Philco 49-1240 horizontal (?) problem

[old_coot88]

..And another engaging, informative, and highly enjoyable resto saga by Phil.
Lovely little set.

[compucat]

It is hard to imagine a 1949 TV can perform that well. Of course, the only 1949 TV I have is a Motorola 9VT1 electrostatic set. It is a good set but not on the same design level as Philco or perhaps RCA. I can't help but think that we are seeing these sets as performing better than new considering the cleaner signal sources from DVD players and converter boxes. I could be perfectly satisfied with a properly restored vintage set over a flat screen any day, even a small screen black and white.

[Phil Nelson]

Weird. After playing happily for days, this Philco suddenly went dark. I could barely see the image with Contrast & Brightness turned up full.

I brought the chassis into the workshop and connected my 5AXP4 test CRT. Picture normal, plenty of brightness, everything works fine.

I checked the 12LP4 CRT on my Sencore CR70 tester and it shows almost no emission. It originally tested very strong. The shorts tests don't show any problems. Resoldering the pins didn't change anything, nor did tapping the tube neck.

It seems odd that a tube would just fall off the cliff like that. The CR70 has rejuve/restore functions, but I'm not sure if it's a candidate for those . . . .

Phil Nelson

[miniman82]

If the tester isn't finding the short, run the tube with the chassis and note the voltages on all the elements (K, G1, G2, ect). You might find that the grid voltage is significantly higher than it should be, or the cathode is very low. In either case, this would drive the tube out of conduction leading to the low brightness situation. I suspect an open has developed in one of the elements of the gun, the voltage measurements will confirm which one. If you find an open, some testers have a 'weld' function which can sometimes repair a fault like this.

[Boobtubeman]

isnt there a 12lp4 on the bay?

[vts1134]

Originally Posted by Boobtubeman

isnt there a 12lp4 on the bay?

Yea Harry is selling it, item number 330678906574. Just in case you need to go that route.

[old_coot88]

Phil,
Have you tried judiciously rapping on the CRT neck (with a screwdriver handle or some such)? Often this will show up a short or open element.

[Phil Nelson]

Originally Posted by miniman82

run the tube with the chassis and note the voltages on all the elements (K, G1, G2, ect).

I tested the element voltages with the old 12LP4 CRT in the cabinet and with my test 5AXP4 CRT on the workbench.

The voltages are basically the same on either CRT for G1 (pin 2) and G2 (pin 10).

On the test CRT, the cathode (pin 11) voltage varies from ~25vdc - ~140vdc depending on the Brightness control setting, with about 40vdc giving a normal picture.

On the old CRT, the cathode voltage is zero under all settings.

Rapping the tube neck doesn't change anything.

The Sencore CR70 manual says that some tubes with open cathode connections can be welded with the "Remove G1 Short" function. Basically, you try this function while rapping on the tube and watching for a bright flash.

Phil Nelson

[miniman82]

Found your cathode open, then. Give the cathode a weld, see if it will work. I think it was banderson that got it to work once, maybe he'll post about it?

[bandersen]

Yes, I was able to re-weld the cathode in a 16KP4 CRT thanks to jeyurkon's help. Here's the discussion thread: http://www.videokarma.org/showthread...7075&page=2#30
Good luck!

[compucat]

So sorry to hear about the CRT failing, especially with such a great picture on it before. One consolation is that you have a perfectly restored set that is well worthy of a new picture tube. I hope you can get a replacement at a reasonable price. I would definitely try the welding technique.

[Phil Nelson]

Well, I was skipping around for a while, anyhow. I tried the welding process, saw several flashes, and after about 15 tries (checking the emission after each attempt), the weld finally worked. The emission bounced back up and the CRT looked good on the life test.

I left the CRT on the tester for about half an hour, curious whether the fix would hold up. When it seemed OK, I reinstalled the chassis and powered it up.



Yay! Wahoo! Looked great for about five minutes . . . until it faded again



I guess the weld wasn't strong enough to last. Perhaps I'll try again tomorrow.

Phil Nelson

[miniman82]

Keep at it, maybe you'll get lucky and one will be good enough? If not, it's not an impossible to find tube...

[Phil Nelson]

I was able to weld it a couple more times, but the repair didn't hold. I even laid the CRT on its face, in hopes that gravity might help. I guess the gap in the cathode is too big to bridge this way.

Time to shop for another 12LP4!

Phil Nelson

[Eric H]

Originally Posted by Phil Nelson

I was able to weld it a couple more times, but the repair didn't hold. I even laid the CRT on its face, in hopes that gravity might help. I guess the gap in the cathode is too big to bridge this way.

Time to shop for another 12LP4!

Phil Nelson

Well that really sucks, especially when the tube is so good otherwise.

I have a 16GP4 like that, open connection on G1, I can bang it around and it works great for a while but it's not going to hold.