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

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

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

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

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.

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

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

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?

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.

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

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.

Hello Notimetolooz,

Thanks for the tip, I will do that.

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.

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

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.

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.

Good evening old_tv_nut,

I did not noticed any distortion, see enclosed pics.

This effect is present with a picture as with a plain raster.

This particular thin white line is transparent, sort of a clear mask and you can see the picture details behind it.

Vertical scanned lines are evenly spaced.

Cheers

Well, that all says it's a blip in the video, or at least on some CRT electrode besides the deflection.

You could try disconnecting C43, .05 video coupling, to see if it's coming in that way. If it's still there, maybe bridging another 20 uF across C32a in case that's bad.

Good day old_tv_nut,

Thanks for your input, I will proceed with your recommandations and will share my findings. Cheers

Good evening old_tv_nut,

Disconnected C43 .05 couping capacitor and also bridged C32a did not erase the thin white line. Cheers

Well, I'm out of ideas then.

Have you tried swapping the video stage tubes and deflection stage tubes. If there is heater-cathode leakage that could cause it and or other strange symptoms.

It might also be worthwhile to feed it a blank raster and check video and deflection waveforms at the CRT. Video should be just sync pulses if you're feeding it a blank raster and deflection should be a clean sawtooth. If you see anything else especially if it is a short pulse then you have found the line the noise is on (then comes figuring out what on the line is causing it).

Caution about checking the deflection waveforms - they are riding on a high DC voltage at the CRT and could fry your oscilloscope. Scope them at the plates of the deflection tubes instead.

Gents,

Thanks for your inputs.

I will check the deflection waveforms directly from the plates of the deflection tubes.

Cheers

Absence of any compression or folding of scan lines would imply the blip's not deflection-related, but is modulating the CRT beam current. It could be a negative-going blip on the cathode (not likely since you've already isolated that), or a positive-going blip on the focus electrode (pin 5) or the accelerator(?) electrode (pin 9).

I have a 7JP4 with a white line across it like yours not quite as vivid as the one on your tube. This is in the tube because the set the picture was taken on has another tube with no line I just hooked this one up to show the line. I don't understand what it is because if it was from no vertical deflection it would be a dark line as stated in another post. just though I would throw this out because of having another tube doing the same thing.

This really is a head-scratcher to me.

Is the white line always stationary with respect to the tube face, or does it move with the vertical centering adjustment? If it moves, does it move the same distance as the picture or a different distance? (Trying to figure out if it's a fixed phosphor defect or electrical.)

Mine is definitely a phosphor defect there is no movement with any adjustment

Hello Jsandlin,

Your enclosed picture depicts the same thin white line as on my picture tube.

Hello old_tv_nut

The white line is not stationary with respect to the picture tube face as it does move with the vertical centering adjustment.

Gents,

As of todate, I've replaced all the resistors and capacitors on the vertical oscillator and balance amplifier and the thin white line remains. I've check both vertical sawtooth waveforms and they are both identical 180 degree out of phase. All the high voltage rated capacitors (6KV) used for deflection were replaced last month. Cheers

Too bad you didn't have another CRT to test it , perhaps it's the CRT at fault

But could the CRT really be causing it? Seems kind of unlikely to me, but then again, I have little experience electrostatic deflection TV sets, beyond working on oscilloscopes.
BTW, I'm NOT a gent! :)

Curiouser and curiouser - one report of movement and one report of none.

:headscrat

Good day Yamamaya42,

Noted !

Cheers

Is there waveforms provided with that schematic? I see something on there marked point V, right out of the video det, before the video amp, and if it is anything like most sets I have seen, and the video signal there is about 1.1v ptp, one could try coupling in an external monitor, and see if the video is clean there.
I did this on my FADA.
It will rule out any problems there and before that point.

Hello,

I will compare with the waveforms provided in the Admiral chassis 19A1 schematic.

Unfortunatly, I don't have an external monitor.

I disconnected the coupling capacitor c43 .05 mf and the thin white line remained.

Cheers

He's already disconnected C43 which decouples the CRT from the vid output, and the line is still there. (Edit. OOPs he just posted this again)

And since the line moves with the vert centering adj., the blip has gotta be modulating CRT beam intensity. I still say look for the blip on pin 5 or 9 (focus anode and accelerator(?) anode respectively.

That pretty much rules out that it's coming from any video source, you could try hooking an oscilloscope to c32a/ the brightness pot, while c43 is unhooked, there SHOULD be nice clean power there, and that will rule out any PWS issue, I doubt that is the problem, but you never know.