RCA 21-D-7177 Horz Drive Issues

[Penthode]

Originally Posted by Kevin Kuehn

But, if the right side "were" compressed, would that also be causing the left side picture to be cut off (no horizontal space bar?) I didn't think it would do that.

Remember that the horizontal output tube only begins to conduct from the middle of the scan to the right of the screen. If the tube is weak then the right of the screen would be compressed. The fact that the voltages are normal and you have a healthy scan otherwise points to the output tube.

Is the 6DQ6 screen voltage correct? Perhaps the screen resistor has drifted upward or the screen decoupling capacitor is faulty?

Originally Posted by Kevin Kuehn

It sure would be interesting to have the actual RCA service literature for this chassis. Possibly there are notes about scan issues.

I do not think the RCA Notes will share this information. However I do recall some good articles surfing the web. I just can't seem to find them at the moment...

Originally Posted by Kevin Kuehn

Does anyone know what was used for pincushion correction (if any) on this TV? Watching this 8" test CRT, the pincusion effect is really bad.

Small permanent magnets can be used on black and white set to correct pincushioning. They will be placed either side of the yoke near the CRT.

[Kevin Kuehn]

Originally Posted by Penthode

Remember that the horizontal output tube only begins to conduct from the middle of the scan to the right of the screen. If the tube is weak then the right of the screen would be compressed. The fact that the voltages are normal and you have a healthy scan otherwise points to the output tube.

Terry,

I'm sorry for the confusion. I don't think I've ever had a problem with the right side of the screen being overly compressed. The reason the right side is currently compressed is because I have the width control turned all the way out. I only did this in an attempt to see the dark space bar on both sides of the picture. If you recall (before I sidetracked the issue) we were discussing that the picture information should start scanning after that dark space on the lefthand side. The width control does have an effect on horiz linearity, but my main concern is that I can not narrow the picture width enough to see the left most edge of the raster. I just thought it curious that the picture is not centered on the 8XP4 test CRT, but it's probably nothing unusual.

I still think that your original theory is correct: that the horizontal retrace time is too short. I think possibly this is also shifting the picture to the left of screen center. Does that combination make any sense, or am I still confused?

[Penthode]

Hi Kevin,

I suppose I am now a little confused. I recall the original problem was that the center of the horizontal scan is to the right. This means that the left side is stretched and at the same time the right side is compressed. Is that correct?

By the width control being all the way out, does this mean the core of the width coil is out so you have minimum width? Then I suppose overall you have the necessary width adustment range but the left side is still stretched.

This would suggest to me you have too much horizontal drive, even with the trimmer capacitor tighted right down. Is this correct?

The Horizontal drive control is part of a capacitive divider. Perhaps there is something wrong with the trimmer? You should be able to adjust the trimmer so that as it is tightened, the left side of the raster slightly compresses and the CRT will dim slightly as the HV drops.

Maybe you should test the trimmer? Or as a test to reduce drive more, you could put a small capacitor (50 to 80 uufd) in parallel with the horizontal drive trimmer?

Terry

[Kevin Kuehn]

Originally Posted by Penthode

Hi Kevin,
I suppose I am now a little confused. I recall the original problem was that the center of the horizontal scan is to the right. This means that the left side is stretched and at the same time the right side is compressed. Is that correct?

Correct. Horizontal center was originally off to the right using the sets 21AMP4 CRT. When I changed to the 8XP4 test CRT, then I discovered the opposite true. Also, with the 8XP4, I'm not able to see the left edge of the horizontal scan. I suppose I should forget using the 8XP4 for these centering tests and just go with the 21AMP4?

Originally Posted by Penthode

By the width control being all the way out, does this mean the core of the width coil is out so you have minimum width? Then I suppose overall you have the necessary width adustment range but the left side is still stretched.

Correct, but really only became an issue when I switched to the 8XP4 test CRT. And this extreme reduction in width was only in an attempt to see the left scanning edge.

Originally Posted by Penthode

This would suggest to me you have too much horizontal drive, even with the trimmer capacitor tighted right down. Is this correct?

The Horizontal drive control is part of a capacitive divider. Perhaps there is something wrong with the trimmer? You should be able to adjust the trimmer so that as it is tightened, the left side of the raster slightly compresses and the CRT will dim slightly as the HV drops.

