A thin white line across the raster Admiral TV 1948 model 17T12

[recepteur]

Good evening gents,

I’m seeking your technical advices on an Admiral TV 1948 model 17T12
Symptoms: A thin white line across the raster half way down the screen regardless if I adjust vertical height and horizontal width, the thin white line remain. See attached pictures.
Known facts:
• Picture tube: 7QP4
• No alignment has been conducted yet.
• Vertical and horizontal sweep and synchronization are good.
• Reception on channel 3 and 4 using a DVD player is good, picture display well and the sound is loud and clear.
Actions taken to date:
• All the capacitors have been replaced with the exact values and voltage rating.
• Few resistors were out of tolerance and were replaced accordingly.
Any inputs and suggestions would be most welcome.
Your help in this matter would be greatly appreciated.
Best regards
Daniel

[nasadowsk]

Does the line move with centering adjustments? Possibly in a past life the set sat with no vertical sweep for a while?

[recepteur]

Hello,

If you move the display on the picture tube, up or down, left or right using the centering, height and width adjustements, the thin white line remain at the centre of the display. Cheers

[rca21d645]

That could be the result of someone leaving the set on with a collapsed vertical for an extended period, causing burn in, in which case there's not really anything you can do to fix it.

[recepteur]

Thanks to both nasadowsk and rca21d645,

Perhaps I did'nt explain the symptom properly. Even If I shrink the display or the raster in this case, move it up or down, the thin white line appears half way in the raster. I was thinking more about vertical/horizontal sweep issues. Cheers

[Phil Nelson]

Are you sure about the CRT type? I thought that Admiral 7" TVs of that vintage used a 7JP4 tube, not 7QP4. But there are plenty of things that I don't know

Do you mean that the thin white line stays at exactly the same place, even if you move the raster up or down in available screen area?

I thought that the classic "ion burn" mentioned by other responders made a darkened shape in the screen: either a horizontal line (created by failure of vertical deflection) or a central dark spot (created by a mis-adjusted ion trap magnet). But the line in the OP's photo is lighter than the rest of the CRT face, not darker . . . .

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

[old_tv_nut]

Phosphor burn would be dark.

I know nothing about this set and was hoping to learn, but since no definite answer yet, I'm willing to display my ignorance with some questions.

This is an electrostatic deflection tube, correct? And the centering is a pot, not a magnet on the neck? If so, could this effect be caused by one of the vertical deflection plates being disconnected and having a floating voltage?

When you have a picture, is there a fold or distortion at the bright region?

Can you link to a schematic?

[old_tv_nut]

More: Is this a push-pull vertical output? If so, maybe the vertical output bias is wrong and this is a crossover distortion due to reduced conduction on one vertical output side or the other or both near the raster center.

[recepteur]

Gents,

This is an electrostatic deflection tube and I believe with a push-pull vertical output. See enclosed schematic. As for if is there a fold or distortion at the bright region, I will check this out right away.

The thin white line does not stay at exactly the same place, even if I move the raster up or down in available screen area.

What could cause a crossover distortion ?.

Cheers

[Notimetolooz]

Look at the area with a magnifier. Do the lines look closer together in the bright area? That would indicate a deflection problem.
Or are the lines there actually more brighter? That would point to an interfering
signal in the video.

[recepteur]

Hello Notimetolooz,

Thanks for the tip, I will do that.

Cheers

[old_tv_nut]

Originally Posted by recepteur

Gents,

What could cause a crossover distortion ?.

Cheers

Crossover distortion happens in class B push-pull amplifiers, where one tube operates on the positive part of the waveform and the other on the negative half. In this type of amp, the tubes are driven in parallel.

The schematic shows the first tube section driving the second section, so I believe both tubes must conduct for the whole waveform - no chance for crossover problems as normally understood.

Could there be some other reason for a distortion in the vertical waveform? I don't know.

[recepteur]

Gents,

I've included some additional pictures with. I physically removed the RF and IF tubes to see any changes in the raster and there is no changes, the white thin line remain. I'm starting to suspect a deflection problem. Cheers

[Yamamaya42]

It's a shame, there is nothing to show you what the waveforms should look like, so there is no guide as to if there is something abnormal in the circuit.

[old_tv_nut]

It's hard to tell from the photos if the lines are closer spaced in the light area or not. Did you look for distorition in a picture?

Is this effect as strong with a picture as with a plain raster like this?

If both vertical output tubes are conduction during the whole waveform as I think they are, it makes it more likely that this is some sort of glitch in the video drive rather than the vertical.