

Maybe that shield got magnetized. Have you tried using a different one ?

[jeyurkon]

It's probably out of adjustment. The tube shield adds stray capacitance which shifts it off frequency.

If it works without the shield I'd leave it that way for now.

There's usually a reactance controller stage before the osc. That tube may be weakening, a component changing, or it might have been adjusted without the shield in place.

-- Sent from my Palm Pre using Forums

[timmy]

well i dont have another shield to try and the set only has 2 and i know there should be more. the sams folder dont say which tube gets a shield so i guess its the little pin at the base of the tube socket that indicates which gets a shield.and there is a reactance coil next to the tube but i have no clue how they are adjusted.

[dieseljeep]

There should be a test point near the tube in question. The proceedure should be in the Sams. You ground out that terminal and adjust the reactance coil for the widest stationary color bars. Remove the ground and see how the color syncs in.

[timmy]

ok well if for some reason it acts up again then ill make that adjustment. eventually the chassis has to come back out for 2 caps to be replaced but its not an emergency at this time. ill gather up a few things first befor pulling the chassis out. thanks for that info.

[ctc17]

jeyurkon is exactly right. My experience is the crystal is bad when you get the warm up color problem. The crystal has drifted and is drifting with temp. Proper repair would be replace the crystal and then do a color afc alignment. Easy fix is tweak the osc coil with the shield on the tube.
One test to verify this is after the color goes funky, tweak the horizontal hold a little each way, just before it rolls and see if the color pops back in.

The crystal will slowly get worse.

[timmy]

well what i found is if i turn the horiz hold to one side just befor the picture distorts i get some blotches in the color and move it back its fine. would you be talking about the 3.58 crystal under the chassis? where would i find that crystal?

[ctc17]

Well wait till it blotches then see if you can bring it back with the horizontal hold. Yes Im taking about the 3.58 crystal and you can get them at most any electronics store. Did you try changing the tube?

[timmy]

yes i tried the tubes that have to do with that circuit and no change. the horiz hold adj will not bring the color back once it blotches, i can get the color to blotch by turning the horiz hold to one side just befor the picture distorts. so far with the shield off the tube its been fine. i have a verticle issue as well when the picture changes scenes it triggers a vert roll and i have retrace lines that intensify when it rolls but warmed up it just about stops rolling. i tried 2 new vert output tubes but same thing, im wondering if another tube may need a shield and is causing this vert problem. other then these issues this set works very good and bright colors,clear.

[ctc17]

I just picked this up last week, is this kinda what you have?
http://videokarma.org/showthread.php?t=251290

The roll could be due to poor high voltage regulation. Sounds like you have the crt bias and screens out of adjustment. Follow the setup procedure in the sams for crt setup. I will get the sams for the 15 next month and should be able to help you with it.

[timmy]

well that could be something like that as the high voltage adjust does nothing, no affect when turned but i do have 24kv and the 6bk4 tube is new. bias adjustment? the screens, meaning red,green, blue, they are done. i have a pic of my set in the silvertone, page 3.the only thing the same in your set and mine is the round crt. if your talking about the agc, its set and the blue and green also set as well a the other 3 colors just what the sams said to do.its like when the picture changes you can see a slight shrink between changes in the left and right of the screen depending on what is being shown. but the hi voltage adj pot has no affect at all and i checked everything, well i thought i did otherwise it would work. the original problem was i could not get more then 14kv at the crt and i found with the help of the sams folder that someone had jumped the circuit to check milli amps and never took the jump off so i did and the hv went up to just shy of 24kv but the pot still dont work. i dont want to try the old 6bk4 because its old and dark in color which tells me it is old unless i got a bad new 6bk4.