1948 Bakelite Admiral Question

[Unimatic1140]

Hi Everyone, about seven years ago I recapped a 1948 Admiral Bakelite television, the model with the 10BP4 picture tube. During my recapping process I found a rebuilt 10BP4 picture tube (which had been never used since re-building) and the set ran beautifully with an amazingly bright and clear picture.

I used the set a few times a year and its always worked perfectly. I powered the set up the other day and it was working great for a few minutes, I left the room for a minute and came back and the screen was dark, but sound was fine.

I pulled the chassis and measured the high voltage at the picture tube anode connector cap, its reading between 10,000 and 11,000 volts. I also measured the voltages coming off of the cap that connects to the neck of the picture tube and they were all close to what the Sams Photofact says they should be. What is odd is when I turn the set off I get a mildly bright dot or two near the center of the screen for about 60 seconds. This makes me think that maybe its not the picture tube that has gone bad.

I'm curious how you guys think I might troubleshoot this issue? What should I be checking?

Thanks in advance everyone!

[Phil Nelson]

If it has an ion trap magnet, have you tried adjusting that? It's on the neck of the picture tube and it can be rotated or slid forward and back.

You can't hurt the TV by experimenting. Note the initial position of the magnet (in case it's already in the right spot). Move it around until you find a "sweet spot" where the screen is brightest and you don't have any shadows on the edges.

There may be other causes of a dark screen, but that's an easy & obvious thing to check.

Phil Nelson

[Boobtubeman]

I usualy get the dot with the brightness turned down..

[bandersen]

If you're getting a dot then the picture tube is most likely good and the ion-trap magnet position is OK. It seems to me that you might not be getting a video signal through to the base cap. Can you get a scope on it ? Might be as simple as a bad video IF, detector or output tube.

[Electronic M]

Also possible that the bias voltages that the tube is receiving at the socket are causing the electron gun to be biased to a strong cutoff. I'd do a voltage check there, and if you can't get the readings close with the brightness and contrast controls then start checking the components that connect to the miss biased element.

[julianburke]

It is not your picture tube, as CRT's don't typically fail like that and it is not your ion trap (if so equipped) unless someone moved it. Try cleaning all of the frontal controls such as the brightness, contrast etc. You may also check for dirty tube sockets in the video circuit. I have an early Philco that did the same thing that was a dirty tube socket and an early Sparton that had a dirty brightness control..

[tvdude1]

Maybe cap under the 1B3 shorted

[wa2ise]

I had a similar problem with my Admiral, think I solved it by cleaning the CRT pins and the CRT tube socket. I say "think" as the problem didn't come back so far...

[jr_tech]

In addition to cleaning the CRT pins and socket, you might want to try re-soldering the CRT pins.
But be prepared for the worst...I had a similar problem in a 14'" DuMont portable that was caused by a bad weld to the cathode sleeve inside the CRT. The picture was very dim and smeared, in fact that I did not see it at all until I darkened the room. I tapped on the neck of the tube, and the image appeared at full brightness, but was quite intermittent. I tried to "spark weld" the connection, but it did not fix the problem.

jr

[Unimatic1140]

Thanks everyone for your suggestions, I will definately try cleaning the CRT tube pins and socket.

It's for sure not the Ion Trap, that was the first thing I tried.

I was under the impression that if something in the video circuit signal went out I would still see a white raster without any picture, but I guess that isn't the case.

I'll keep you guys posted to what I find. By the way how does one clean the front controls? I assume that they shouldn't be opened in any way.

[bandersen]

If the video goes out such that the signal to the CRT is down in the black or lower voltage region, your screen would be dark. Although, I would think you could still see a raster by cranking up the brightness and decreasing the contrast.

If your set is like my Admirals, there's a gap between the metal control housing and the phenolic wafer the pins are mounted on. You can squirt a little DeOxIt up in there and rotate the control to clean the contacts.

[Electronic M]

Also hit the terminals of the control with a tooth brush to remove any tin whiscars that might have grown.

[Tom Albrecht]

You might try subbing a different ion trap. Just because you get a dot when you shut the set off doesn't mean the ion trap is working properly. When the set is shut off, anode voltage may be down quite a bit, and the function of an ion trap is affected by the acceleration voltage of the electron gun. A field that will pass electrons at low anode voltage may not do the same at proper operating anode voltage.

Otherwise, try shorting cathode and grid together at the CRT socket. If you have proper screen voltage and anode voltage, and a working ion trap, this should give you a full brightness raster. If you don't get it, it would suggest something came disconnected in the CRT socket or gun. It's not unheard of to develop an open circuit in the electron gun.

[Unimatic1140]

Thanks everyone, Tom I like your test of "shorting cathode and grid together at the CRT socket". I just want to be sure I understand how to do this without creating sparks and explosions lol.

Here is an image of the schematic, is the cathode Pin 11 and if so which pin is the grid pin 2 or 10 or both 2 and 10?

If I understand you correctly I should put the cap on the picture tube, but not all the way down. Then take a jumper cable and short pin 11 to pin 2 or 10?

Thanks again for you help.

[old_coot88]

He means jumper pin 2 (control grid) to pin 11 (cathode).