Pincushion distortion on Sears 528.41881220
 

I'm working on one of these 1973 Sears 19" sets and have it working pretty well except for noticeable pincushion at the top of the screen. The pincushion adjustments have some effect, but not enough. It gets really noticeable with titles or DVD player menus. The CRT is the original, not a rebuilt or replacement. Anyone have some idea of what to check for? The circuitry appears pretty minimal for pincushion correction.

First thing that comes to mind is out of tolerance resistors in the pin amp circuit. Possibly something connected to the pincushion correction potentiometer. Do you have the schematic on this tv? I have noticed with some sets the pincushion correction potentiometer has more affect on the top than the bottom.

Does the its potentiometer adjust all the way to one side and acts like it's not enough or does it adjust to a certain point and then distort the pincushion altogether? Also looks like the yoke may be ever so slightly out of position from 90 degrees.

This is my opinion of course, but it's not that terribly bad. You might check some components around pin amp like suggested, but you won't get it perfect. Just like convergence was always a compromise because it got close but never quite like you want it which was perfection. I don't think it's that horribly bad.

True. Sometimes, I have found that grommets and mechanism that rest the yoke on the crt neck housing deteriorate and cause the sag.. This will also cause the pincushion off on the top more than the bottom.

Same here. There's an old saying, "Perfectionism is the manic form of fuddyduddy-ism."

:D:tongue:

I don't have a schematic as of yet, but it looks like this one uses passive components in the pincushion correction circuit. I did correct the picture tilt with the yoke. The convergence, if not perfect, is really good. The horizontal edges are pretty straight, it's just the top and bottom that look off. There is not enough range in the pincushion adjustments to take care of the top and bottom

I remember my grandparents' old Magnavox console, they had a rebuilt CRT (25V delta gun) installed in the late 80s and it never could be adjusted for good purity, convergence, or pincushion; I sure as heck tried, and that was after the TV repair guy put it in.

Possibly he left the degausing field coil disconnected? I did this once. Drove me crazy until I figured it out. surprisingly it was a Magnavox too lol

I think the real problem is all of that test equipment setting on top of the TV. :D

Good call! Something could be creating a magnetic field. I moved a Zenith Chromacolor around the house. I noticed the deflection on the CRT changes slightly from the usual elements of the earth's magnetic field.

External fields would screw up the purity long before having a noticeable effect on pincushion.

Late night humor attempt. Judging from the width of that signal generator I'm pretty sure that gear is back behind the set on a shelf, although it could be closer than I realized.

Once had a neighbor whose kitchen TV picture was screwed up by the thing under it - the refrigerator.

Haha I just noticed that after taking a second look. Honestly I've gotten pincushion almost good enough on older sets, but never perfect. Take a look at some of my old posts on this. Thi gs bug me too much I guess. Like convergence. I found once a set cools off, convergence on R/G horizontal and blue lateral is off on my CTC39, pincushion on the top is a little off too. Once it warms up for at least 5 minutes, all back to my previous adjustments. Possibly some degree of expansion on the shadow mask.

I remember in the days at the shop when rebuilt jug was put in we at least waited for about a twenty minute warm up so the tv was good and ready for adjustments of purity, grey scale, and convergence.

I have a feeling the yoke on this set has problems since it has ringing at the left and top of the picture. The resistor and capacitor in the yoke check out OK. There isn't much I can do about this one at this time short of finding a replacement yoke, unless there is a weird problem with the set itself causing this.

Have you checked that the AGC is working correctly? It almost strikes me as slight signal overload. Turning your signal generator output way down should verify.

The up and down wiggles on horizontal lines at upper left may be the yoke. (But I'm wondering why they aren't the same from top to bottom - maybe the pincushion circuit?)

The left and right wiggles at the top are probably just the horizontal AFC. (Reacting to non-standard sync from your cross-hatch generator? It would be interesting to check if you get the same result with a DVD source.)

I experience that very issue with this Heathkit IO-101. Drove me about nuts trying to figure out the issue.

http://videokarma.org/showthread.php?t=268858

The crosshatch is actually from a DVD that I made with a few test patterns on it. Some time later on I'll get a schematic for this and see if there are problems with the sweep waveforms. I did have to put in a new 36KD6 since the one that was in the set when I got it was broken.

I finally found the problem, but don't have a really good fix at this time. The top core of the pincushion saturable reactor had fallen off and is lost. It was simply glued on when it was made. I did a "fix" from a few small ferrite cores glued together and then glued to the saturable reactor. The pincushion control has some effect now, just not enough. The problem is these transformers use a special core material that saturates easier than the ferrite I used, that's why I have only partial control at this time.