RCA Victor 21-T-8205 White line in middle of picture

[ThePlague]

Originally Posted by Penthode

It does not appear to be a horizontal drive line. The line is most likely due to insufficient horizontal retrace blanking. During horizontal retrace there may be a positive overshoot of the rear of the sync pulse.

If it was a drive line, horizontal scan linearity would be affected with evidence of picture foldover across the line. I see no evidence of this in your photos.

To confirm, adjust the horizontal sync phase by adjusting the horizontal hold control. If the line moves, it is due to a blanking problem. If it stays fixed, it is a drive line.

You should be able to minimize it by adjusting the fine tuning. But there are means to improve retrace blanking if that is the problem which I believe it is.

I haven't had a chance to open it up and make adjustments per the service manual yet, but I can confirm that the line moves left or right in accordance with the user control of the horizontal hold.

[Penthode]

There are no adjustments necessary unless you check the RF/IF alignment. The horizontal drive adjustment, if the set has one, will make no difference.

What may be useful to post the schematic of the horizontal sweep/ video amplifier section to see if there is a blanking circuit present.

It may also be due to RF radiation of the horizontal retrace spike being picked up by the tuner. It may help selecting another channel to feed the set. It also may help to move or shield the wires leading to the deflection yoke.

Other things to consider are consumer box sync generators which exhibit too fast a fall and rise time. You may wish to try another set top box. Also alignment of the IF where the video carrier is to low on the Nyquist slope or a faulty IF tube (check by substitution one-by-one ensuring the original is replaced in its original socket).

[jr_tech]

Very interesting, certainly explains those stubborn "drive bars" that cannot be adjusted out by the usual horizontal drive adjustment. I posted a link to the schematic in post 2 of this thread... do you see a horizontal blanking circuit?

jr

[Penthode]

I reviewed the schematic. There is not horizontal retrace blanking only vertical blanking. This means that a negative going pulse is accompanying the video to cause the line.

I see that there is a common Horizontal sweep- video amplifier 100uF electrolytic across the +260v B+ line. I would put a scope probe across it to ground and see if the if a negative pulse dip is be coupled back to the video amp plate circuit.

It could be the pulse is picked up by the antenna. If you switch the rf modulator to another channel, does the line go away?

Look at the sync pulses of the set top box. If you try another set top box, does that remove or subdue the line?

As to experiment to remove it, I might try wrapping a few turns of one end of an insulated wire around one of wires leading to the horizontal deflection coil leads and connecting the other to the CRT grid. This would effectively couple the retrace pulse to the grid to cutoff the tube. This may take a bit of experimentation and polarity is important so you would have to try each lead.

[ThePlague]

Originally Posted by Penthode

It could be the pulse is picked up by the antenna. If you switch the rf modulator to another channel, does the line go away?

Look at the sync pulses of the set top box. If you try another set top box, does that remove or subdue the line?

Switching the channel provides no improvement nor does switching the box as I have run it through both my S-VHS player and Betamax player, both exhibit the same issue.

Originally Posted by Penthode

As to experiment to remove it, I might try wrapping a few turns of one end of an insulated wire around one of wires leading to the horizontal deflection coil leads and connecting the other to the CRT grid. This would effectively couple the retrace pulse to the grid to cutoff the tube. This may take a bit of experimentation and polarity is important so you would have to try each lead.

This will take some looking into as I have not experimented much with what you suggest. By the CRT Grid, are you referring to the control grid portion of the CRT?

[old_coot88]

Originally Posted by Penthode

As to experiment to remove it, I might try wrapping a few turns of one end of an insulated wire around one of wires leading to the horizontal deflection coil leads and connecting the other to the CRT grid. This would effectively couple the retrace pulse to the grid to cutoff the tube. This may take a bit of experimentation and polarity is important so you would have to try each lead.

Sounds like a great idea. I did a similar 'gimmick' to derive the AGC keying pulse when rigging a non-standard flyback that lacked the pulse winding.

But here, it'd be treatment for a drive bar, and drive bar has already been ruled out. Or maybe I'm missing something..