Electrostatic Deflection "Bunching" - Solved

[Kevin Kuehn]

Could you capacitive couple the probe by clipping it directly over the insulation of the wire going to the plate?

[Tom Albrecht]

That may be worth a try. I know you can pick up a nice horizontal waveform that way. For the vertical, the frequency might be too low for that kind of coupling. Will give it a try when I do the Sentinel and report back.

[earlyfilm]

Originally Posted by Tom Albrecht

It does indeed happen with brand new ASC brand .0047 uF 6000 V tubular capacitors.

Interesting and completely unexpected!

In the late 1950's, some of these electrostatic sets that had been retired, were wanted each fall by college students going off to school, so their parents would bring them into the shop to be serviced.

We used to see this problem on many of the sets, but not nearly as bad as your pictures show. We tried capacitor* replacement, but we saw no change. The thinking back in the day, was this was caused by the CRT's going gassy and generating secondary emission from the deflection plates. This was sorta confirmed by many of the sets picture improving somewhat after a few weeks of use.

I am at a complete loss to explain how if this were from a gassy tube, why the heck it would cause this symptom on the vertical sweep and not on the horizontal?

James

* (Black molded Sprague caps were used for testing that looked like the RMA banded ones, but were ink stamped and the black plastic was not as shiny.)

[Tom Albrecht]

After some further experimentation and verification, it turns out that what I reported above is not correct. Sorry to have caused some confusion (and perhaps some unwarranted purchases of .01 uF 6 kV tubular capacitors).

The problem with the first set of capacitors (the .005 uf 6 kV ASC tubulars) used above was that one of them was in fact bad, with basically an open circuit. On my capacitor checker, it read 50 pF. So.... when your picture looks trapezoidal with pretty bad bunching, the problem is that one of the two vertical plates is floating, and the other is doing all the work. That's seems to be the cause of the trapezoidal shape, and why there was generally not enough vertical deflection available.

And wait, it gets even more confusing. To redeem myself, I figured I would do some experiments comparing ceramic disk and tubular capacitors, to show everyone what they already know -- that ceramic caps cause problems. Well, the results show otherwise, at least in this particular set.

The various capacitors I tested are shown in the attached photos here. These are used in pairs, one for each deflection plate:

1. series array of two gold .001 uF 3 kV ceramic disk caps (should be equivalent to .0005 uF 6 kV)

2. series-parallel array of four gold .005 uF 3 kV ceramic disk caps (should be equivalent to .005 uF 6 kV)

3. single red .005 uF 4 kV ceramic disk cap (I'm actually a little over 4 kV on the HV, so this is pushing my luck a little for voltage rating, but it survived fine for the duration of the test)

4. ASC .0047 uF 6 kV tubular cap (what I normally use in the set)

5. big blue .015 uF 6.5 kV tubular cap for a laser power supply

Results in next post (not room for enough pics on this one).

[Tom Albrecht]

I had first convinced myself that ceramic caps didn't work when I restored my very first electrostatic set about 15 years ago. It was a Motorola VT-71, and maybe the results would actually be different on that set (maybe I'll try it sometime; I still have the set). But on this Sentinel 400TV, the results are as shown below.

The pictures show the raster obtained with a test pattern so that scan nonlinearity can be seen fairly easily. I had to readjust the vertical linearity and vertical height controls quite a bit between the various capacitor choices, but the bottom line is that I was able to get a pretty decent raster scan with any of them within the adjustment range of the controls.

1. Raster with the array of four .005 uF 3 kV gold disk caps (equivalent to .005 6 kV) on the vertical plates. This required significant correction of vertical linearity, but looks OK once adjusted. These are in fact the exact same capacitors I had installed in my VT-71 15 years ago that I thought showed bad behavior in that set!

2. Raster with the single .005 uF 4 kV red disk caps on the vertical plates. This required much less correction of vertical linearity.

3. Raster with ASC .0047 uF 6 kV tubular caps on the vertical plates. This was the original case for which the vertical linearity had been adjusted.

4. Raster with large .015 uF 6.5 kV blue laser power supply caps on the vertical plates. This required a little vertical linearity correction in the opposite direction, but not much.

Note that they actually all look pretty good! The only difference is some modest improvement in the linearity at the very top and bottom of the raster when the tubular caps (especially the large value .015 uF) are used.

I also tried to see if there was a difference in "bunching" behavior at high contrast with the various caps. Bottom line is that this set, for whatever reason, does not show bunching until the contrast and brightness are high enough to show other problems with the picture (a little blooming, focus suffering), and there was NO notable difference in this behavior between the various cap choices above.

