Philco 50T-1401 Ion Trap & Low Contrast - Page 2 - Videokarma.org TV - Video

bonanzaman · #16 03-13-2014, 05:14 PM

Quote:

Originally Posted by bandersen

I concluded the opposite from reading that. Perhaps a really strong magnet could bend some of the ions towards the screen, but the set wouldn't be watchable so why would leave it running like that ?

Fair enough, so, I guess my question is, then, what caused the ion burn in the 1st place? I don't mind the spot that is there now, as it is pretty faint, but I sure don't want it to get worse just by operating the set!

bandersen · #17 03-13-2014, 05:21 PM

My best guess is that every time the set was turned off the raster collapsed to a bright spot in the same area. Also maybe a manufacturing or rebuild defect.

bonanzaman · #18 03-13-2014, 06:26 PM

Quote:

Originally Posted by bandersen

My best guess is that every time the set was turned off the raster collapsed to a bright spot in the same area. Also maybe a manufacturing or rebuild defect.

Bob, I think you guessed it! On shut off, the raster first snaps to a horozontal line, which then collapses to a bright point.......(I've observed this on many old sets; filter caps in the power supply draining off?), and danged if it is not in the same neighborhood as that smudge!

So how about a piggybacked pre-switch of sorts to cut the filament to the horozontal OP tube, before powering down the whole set? Or perhaps switching the hi-voltage off somehow before turning the power switch? With good 12LP4's not extinct, but not exactly growing on trees, I'd like to preserve this one as long as possible!

bonanzaman · #19 03-15-2014, 12:42 PM

Came up with a waaaay better solution......just turn the brightness control all the way down before shutting the set off......no bright spot! duh!

wa2ise · #20 03-15-2014, 02:08 PM

Quote:

Originally Posted by bonanzaman

.just turn the brightness control all the way down before shutting the set off......no bright spot! duh!

You can do a similar thing automatically by connecting an electroytic cap between B+ and the brightness line, if the brightness changes the voltage on the G1 of the CRT. When you turn the set off, the charge in the cap will push the brightness negative as the B+ supply collapses, thus pushing it negative, cutting off the emissions of the CRT cathode. Thus no or greatly reduced spot.

bonanzaman · #21 03-15-2014, 02:28 PM

Quote:

Originally Posted by wa2ise

You can do a similar thing automatically by connecting an electroytic cap between B+ and the brightness line, if the brightness changes the voltage on the G1 of the CRT. When you turn the set off, the charge in the cap will push the brightness negative as the B+ supply collapses, thus pushing it negative, cutting off the emissions of the CRT cathode. Thus no or greatly reduced spot.

That would be a great to add feature for the set; looking at the schematic, the center wiper of the brightness control goes directly through a 100k resistor to
the cathode of the 12LP4, so not sure if that would work, however.

Tubejunke · #22 03-16-2014, 09:02 PM

I never really understood the whole double ion trap "beam bender" magnet thing. I mean any tube guide shows that application on 10BP4s, but I have one with only a single trap and have seen others. I would have to take a look at my 12LP4 in a '50 RCA to see what type it has.

Back to the topic: That positioning you mention of having your trap all the way back against the tube plug is something that I have seen more than once. Either way you look at it, it's wrong. The last set that I can remember specifically the conditions that were at hand was a mid/later 50s Magnavox 21". The set worked fine till the flyback melted. I did some repair attempts at piecing together the fly as opposed to troubleshooting and repairing what was causing the fly to heat up. Long time ago.

Anyway, at some point it seemed that the set might work and was producing some level of high voltage. I STILL need a H.V. probe BADLY! LOL! The voltage was low, but for some reason of I pulled that ion trap all the way back to the plastic plug at the very end of the glass I produced a raster. Not specifically bright, but a raster.

Years later after the Internet came into being a household fixture, I was able to locate a flyback which probably never would have happened without the WWW. Put that in and changed a few caps and I had closer to correct high voltage. Again, no probe but you can sort of tell. Long story short, at that point I had to move the trap back to its correct position about .5 inches in toward the yoke.

bonanzaman · #23 03-17-2014, 11:58 AM

The Sylvania data sheet for the 12LP4 does not give a position for the ion trap magnet, but the GE & Tung-Sol sheets do:

https://www.dropbox.com/s/507h6hokl4...2007.18.07.png

I'd really like to find the correct double magnet unit, but so far, no luck. Looks like the magnet should be approx 2" forward of the base key end....mine is at about 1-3/4" and is the best it gets. The search continues.....

Tubejunke · #24 03-17-2014, 09:01 PM

Wow, I've never noticed a given position for a trap. I have seen approximate placement pictorials in old "how to" books and school textbooks. Really all you have to do is turn the brightness down a bit, I think it may be halfway, but I'm not positive. Really it's no matter, but when you turn that magnet around that tube neck in various spots while moving position between the yoke and the tube plug you will find your raster as there will be none with the magnet grossly mispositioned.

Once you find the raster, then you fine tune for maximum brightness while at the same time micro-adjusting for corner shadows. The proper position will be somewhere in between the yoke and tube plug; not against either.