Circuits for removing retrace lines

[bigaudioal]

Anyone know of a source for procedures or schematics for adding circuits to remove retrace lines for Admiral chassis 21K1 and 21L1?

I found this procedure below with attached photo, but it was kinda hard to follow what exactly needs to be done. Specifically how/where to wire in the pot and connecting the voice coil winding in series with vert. deflection yoke?

ADMIRAL 21- and 22-series Chassis

Retrace line elimination
(see Fig. 5 attached).

Retrace elimination by means of fixed values has often proved unsatisfactory mainly due to tolerances used in manufacturing and aging of tubes. The following method provides an adjustable control so that a pulse of correct amplitude may be applied to the picture tube.

1. Connect a 100,000-ohm potentiometer (part No. 75B13-12) across the primary winding of a speaker transformer (part No. 79C33-1).

2. Connect the voice coil winding of the speaker transformer in series with the vertical deflection yoke as shown in the figure.

3. Cut the green lead going to pin 2 of the picture tube. Connect the portion of the lead remaining on pin 2 to the center arm of the potentiometer, and connect the other portion of the green lead to an outer terminal of the potentiometer. Adjust the potentiometer until the retrace lines are removed. If the retrace lines do not disappear or they become brighter, change the green lead from one outside terminal of the potentiometer to the other.

With the contrast control set at minimum and the brightness control at maximum, the potentiometer should be adjusted to the point where the retrace lines just disappear.

[Electronic M]

I believe you need to obtain an additional separate audio output transformer to build that circuit. I can't see any way the set's own audio output transformer could be connected in that manner while still using it to drive the speaker with any sort of remotely acceptable audio.

I think the pot you are adding is the 100K on the secondary of the audio transformer (which would be the tube side if it were being used for audio). I think the pot on the right is part of the original brightness(?) control circuit. I believe the dashed line on the right pot is an original wire you are supposed to cut so you can wire the new pot/transformer blanking circuit in series.

Hope that makes sense.

[bigaudioal]

Originally Posted by Electronic M

I believe you need to obtain an additional separate audio output transformer to build that circuit. I can't see any way the set's own audio output transformer could be connected in that manner while still using it to drive the speaker with any sort of remotely acceptable audio.

I think the pot you are adding is the 100K on the secondary of the audio transformer (which would be the tube side if it were being used for audio). I think the pot on the right is part of the original brightness(?) control circuit. I believe the dashed line on the right pot is an original wire you are supposed to cut so you can wire the new pot/transformer blanking circuit in series.

Hope that makes sense.

I am glad I am not the only one who could not see how it worked as described. Anyone have any other procedures or ideas about removing retrace lines on these sets?

[Phil Nelson]

I don't know offhand if it's appropriate for your Admiral, but I used the circuit described in the following article to minimize retrace lines in my Philco 49-1240:

http://www.earlytelevision.org/vertical_blanking.html

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

[Steve McVoy]

Phil, the circuit on our site won't work with the Admiral, since the video is fed into the CRT grid and the pulse will be the wrong polarity.

The circuit posted by bigaudioal is a nice solution for sets like the Admiral. It should be noted that it can produce either a negative or positive going pulse, depending on which wires in and out of the transformer are used. You will have to experiment with this to produce the proper polarity pulse (negative going for a set that feeds video to the CRT grid).

[Kamakiri]

Remind me when you come up Al, I have a spare audio output transformer you can have if you want to try to build the circuit.

[wa2ise]

This circuit looks better because there is less risk of the vertical blanking pulse leaking back into the video amp plate, which would happen if you connected that vert blanking pulse to the CRT cathode. The set's sync separator is connected to the same video amp plate. Connecting the vertical blanking to the CRT grid (assuming the video amp feeds the CRT cathode) is a better approach, in that this undesired leakage path won't happen.


Most any audio output transformer from an AA5 tube radio should work.

[bigaudioal]

Thanks for the replies everyone. Not sure I have come far enough to build this circuit yet without some guidance? I will take you up on that spare audio transformer next week when I visit, Tim. Thx!

[Tom Albrecht]

I often simply hunt for an appropriate waveform somewhere in the vertical oscillator or output stage and couple it through a small value cap to the brightness control of the set. If the video signal is on the CRT cathode and brightness control on the grid, then you want a sawtooth with a rising ramp and a vertical drop. If the video is on the CRT grid and the brightness is on the cathode, then you want a signal with a falling ramp and a vertical rise. Depending on the amplitude of the signal you are grabbing, something in the range of 50 - 1000 pF will usually do it, and typically you need to add about 50-100 K in series with the lead to the cathode or grid, and couple the signal to the cathode or grid end of the resistor.

I've had a few sets where this was not successful. Sometimes the problem is that the sawtooth rise or fall is too fast, and the retrace on the screen isn't done before the blanking pulse ends. The method shown here should solve that problem, since it pulls the blanking pulse off the actual current waveform in the vertical deflection yoke. I'll keep this circuit in mind for future reference!

[old_coot88]

Originally Posted by Steve McVoy

Phil, the circuit on our site won't work with the Admiral, since the video is fed into the CRT grid and the pulse will be the wrong polarity.

If a person wasn't averse to having a SS device in a vintage set, i wonder if a phase inverter such as this might be useful. The transistor would be an NPN video driver (high voltage) type, and the R and C values chosen as needed...

[old_coot88]

Originally Posted by old_coot88

If a person wasn't averse to having a SS device in a vintage set, i wonder if a phase inverter such as this might be useful. The transistor would be an NPN video driver (high voltage) type, and the R and C values chosen as needed...

Minor error on vert. out connection. Doh.

[Steve McVoy]

The transformer circuit solves the polarity problem, since either phase can be obtained by reversing either of the transformer windings.