So then I got to wondering... Was the problem actually with the horizontal deflection when using ceramic caps? So I tried it. For these experiments, I left the big blue laser caps on the vertical and swapped out the ASC .001 uF 6 kV tubulars that were on the horizontal plates with the following:

5. Raster with two .001 uF 3 kV gold ceramic caps (equivalent to .0005 uF 6 kV) on the horizontal plates - note this is less capacitance than originally there. Overall raster width just a little decreased from what I had with the tubulars, but no other obvious problem.

6. Raster with the array of four .005 uF 3 kV gold ceramic caps (equivalent to .005 uF 6 kV) on the horizontal plates. Same width as I had with tubulars, and no obvious problems.

So there. What does it all mean? Do different sets and different CRTs behave differently with different kinds of coupling caps?

In any case, we certainly cannot conclude after this experiment that "ceramic caps always cause problems when used as deflection coupling capacitors in electrostatic deflection sets."

On the other hand, I'm not about to start recommending people to use ceramic caps for this purpose, or to start using them routinely myself in future restorations. Will probably do more experiments on other sets at some point.

Perhaps the key point is that the ceramic caps needed significant correction of vertical linearity, which on the Sentinel set is a provided adjustment. I believe quite a few other electrostatic sets don't have a linearity adjustment, so perhaps without the capability to adjust linearity, the ceramics result in poor vertical linearity. Maybe that's the story.

Sorry for the confusion, everyone!

[bandersen]

Thanks for the last couple posts. I've restored a few electrostatic sets and hadn't encountered the problems you initial described and thought maybe I'd just been lucky

As for ceramic caps, I've picked up a few sets that had ceramic doorknob caps substituted for the horizontal coupling caps that seems to work OK.
It was the vertical where I figured the piezoelectric effect would cause some major linearity issues, but your results seem to indicate otherwise.

So it sounds like this set has a linearity control ? I wonder how ceramic caps might work in a set like VT-71 that lacks this adjustment ?

[dtvmcdonald]

Its quite OK, indeed very excellent, to use ceramic caps in this
coupling application. Use paralleled 6KV 0.01 uF caps, usually
two of them to make 0.02 uF. Using the specified 0.005 uF
does not work well with the ceramics ... or anything else for that matter.

The caveat is that using 0.02 or higher, you have to remove or
at least tone down any circuit that is supposed to "correct" linearity
for using caps that are WAY too small.

[bandersen]

I've been looking around at the usual places for high voltage ceramic caps and I'm finding them to be rather expensive.

Mouser only has two in stock that are 4700 or larger at 6,000 volts.
0.01uF @ 6kV = $5.23
0.0047 @ 6kV = $2.20
http://www.mouser.com/Passive-Compon...z0wpzoZ1z0wx8i

The largest Digi-Key has is 0.002 @ 6Kv for $1.02
http://www.digikey.com/product-detai...9PH-ND/2356805

Allied Electronics
0.0047 @ 6kV = $2.01 ( currently out of stock )
0.01 @ 6kV = $3.44
http://www.alliedelec.com/passive-co...673-4294959805

So two 0.01 from Allied would be about 7 bucks. Might as well just use the less expensive 0.0047 ASC caps.

Does anyone know of a cheap source of HV ceramic caps ?

[compucat]

When I first restored my Motorola 9VT1 with the TS-18 chassis, I used ceramic disk caps not knowing any better. The HV and horizontal were fine but I could not get good vertical linearity without putting two or three in parallel across each deflection plate. i have since replaced them all with the white tubulars from Allied of correct value and all is well. The Motorola has no linearity controls, just size and position much like an oscilloscope which is more or less what an electrostatic TV is. I have not run into the vertical bunching like you describe but one open cap makes sense that it would cause that. Very interesting thread and experiment. I personally really like electrostatic sets and find they are fun to work on.

[Tom Albrecht]

I'm busy with some other things at the moment, but next weekend I'll try some larger value ceramic caps and report how they work. I'll intentionally leave the linearity adjusted for the tubular caps, just to see how ceramic caps would function in a set without linearity adjustment. Based on the experiments so far, it seems clear that the linearity would be quite poor with .005 ceramic if there were no linearity adjustment. If simply using a larger value of ceramic solves the problem, that's a nice option to have (although probably not cheaper than using tubular caps if you have to order new caps).

