Sparton 16A211 Width Problem

[teevee]

From your earlier, you had stated that the drive waveform (W18) was per the schematic (120V p/p) If so, it would be worth scoping the cathodes of the 6BG6's. (Just to confirm that C131 is doing it's job.) Also, it's interesting that the schematic is showing these freq's as 7875 cps, rather than 15,750. Maybe that's a ref to showing 2 cycles on the scope? Yes, the "capacitor across the damper" trick will increase width some, BUT will result in lower HV, boost, etc.

[Mal Fuller]

If there is a "surplus" of high voltage, presently being regulated by the shunt regulator, there may be no decrease in voltage at the 2nd anode with the addition of a capacitor across the damper.

[Mal Fuller]

Any progress Steve?

[earlyfilm]

Originally Posted by Steve McVoy

I've been trying to find a width problem with this set with no luck. It lacks about an inch on each side. Width and Hor. Linearity work properly. I've checked all the tubes (by substitution) and all caps are new.

I have the proper waveforms and p-p voltages at W17 and W18 (see the schematic). Power supply busses are within 5 volts. On the 6BG6s, screen voltage is right, but the cathode voltage is 12 vs 18 on the schematic. Grid voltage is -25, but I noticed that setting of the hor. drive changes it from about -15 to -30. I can just barely get 20 kv on the CRT with the HV control full up. Boost is 550 vs 620 on the schematic. Damper plate voltage is right.

I have checked all the resistors and capacitors in the H. out, flyback and damper areas. I've even changed the yoke.

. . . . . Schematic is here (better to download the pdf so you can zoom in):

Steve,

I assume that you have done all these, sometimes, fresh eyes looking at a circuit might spot a possibility. From a schematic analysis, here are some simple checks that could have been easily overlooked:

Is the optional R158 which is parallel with R157 present? (This circuit should be either a single or a pair of resistors. The set should have either two 11000 ohm two watt resistors in parallel or one 5600 ohm five watt resistor. (It us easy to misread the schematic here if one missed the check-parts-list symbol! These resistors provide screen voltage for the 6BG6's.)

Check R178 for value. It is off the high side of the R-G-B balance controls. (It is not shown on your schematic, so I've added a complete schematic to this post.)

If your CRT is weak, has someone set the R-G-B balance controls excessively high to compensate? Are the three .022 caps off the wipers of R-G-B balance controls leaking?

Since a prior owner had extensively worked on the set:
Is the yoke a Sparton XD-2071-J or RCA 223D1?
Is the flyback Sparton XD-2166-J or RCA 240 T1?
Since that flyback has so many unused terminals and may be a replacement, did you check to see of the correct terminals were used?

Have you verified that the voltage on the on the HV regulator grid is within the specified range?

Verify the replacement component values and wiring connections of all the work done by the former owner of the set, preferably by someone who has not previously worked on this set.

James.

[Steve McVoy]

James, here is what I've found in reply to your excellent suggestions.

Is the optional R158 which is parallel with R157 present? (This circuit should be either a single or a pair of resistors. The set should have either two 11000 ohm two watt resistors in parallel or one 5600 ohm five watt resistor. (It us easy to misread the schematic here if one missed the check-parts-list symbol! These resistors provide screen voltage for the 6BG6's.) --- Yes, both resistors are there. I've removed one (lower width) and shorted out both (B+ directly on the screen grid, greater width, cathode current close to normal).


Check R178 for value. It is off the high side of the R-G-B balance controls. (It is not shown on your schematic, so I've added a complete schematic to this post.) --- I've checked it.

If your CRT is weak, has someone set the R-G-B balance controls excessively high to compensate? Are the three .022 caps off the wipers of R-G-B balance controls leaking? --- The CRT is excellent. There is no difference in width in any position of the R-G-B balance controls, so I think that rules out leaky .022s.

Since a prior owner had extensively worked on the set:
Is the yoke a Sparton XD-2071-J or RCA 223D1?
Is the flyback Sparton XD-2166-J or RCA 240 T1?
Since that flyback has so many unused terminals and may be a replacement, did you check to see of the correct terminals were used? --- The yoke that came with the set is a Sparton, but I've tried a RCA replacement, with identical results. I think the flyback is the original Sparton - there is no evidence of it ever being removed. I don't have a spare, so I can't replace it. I've checked the connections.

Have you verified that the voltage on the on the HV regulator grid is within the specified range? --- Yes, and I've removed the cap to the regulator tube to isolate any problems in might cause. Voltage is just 20kv with the regulator out of the circuit.

This week I will take a fresh look at it, after having been away for a few weeks.

[roundscreen]

You must have done this but I re read the thread and did not see if you checked the horiz centering control?

[miniman82]

It's not centering, it's width. Shorth on both sides will not be cured by the centering pot.

[Big Dave]

Steve,

This is a longshot, I would try subbing out C137 through C141. I have had doorknob caps go soft. If you're only getting 20kv without the regulator, one or more of these may be the culprit.

[earlyfilm]

Steve,

I was looking at the pictures you have on ETF of this set and spotted what to me looks like an old-stock molded wax paper capacitor in sweep chassis, that looks from your picture like a unit that historically was prone to moisture penetration due to poor seals on the leads. (It also could be a new type that I'm not familiar with.)

I can tell from your picture that it goes near or to the horizontal output, but can't see tell how it connects, however, the fattest condenser near that tube is C130, a .22 at 400 v, that filters stray damper hash out of the horizontal output screen voltage. This capacitor normally has 320 volts across it, plus whatever spike comes from the damper and centering circuit.

I think that a minor leakage in this capacitor would not cause the problem that you are having, unless it dropped the screen voltage, which you said was normal. It is a long shot, but it might be worth a test or a substitute.

James.

[Steve McVoy]

I spent several hours working on it today, rechecking everything. I could find nothing wrong.

John, I experimented with cathode resistor values and found that the original 180 ohm worked best.

So, I experimented with capacitors across the damper. I don't have a good selection of high voltage low value caps, but I tried 40pf, with a small increase in width, and 1000pf, with a very large increase (too much, probably 2 inch overscan on each side). With the 1000pf I still had 20kv, so I'm sure a smaller cap will give me the width I need with plently of HV. I'll order some smaller values.

So, it is still a mystery, but at least I have an operational set. Everything works well now, including the color circuits, and I have the RACS 15G in it (still working fine).

[old_coot88]

Originally Posted by Steve McVoy

... With the 1000pf I still had 20kv, so I'm sure a smaller cap will give me the width I need with plently of HV. I'll order some smaller values.

WOW! That's amazing. The biggest value i ever tried in that application was 120pf (or mmf as we called it in those days).

[Mal Fuller]

I was hoping you'd try that suggestion Steve. And none of us here at Video Karma will ever tell anyone that your set isn't stock!
All's well that ends well.

[old_coot88]

The problem with the damper cap trick lies in the old adage "there is no free lunch". The higher the value of the cap, the more width you get, but the plate current of the H.output tube diminishes. This reduces the "stiffness" of the HV supply, and the HV will sag with CRT beam current and the raster will start to bloom as you turn up the brightness.
So the cap trick is a balancing act between width and HV regulation. Often you have to increase the G2 voltage on the output tube to improve HV 'stiffness'.

[Steve McVoy]

400 pf seems to result in enough width and high voltage. I now have a good picture, but I need to do an alignment of the color section to get the proper colors displayed:

http://www.earlytelevision.org/spart...storation.html

[old_coot88]

Man, that's a cool bit of adventurism 'pushing the boundaries' with the damper cap trick.