Philco 50T-1401 Ion Trap & Low Contrast
 

Hello, all....I have been on the ARF's for a few years now and have enjoyed lots of useful radio feedback and info from the great guys over there ....however I've started my 1st TV resto, so I thought it might be most helpful to post here for this set's remaining issues.

After a complete re-cap, and contacts and controls cleaning I have a pretty decent picture with the exception of a general lack of contrast, some stubborn retrace lines, and an ion trap that I can't adjust for optimum brightness.

https://www.dropbox.com/sh/vsodavszt...030988.JPG?m=1
I combed thru all the video circuits looking for out of tolerance resistors and found a half a dozen or so and replaced them, but no change, so I am shelving the contrast issue for now as I'd like to check the associated tubes before proceeding.
(I am currently out of the country, and don't have my tube tester and other test equip with me....DMM only). As a matter of interest I went thru the Sams voltage checks and found only 20VDC on the cathode of the 12LP4....Sams shows 130VDC??? The Philco factory schematic shows no voltage measurement here.

Same drill for the retrace lines...I checked the DC resto area, and found nothing amiss, so further ideas here would be welcomed.

The ion trap magnet adjustment is a bit of a puzzle....the brightest screen occurs when the trap is pulled back aginst the CRT base, but at a cockeyed angle that won't stay put....the spring tension pulls the trap back straighter as soon as I let go of it!

https://www.dropbox.com/s/7ryx2cqn29ntwuw/P1030987.JPG

As you can see, it is still tilted a bit, but the picture actually gets brighter if I twist it a bit more....it just won't stay that way. Does this seem like a defect in the internal
crt element arrangement? I thought of removing the spring and super gluing the trap back on, but that's not a very confidence inspiring fix! Ideas here as well???

Thanks!
Mark

Is the spring stuck to the glass? It should slide readily to wherever needed.

The Sylvania 12LP4 requires a double ion trap. The OEM Philco CRT used a single ion trap, which seemed a little strange, because all tube manuals specify a dual ion trap.

OK, that makes sense...I also have a faint Ion burn......pic tube replaced, 2nd trap never added!

You wouldn't get ion burn from an incorrect trap magnet. The electrons and ions shoot out the side of the neck. The magnet bends the electrons back towards the screen.

If the entire viewing area is discolored, it's probably just from years of heavy use. If you have a circle burned in the middle, it's probably been rebuilt with a straight instead of bent gun.

If your set has an AGC control try adjusting it for better contrast. IIRC the optimal setting for AGC is to turn it up until the picture looses synch and is very contrasty, then back it off a bit.

Thanks, Bob....the burn is a faint thumbprint near the center.......actually hard to see much of the time........so If the tube was rebuilt with the incorrect gun then the burn is going to continue to get worse with continued use, yes? If I can find a proper dual trap, my thought is I may be able to turn the brightness down which should weaken the retrace lines as well.

No such luck, Tom, thanks for the info for my future reference.....boy I got a bunch to learn here;)

Have you tried flipping the Ion trap front to rear? You will probably have to rotate it 180 degrees as well but it might help but I think you need a double trap also.

The retrace lines are common on older sets and are usually more prominent when the contrast is low.
I would suggest that you may have some weak tubes in the video i.f. strip, if you have any extras try replacing them one by one and see if it improves.

Thanks, Eric...I will try flipping/rotating the trap. I probably have some 6AG5's in my tube stash at home, but I may order 4 of em anyway since I have checked the vid if section pretty well at this point and I'd like to see this set working well before going back home next month.

The ion trap I mentioned in the earlier post is a single unit double ion trap. It seems like it has two different magnetic strengths. You should see a picture of one on Bob Anderson's posts of the old Admiral chassis.

I flipped the trap and it looks pretty much the same except the brightest spot is now a bit forward of the tube base, 180 deg off. It looks to me like I need to find one of the double magnet traps. The retrace lines are a bit less in this position....

https://www.dropbox.com/s/3r717qe9hn...1030990.JPG?m=

The Admiral 20B1 combo that Bob is doing looks to have 2 separate traps. If I can find a small PM somewhere, I'll experiment with a second, even if it is taped on temporary while I locate another

An interesting thread on ARF on this very subject, from which I take an incorrect or misplaced magnet would indeed cause ion burn.:scratch2:

http://www.antiqueradios.com/forums/...hp?f=3&t=12606

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 ?