The bunching effect is a real effect, and may or may not be related to the vertical deflection coupling caps as I originally reported. In the past, I have had the impression that some sets and certain CRTs are more prone to it than others, ranging from fairly problematic to completely unnoticeable. The Sentinel 400TV I have on the bench now, with the CRT I now have in it (I swapped it out recently) doesn't show this problem until the brightness and contrast are high enough to cause other problems (so I don't consider the bunching to be a problem at all on this set now). Next time I have an electrostatic set on the bench that shows the bunching problem, I'll go after it some more.

A few of you have reported seeing the bunching problem in your posts above. Any insight you can add would be helpful.

[compucat]

I remember when I first used the .005uf ceramics in the vertical I could not get the vertical to fill out all the way without the linearity going way off. The picture would stretch out severely at the top giving a cone head effect. The only way I could get correct linearity was to have the vertical height control set to where the picture was about half height. It is amazing to me how well these sets actually perform when they are correctly restored and adjusted. Mine is in regular service and has been a dependable daily watcher for a while now. Not bad for early TV technology.

[Tom Albrecht]

To add a bit more to the topic of vertical linearity using different types of coupling capacitors, I did a few more experiments.

In this case, instead of readjusting the vertical linearity control to optimize the picture with each type of capacitor, I set the linearity control for best linearity with the .005 uF 6000 V ASC tubular caps which are generally thought to work well in these types of sets. Then, to simulate how other types of capacitors would behave in sets that don't have a linearity adjustment (e.g., Motorola VT-71 and many others), I substituted different capacitors and took pictures to make the linearity changes apparent.

Here are the five types of capacitors and their corresponding images:

1. ASC .005 uF @ 6 kV tubular cap (linearity adjusted for optimal picture with this type of capacitor):



2. Network of four gold ceramic caps with Z5U dielectric, .005 uF @ 3kV (series parallel configuration, with net capacitance of .005 uF @ 6 kV):



3. Red ceramic with Y5S dielectric, .005 uF @ 4 kV:



4. Two of the above red ceramic Y5S caps in parallel, for .01 uF @ 4 kV:



5. Blue .015 uF @ 6.5 kV large tubular capacitor from laser power supply:



Comments:

The really bad actor is the gold Z5U capacitor, with bad compression near the bottom of the image. Note that the red Y5S capacitor is much better, with just a little compression at the bottom, even though both are ceramic disk capacitors.

Looking at the specs for the two types of dielectrics, we have the following:

Z5U: +10 to +85 C, with capacitance variation of -56% to +22% over this range
Y5S: -30 to +85 C, with capacitance variation of -22% to +22% over this range

Now of course we aren't varying the temperature here, but it is also known that ceramic capacitors have ferroelectric (piezoelectric) dielectrics, whose capacitance changes with the applied voltage. That's the effect that's causing the problem here. If we assume that the capacitance change with voltage bears some resemblance to the capacitance change with temperature (which may or may not really be true), maybe the Y5S rating is a better choice here than Z5U. To really check, one would have to look at some other examples of these types of capacitors and compare.

It has been mentioned above that using a higher capacitance can provide better linearity when using ceramic capacitors. Note that when two of the red ceramic caps in parallel are used, for a .01 uF capacitance, the linearity problem actually goes the other way (compressed at top), suggesting that some capacitance between .005 and .01 uF would actually give the same performance as the original .005 uF tubular cap used here.

I don't have enough of the gold ceramic caps to see what would happen with adding extra caps for more capacitance with that particular type.

Finally, when an even larger capacitance is used (this time with another type of tubular cap), the linearity goes even farther in the other direction (compressed at top).

This all goes to show that the .005 uF tubular cap gives a somewhat nonlinear scan, but the set has some compensation in the circuitry to linearize things. This makes sense, since these caps were expensive and they could save money with a not-too-huge cap and a bit of compensation. Putting in too large a value today requires some modification of the linearity correction (either adjustment of linearity control, or modification of compensation circuitry if no control is provided).

So the bottom line is that ASC tubular 6 kV caps seem to do a good job of emulating the original tubular caps of the same value (based on experience in many different types of sets), but ceramic caps can indeed be used if some measures are taken to adjust the linearity. One simple approach is to increase the capacitance by the right amount.

There are some hints here that Y5S dielectric is better than Z5U, but that needs to be checked further before we can really recommend it with any confidence.

[ChrisW6ATV]

Tom, beside your valuable work testing the different capacitors, I am impressed with the quality of that set's raster/alignment in general. I did not know a 7-inch set could look that good.