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  #16  
Old 04-11-2016, 08:05 PM
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Jumping into this discussion a little late, I think it important to consider the variability of HO Tube characteristics which may lead to a drive line. I have jotted down my practical thoughts to see what others think because of the frequency this topic is discussed.

Recall that in the flyback sweep the left hand portion of the sweep after retrace is from the energy recovered from the damper. At initiation of retrace the HO Tube is sharply cut off and the energy recovered from the resonant negative voltage swing powers the first half of the horizontal sweep. When the recovered energy is dispelled, the HO Tube begins to conduct linearly until the tube is cut off at retrace and the cycle begins again.

The white line would be the result of the transition when the HO Tube conducts. If the HO Tube does not begin to conduct before the recovered energy is fully dispelled, the sweep will be momentarily halted resulting in a vertical drive line. Increasing the horizontal drive beyond a certain point will cause the HO Tube to remain cut off longer hence the conduction will not pickup in time resulting in the line.

Changing the HO Tube for one with a characteristic with slightly less transconductance I believe will alleviate the drive line. Reducing slightly the horizontal drive by perhaps replacing the horizontal oscillator tube or else reducing the horizontal oscillator output by some means (resistive divider or shunt capacitance) would do the same thing. Later sets did not have a horizontal drive control to manage the oscillator level: I suppose this was to save cost.

These are just some of my random thoughts. I am glad the problem was resolved by simply swapping a tube.
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  #17  
Old 04-11-2016, 09:57 PM
old_coot88 old_coot88 is offline
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And kudos to Jr_tech for posting Pete Deksnis' excellent horizontal scan treatise..

http://www.earlytelevision.org/Deksn...implified.html

Generation of the horizontal sweep sawtooth is "taken for granted" without much thought to the arcana behind it. Pete's tutorial translates easily to B&W tube topologies as well (except for a few of those early oddballs that used a single tube for both oscillator and sweep and no damper).

Last edited by old_coot88; 04-11-2016 at 11:17 PM.
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  #18  
Old 04-11-2016, 11:29 PM
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Quote:
Originally Posted by Penthode View Post

...The white line would be the result of the transition when the HO Tube conducts. If the HO Tube does not begin to conduct before the recovered energy is fully dispelled, the sweep will be momentarily halted resulting in a vertical drive line. Increasing the horizontal drive beyond a certain point will cause the HO Tube to remain cut off longer hence the conduction will not pickup in time resulting in the line.
I wish we had someone very familiar with the details of HO tube operation. For instance (figures for 6CD6), I do not know why the maximum rated peak cathode current (700 ma)and maximum rated average cathode current (200 ma) are different. I also do not know why the peak plate voltage (7000 volts) is only allowed for 15% of the horizontal period. In transistor circuits, it was common to make the retrace time longer (and the pulse wider) specifically to reduce the peak voltage, which was much more critical to the transistor specs than pulse width. In fact, IIRC, some CRTs were narrower than 4x3, so the shorter trace time would not produce fat people.

Was something gained by keeping the tube HO drive on-time to the minimum necessary? Lower G2 dissipation? Lower average cathode current? Could the design-center on-time simply be increased and thereby avoid the limit-tolerance cases where it wasn't enough? I don't know.
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  #19  
Old 04-12-2016, 10:29 PM
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A lot of good questions are raised here. I believe vacuum tubes are more tolerant to voltage spikes hence the shorter retrace period. Vacuum tubes are perhaps able to accept the punishment of higher peak values?

However I am curious the comparison between retrace times between tube vs transistor horizontal output amplifiers. What is typically the retrace time when using a transistor?
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  #20  
Old 04-12-2016, 11:28 PM
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The retrace time in transistor sets probably got shorter over time as higher-voltage transistors became more readily available, but I don't know actual numbers. Excessive overscan was a constant complaint of Consumer Reports over the years. The excess was necessary in tube sets because of the variability in components including tubes, and the unregulated power supplies. Later solid-state sets with regulated supplies and much less variation due to the H output device could reduce the overscan without danger of producing black edges. Before the regular use of letterboxing, manufacturers avoided underscan at all costs because black edges would bring warranty claims.