Maybe you should test the trimmer? Or as a test to reduce drive more, you could put a small capacitor (50 to 80 uufd) in parallel with the horizontal drive trimmer?

Terry

I'll check into reducing the horizontal drive even more and report back. I've never seen any noticeable compression on the left side, even with the trimmer completely closed. I'm pretty sure I've never seen the CRT dim with it fully closed. I know the trimmer checks ok, but I've yet to check the value of C105 (mica 560 pf). Using the sets 21AMP4 CRT, I generally get the best horiz linearity with the trimmer fully closed. This would seem to indicate there is still too much horiz drive.

[Kevin Kuehn]

Originally Posted by Kevin Kuehn

I'll check into reducing the horizontal drive even more and report back.

I'm still trying to make sense of this horizontal scan issue.

I did try lowering the horizontal drive some more. That did improve the horiz linearity some, but then the picture collapses before it reaches optimum linearity. The poor linearity doesn't bother me so much as the wiggle in the horizontal scan lines over on the left screen.

I've also installed a NOS Merit flyback and yoke. Amazingly nothing changed with the picture, other than the Merit yoke causes significant barrel distortion in the picture. Not sure what causes that, so I'm currently using the original yoke and the new Merit flyback. I've even tried changing value of the yoke balancing cap across the top half of the vertical winding. Also tried replacing the damping resistors across the horizontal windings, even with lower values. At least a half dozen NOS damper tubes have be tried, and they show very similar results.

Not sure where to go from here.

Pictures below are with the horizontal width reduced. It does look like there is horizontal blanking on both the left and right sides of the picture. You can also see light and dark vertical lines, typical textbook examples of horizontal ringing on the far left. It's hard to see in the pictures, but I can also see wider light and dark vertical columnes all across the screen.

[Penthode]

Kevin,

My guess is the wavy scan on the left is due to imbalanced distributed capacity on the horizontal deflection coils. Check the small capacitor: it should be between 50 to 100uufd and its value cancels the waviness.

The vertical stripes you call ringing I guess may be the unblanked burst on retrace. You can confirm this by adjusting the horizontal hold back and forth: if the stripes move in the opposite direction to the picture as you turn the control, then it is the burst.

All the best,

Terry

[Kevin Kuehn]

Originally Posted by Penthode

Kevin,

My guess is the wavy scan on the left is due to imbalanced distributed capacity on the horizontal deflection coils. Check the small capacitor: it should be between 50 to 100uufd and its value cancels the waviness.

The vertical stripes you call ringing I guess may be the unblanked burst on retrace. You can confirm this by adjusting the horizontal hold back and forth: if the stripes move in the opposite direction to the picture as you turn the control, then it is the burst.

All the best,

Terry

Hi Terry,

I've tried adjusting the horiz yoke balance capacitor from its stock 150 pf value, all the way down to nothing. Making it smaller does lessen the waviness, but it's still there. This TV has its keyed AGC pulse taken from the yoke windings. I'm starting to wonder if those associated capacitors aren't the cause of this whole issue.

Is there a practical way to completely blank the retrace burst you speak of?

[Kevin Kuehn]

Terry,

Those vertical stripes do not move with the horiz hold control. I suppose you could say they move opposite relative to the picture movement, but the lines themselves do not change location on the CRT.

[old_coot88]

Man, that is a brain teaser. From the FWIW department regarding the 'ringing', here's what I would try at the present stage:
Get a disc ceramic cap rated at at least 4KV, around 82pf (anywhere from 68 to 100pf will do). put it directly across the damper, plate to cathode. This will increase the width just a smidgin*. It may (or may not) have some noticable effect on the 'ringing'. Now connect the cap from the cathode (pin 3) to the other side of L39 and see if there's any effect (other than the increased width). If nada, try the cap directly across L39.

If any of those three arrangements had any effect on the ringing, either increasing or decreasing it, it will have at least localized the problem area.

*Besides increasing width, this will also reduce H.output tube cathode current slightly, and reduce HV slightly. Thus you can't go much higher than 100pf with the cap.
oc

[Kevin Kuehn]

OC,

Will try your suggestions.

Last night I unhooked the keyed AGC coupling components form the yoke socket, then hooked a battery to the AGC line. Ringing still persisted, but it did change the frequency slightly.