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Old 01-25-2018, 06:25 PM
Jon A.'s Avatar
Jon A. Jon A. is offline
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Blue pulse shaping circuits

This is about my Heathkit GR-2000 but it isn't my only set in which this has been a problem area.

Most of the controls on the convergence board are unmarked, leading me to mess with this one which the manual says not to adjust. The form was cracked when I got the set and is now broken completely off. Unlike the service manual for the other set I'm ironing some convergence bugs out of none of the books tell me what to do if the adjustment gets thrown off. I re-wound the loose windings as best as I could and tacked the wires back onto the base.

Since I had no adjustment data it was best guess Mr. Sulu...

I finally decided to try to equalize the waveforms on both sides of the coil. Yellow and blue are no longer splitsville on the right side as before but I have no way of knowing if I did the right thing. Attached is a partial schematic of the blue convergence network including the blue pulse shaping circuit, I'm hoping someone can chime in on how to properly adjust these.

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File Type: jpg Blue shaping.jpg (53.8 KB, 33 views)
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Old 01-26-2018, 10:27 AM
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Don't know what you mean by "equalize the waveforms on both sides of the coil."

The purpose of the second harmonic circuit is to add a ripple to the vertical deflection of the blue beam. The basic blue dynamic convergence waveform for a round tube has two parts: a sawtooth that makes a difference from left to right, and a first harmonic parabola that makes both edges different from the center. On a rectangular tube, the second harmonic is added, to make a difference between (center and edges) and (halfway between center and edges). In other words, without the second harmonic, you could have both the center and the left and right edges converged, but the left-center and right-center would be off. Or you could have the center and most of the width OK, but the extreme left and right would be off.

If the second harmonic circuit is way off (coil broken), it will not produce symmetrical effects at left-center and right center, because it may introduce less second harmonic, or introduce some first haormonic, or who knows what.

If you are now getting acceptable blue convergence all the way across from left to right, you have found the right adjustments.
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Last edited by old_tv_nut; 01-26-2018 at 10:32 AM.
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Old 01-26-2018, 12:19 PM
Jon A.'s Avatar
Jon A. Jon A. is offline
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Oh, I checked the waveforms at both ends of the coil windings and just dialed it in until they matched.

Anyhoo, thanks for the info. After making this topic I connected the scope to the other set I was having trouble with; turns out the very weird waveform I was getting before was due to a bad probe ground. Finally I hooked onto the yoke clamp and was able to dial in the proper waveform. However, it isn't quite equal to the waveform I get at the other terminal which lacks the horizontal harmonic "bump" of the former. I probably have to fine-tune the Heathkit's coil (or not, I have to wind a new one anyway so why bother).

Here's the waveform at the other set along with an excerpt from the manual. It's as close as I could get.


Last edited by Jon A.; 01-27-2018 at 10:45 AM.
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