Regarding variation due to the output device: tube HO's have significant voltage drop at peak current (50 to 100 volts), whereas a transistor will have a reliable saturation voltage on the order of a volt. Moto had trouble with its first transistor sets in New Orleans due to this: humidity would infiltrate the flybacks as the sets were in the warehouse. A tube (or 4-tube hybrid) set would refuse to draw terribly excessive current, and would heat up the flyback slowly enough on first use that the moisture would be driven off safely. The high voltage and scan might not be right, but would gradually correct itself. Solid state sets, however, would merrily draw all the current demanded, turning the flyback into a small pressure cooker that might crack the secondary winding. There had to be some careful fiddling with the potting compound and coil design to fix this problem while still maintaining the required fire-retardant properties and decent Q factor for efficiency.
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  #21  
Old 04-12-2016, 11:54 PM
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Getting slightly further off-topic (sorry), this also relates to safe action area and safe title area in production. The safe title area used to be a reduction in width and height of 10% on each of left and right edges, the top, and the bottom (20% total each way).
The latest EBU recommendation
https://tech.ebu.ch/docs/r/r095.pdf
reduces this by half to 5% on each edge, but a web search shows many continued recommendations for 10%, which seems excessive these days.
Flat panel sets do incorporate some overscan in order to hide any black edges that might arise from legacy analog video having excessive blanking time.

Early color cameras would often have round-tube outlines drawn on the viewfinder inside the safe title or safe action area. Image orthicon cameras were often set up with enough overscan that the edges of the target appeared as dark areas in the four corners of the raster. No one expected these to ever be visible, even on rounded-rectangle monochrome sets. Nowadays, these can be seen on home flat screen sets when viewing DVDs of classic shows.
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  #22  
Old 04-13-2016, 01:24 AM
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Of course safe title area is becoming more and more a thing of the past since the flat panel displays now have a fixed spatial dimension. This contrasts with a CRT where the raster size is dependent upon surround sweep circuit stability.

The commercial networks for many years made the HD 16x9 transmission 4x3 safe: that is safe titling was 4x3 protected for legacy 4x3 displays. The last 4x3 displays were sold in about 2005-06 and so I wonder if this is now becoming a thing of the past? (I do not watch much commercial TV these days).

Note that PBS member station broadcast of SD Create and World are now 16x9 and are not produced 4x3 safe. This brings about a new topic which deserves a thread in itself: "How Do You Cope with 16x9 Produced Content on a 4x3 Display?"
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  #23  
Old 04-13-2016, 01:37 AM
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Going back to the drive line business, I think it is important to understand the drive line is a result of a horizontal scan pause during the active trace eg left to right. The HO Tube is generally dependent on grid leak bias from the drive of the horizontal oscillator. If the amplitude of the oscillator signal is excessive, the HO Tube will have excessive bias due to control grid rectification hence will not begin conduction until too late. Hence the line.

I earlier suggested the 6CD6 with higher than normal transconductance may resulted in the drive line because the control grid negative cut off potential would be less. I would be curious if Bob could subject the offending HO Tube to a transconductance checker to determine if this is true.

It therefore appears that the appearance of the drive line is due to either or a combination of these two factors: excessive horizontal output amplitude output and/or HO Tube cut off characteristics.
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  #24  
Old 04-13-2016, 04:18 PM
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Quote:
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It therefore appears that the appearance of the drive line is due to either or a combination of these two factors: excessive horizontal output amplitude output and/or HO Tube cut off characteristics.
Another of my old books (Horizontal Sweep Servicing Handbook) seems to agree with you:

"If we overdrive the output tube, we will cause distortion of the drive waveform, by grid-clipping, etc., and upset the normal timing of the plate current pulses. If we make the output tube cut off at the wrong time, we upset the resonance relations in the secondary and distort the sweep waveform of the yoke. The invariable symptom of this condition is a foldover in the center of the screen, in the transition area between damper and output tube conduction. This appears as a vertical white line in the raster, which is called a drive line. Of course, it should be called an overdrive line, but you know how we are."

The book goes on to say that you can use the drive adjuster for diagnosis. If you can't make a drive line appear, the horizontal oscillator output signal may be weak and should be checked out.

Phil Nelson
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  #25  
Old 04-14-2016, 12:50 AM
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I remember a Stromberg Carlson set we had many years ago had a doughnut shaped spiral piece of metal that was fitted on the HOT. Surrounding the midsection of the tube. Not sure what it did, but maybe to prevent snivits? Maybe it was magnetized?

You may see line drives if a baseball game is on...
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