RCA 21-D-7177 Horz Drive Issues
 

I restored this set back in December of this year and have been watching and trying to learn proper setup with it ever since. Overall it's working well, but I've never been able to adjust the horizontal drive to my satisfaction. Regardless of the Horz drive cap adjustment, I end up with what looks like ringing, or a ripple in the left side of the picture. I mostly notice this on the bottom left screen while watching program material. Pictures are taken with test CRT, but the same issue persists with the sets CRT. I also see faint light and dark vertical streaks (drive lines?) in the picture. With the fine cross hatch pattern, I get the bright band as can be seen in the pictures. In general the picture looks best with the HORZ drive cap turned all the way closed. Adjusting the Horz drive cap moves the white band seen in the cross hatch across the screen, but I can't make it go away. All my horizontal waveforms look correct as shown in SAMS.

Would appreciate any ideas on where to look for the problem.

Sure the horizontal wave coil has a correct looking waveform on it? You should have equal peaks.

I think it's real close. This is what it currently looks like with the fine cross hatch pattern. This is looking at point I on the schematic.

To offer a WAG, possibly leakage between the V and H windings of the yoke, with the potential for developing into the classic trapezoidal-shaped raster. oc

Here's what the Horz drive waveform looks like, and a screen shot that shows the horz wave over on the far left (look where it says QVC price $13.98 in the black bar). I think I can actually see the small dip in the bottom of the drive waveform? Also some ringing way at the bottom. My problem is I don't know how much of this distortion is considered normal for this set.

I wondered about that. I guess I will need to find a new yoke in order to test that theory. I also wondered about the cap directly across the one yoke winding. I have not replaced that. I think it's rated at 2500 volts.

I've has the same issue with a few sets. It's usually something in the damper circuit. I'd check that yoke capacitor too.

These are my few thoughts.... I do not think there is a horizontal drive problem. Horizontal overdrive leads to non linear scanning where the beam slows during forward horizontal trace before the Horizontal output tube conducts. Instead I believe the line is displayed during the horizontal retrace.

When you adjust the horizontal hold, does the line move? If so, the problem is too slow a horizontal retrace. What you are seeing is the first vertical line in the cross hatch which should be on the left of te picture but is being displayed in the middle of the screen as the beam too slowly swings back to the left.

There are a few reasons why this may be so.

1) Is the test generator horizonal blanking wide enough. Examine the horizontal sync pulse with the scope. Horizintal blanking is supposed to be 10.9us. If the video starts too soon on the generator, you will see the white line. See the link below I found on the web:

http://www.maxim-ic.com/app-notes/index.mvp/id/734

2) The horizontal retrace is too slow due to a circuit fault. The horizontal output circuit is tuned to about 70kHz to facilitate the fast timed beam swing back t the left. Anything which may affect the horizontal "tuning" eg bad capacitor on the linearity coil, bad damper tube, shorted horizontal output transformer, shorted yoke etc may slow down the retrace.

Do you see the line on normal pictures? If not, I'd have a closer look at the cross hatch generator.

Thanks for the suggestions.

The line definitely moves while changing the Horz hold control.

Don't see a line on normal pictures, but something is bending the horizontal scan lines over on the left-hand side of the picture. It's not bent all the way top to bottom, just at certain points in the picture. Moving the hold all the way to the right seems to help minimize the horizontal bending. I'm not sure if the two symptoms are related?

With the RCA Synchroguide horizontal hold setting, I generally set the horizontal frequency (transformer core adjustment) so that with the full right setting of the front control it is on the verge of losing horizontal sync. At this point the horizontal oscillator is most firmly locked

The reason the white bar disappears when turning the knob and the video to the right is because the retrace is starting earlier hence the left portion of the video (vertical bar) moves to the left side of the raster where it belongs. This is further indication of too slow retrace.

Have you changed the capacitors in the horizontal sweep section? If so, recheck the values. If a capacitor is too large in value, it conceivably could reduce the resonant frequency of the flyback system, leading to too slow a retrace.

I shrunk the horizontal and vertical picture size down so I could see the edges of the picture, in an attempt to get a better idea of what the deflection circuits are doing. The horizontal linearity is off pretty bad and the horizontal lines still have a lot of wave over on the far left. For some reason the designers chose not to put a horizontal linearity control on this set. The only way to get decent horizontal linearity, is to adjust the horizontal width for a much wider than needed picture. Any idea's how I can adjust the horizontal linearity? I’ve tried several horizontal output and damper tubes with no change. :scratch2:

Hi Kevin,

Typically, with good tubes and good components (flyback transformer, yoke, resistors and capacitors), the linearity should be okay. From your earlier posted photos, the linearity did not look too bad to me. Can you outline or show by perhaps with a photo where the linearity is bad?

With the flyback horizontal sweep design, the damper provides the first half (left hand side) of the sweep and the horizontal output the second half (right handside) with the horizontal drive facilitating the best crossover in the middle of the screen. With this in mind, the pursuit of better linearity can be focussed on the particular part of the circuit.

Cheers,

Terry

Terry-

I'll post a few pictures using the sets 21AMP4B CRT. Will probably be an hour before I get them posted.

Ok...these pictures are all with the vertical and horizontal size brought down so we can just see the edges of the raster. The line patterns are from my B&K 1250 NTSC generator. The B&K target pattern is actually being played on DVD.

All the patterns seem shifted right from center when I have equal spacing on the outside edges of the raster. In the last target picture, you can see what happens when I center the image on the screen, the horizontal linearity issue becomes much more noticable as the target is move to the left.

Now if I adjust the width control for a wider horizontal scan, linearity does get better, but then the edges of the picture are also being cut off. Sams tells me to adjust the picture size until the image just fills the screen, but then the horizontal linearity issue is very noticeable.

I'm also still concerned about the vertical lines over on the left side of the screen. I'm guessing all these issues are related to the horizontal flyback system not operating at the correct frequency.

Kevin,

Sorry for the late reply. To be honest, the linearity does not look too bad to me. The horizontal sweep system in the early '50's is pretty crude by today's standards. However, the picture is pretty cramped on the left and I would suggest that the horizontal output tube is either a little weak or else some of the resistors around the horizintal output tube have drifted in value a bit.

Reducing the horizontal drive and increasing the width would probably provide the best compromise. The stretch on the left may also be due to too much horizontal drive.

I would be curious to see how the video fits properly within the video signal lines up on the raster eg. video starting 10us after horizontal sync. This would indicate if the retrace is timed correctly, that is the beam arriving at the left with a slight margin prior to scanning the active picture.