Currently working on a philco 50-T1400. So far I have a raster on the screen, however there is a strange black circle that shows up when the brightness is high, when the brightness is low it seems to go away. Is this a high voltage issue?

First thing that I would suspect was that if it has an ion trap, it is misadjusted. If not that, possibly ion burn on phosphorous.

Cant play video but it may be a weak HV rectifier tube causing it
to bloom a little. Dont trust checker, sub in a new one.

73 Zeno

I’m not entirely sure where the ion trap is located on this crt, all I see is the focus coil and the yoke. I don’t see a place where one would go anyway. Maybe it’s inside of the focus coil assembly. That aside, I don’t believe it’s ion burn because it does change shape and size when I adjust the brightness.

It would have come with a 12LP4 that used and ion trap. The ion trap magnet would be between the base and the yoke. It's possible it was rebuilt with a straight gun and now had an ion burn. Or possibly the original CRT was replaced with an aluminzed 12" CRT? Is there are designator on the CRT?

As you have already correctly reasoned, this ain't ion burn, since that would show up the same with brightness set low or high.

Some possibilities I see:

1. As others have noted, if there is an ion trap, check to see if adjusting its position changes this phenomenon.
2. Check the series resistor between the high voltage rectifier and the CRT anode (R119 on SAMS schematic, 680 K). If it has gone up a lot in value (not uncommon), it can cause high voltage sag when brightness is high, and this can upset the CRT electron optics (usually seen as focus change, or raster growing with increased brightness, which I might be seeing in your video)
3. Weak HV rectifier tube, as others have noted. Effect similar to above.
4. Tired CRT, with center of cathode getting weak, and more emission from outer ring area of cathode at higher cathode current, which might cause something like this.

All that being said, see how it looks with a real picture on the screen. You may not have to turn the brightness so high to get a normal picture, in which case, the brightness control has excess range on the top end, which could result from various causes, including original design.

You probably know this, but Incase you don't:

Sets of this era weren't designed to be watched in well lit rooms. Usually they were used at night with a 25-40w Incandescent TV lamp on top that fired it's light onto the wall behind the TV and gave just enough light that you could avoid tripping over the coffee table and stepping on the cat on the way to the fridge...You can usually get a bit more out of a set and run it in a less dark room but they weren't meant to be used in sunlight or retail lighting levels, and if you push them to do that the results often are sub-par at best.

That was mainly a problem with pre-war TV's which had low screen energy due to low range EHT voltages.

In the post war period the EHT was raised significantly and the RCA 621TS as one example with a 7DP4, produced a very bright high contrast image in a normally lit room. I found this out when I installed a NOS tube in my set. So did the 12LP4 in sets that used it, when it was new that is. We might get the wrong idea inspecting them with used tubes.

It went a step better with the 12KP4 which is an aluminized version of the 12LP4, and even more light comes out of the tube face rather than going back into the CRT. With non-aluminized CRT's the raster scan always appears brighter looking at the rear of the screen (when you can see it through the side of the bulb). This is because the thickness of the phosphor limits some of the light coming out of the face. There is an ideal phosphor thickness for max light output. Oddly Dumont did not appear to subscribe to this or the same P4 mix as RCA and often the layer is thicker and the light color on the average more sepia tone than RCA tubes.

It is an upgrade to any 12" set with a 12LP4 to change to a 12KP4 for this reason. Knowing this I bought a large number of NOS 12KP4's some years ago, just for this application. They were RCA's last production run and I imported them from a seller Switzerland. The shipping cost a small fortune. I also have a 12LP4, which was re-built into a 12KP4, complete with a new gun and new screen phosphor. That was in the early 1990's when Thomas Electronics were still providing the service in Australia.

For more info on mysterious dark spots on CRT faces, read the below Radio Electronics TV Service Clinic article beginning on page 64. Especially the section on "Adjusting the ion-trap magnet". It's possibly from vaporized electron gun aperture material having been re-deposited on the back side of the phosphor. The permanent damage is caused by an improperly adjusted ion trap(beam bender).

https://worldradiohistory.com/Archive-R ... 955-04.pdf

Could be a computer game spot.

Dave

I agree with the post of the brightness being too high and the possibility of a weak high voltage rectifier tube.

I agree with possibility of weak HV rectifier tube. Also notice it doesn't have full sweep till the blooming starts, points to insufficient HV. Could have weak horiz output/damper tube, or other issue horiz in output circuit.

Well, it could be. It would require that Marty McFly remembered to take one of his Atari video games back to the 1950's. Most sets that used CRT's like the 12LP4 were long defunct before video games arrived in the late 1970's and early 1980's. The incentive to restore these old sets back to working order probably only came 10 to 30 years after that.

The Magnavox Odyssey came out in 1972 that Philco would be 22 years old then and could have still been in service. Sam's used to run an ad where they showed a an RCA 630 or 8TS30 where they said something to the effect of "It's old enough to vote, the owner wants it repaired and you have to fix it. Where do you go for service info? Sam's photofacts..." Those split chassis Philcos can be long lived. I know a collector that has one that TO THIS DAY is still running perfect on its original caps and only has seen some tube changes since new.

It's much, much more likely this damage is from a misadjusted ion trap burning the gun material away onto the screen, but it's still perfectly possible that this set was used for videogames.

Try adjusting the ion trap, focus coil (or magnet), and the deflection yoke. The ultimate test would be to try it on a test CRT, such as a 5AXP4.

Since the problem only occurs at high brightness setting, don't you guys agree that screen damage is completely ruled out? I see this idea coming back again and again...

Well the symptom looks very similar to this 8AP4 I own. There is zero indication on the unlit CRT face. Dark area shows when there's a bright background in the raster. Was typically rumored to be ion burn, before we knew better. I have a 19AP4 with a similar dark area. Also a 12LP4.

I played the video.

What I am seeing is "blooming" - that's the effect you get when the HV drops under beam current load. (I can't see any sign of screen burn).

The width appears to be down too and the picture centering is way off.

I'd check the cause of the low width first. Turn the brightness down until the screen is barely lit and see what width you get.

That does look like a typical ion burn. Though perhaps larger than one might expect, unless your raster is significantly over-scanned. Your spot looks about 20% of the screen size, larger than one would expect.

The negative ions emitted from the cathode are deflected less than the electrons because of their mass, their velocity is lower than the electrons,so this reduces the relative ion current, which has a smaller associated magnetic field, to interact with the field from the yoke.The electrons with the higher velocity, produce a higher current, hence a higher magnetic field to interact with the deflecting field produced by the Yoke.

(It is interesting, that in electrostatic deflection, the ions are deflected the same amount as the electrons, because they are deflected on the basis of the charge they carry, not the magnetic field of the ion or electron stream. So the only reason there is no ion spot visible, is that the ion contamination of the phosphor, which still happens, is spread everywhere the scanning raster is. But, the phosphor is still degraded with this over time, it is just more uniform).

Two ways to tell if the spot is an ion contamination spot:

Typically it is about 5% of the screen size, or a little more if the raster was significantly over scanned.

Also, if you rotate the yoke, the phosphor de-sensitization, stays in the same place on the tube face.

Once tubes were aluminised, the ion spot issue largely went away and ion trap magnets were abandoned. The ion trap magnets were not 100% effective at removing all the ions in the beam. Likewise, aluminisation of the phosphor was not 100% effective either, I have some aluminised tubes that show some evidence of ion contamination to the phosphor.

With color TV sometimes you had to Degaus aka (decreasing or eliminating a remnant magnetic field)

It simple and impressive for home owner too.

The Olds the TV tech would it just bad spot want new tube.

Dave