Sparton 16A211 Width Problem

[Steve McVoy]

OK, my turn with a 15 inch color set issue. 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. Like all 15 inch sets, the major components are RCA.

This thing is driving me crazy. Anyone have any ideas?

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

http://www.earlytelevision.org/image...ton_16a211.pdf

[Pete Deksnis]

Since the boost is developed during the first third of the sweep when the horizontal output tube is off and the damper is conducting current generated by the collapsing magnetic field and the boost is low, and the magnetic field generated by both yokes you tried is inadequate, I'm going to take a wild stab and wonder if the fly isn't the cause. Maybe something easy to rule out like conductive crap of some kind across the gap in the core.

[Penthode]

Originally Posted by Steve McVoy

OK, my turn with a 15 inch color set issue. 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 had a recent problem trying to get adequate width with a set using a 6BG6. I use an AVO Mark III transconductance tube tester and a bad tube will give only just adequate transconductance. The checker showed that the plate current was a bit low indicating cathode emission was down. Keep an eye that the plate current under test is correct and not low. I would try a few 6BG6's by substitution.

I suspect the Sparton's 6BG6 emission is low since the cathode voltage you indicated is low.

Question: Why did Sparton use a 6BG6 which such an old design tube in a color TV? I would consider substituting a 6CD6 as I believe it has a beefier cathode? (My guess after only just visually comparing tubes).

[Steve McVoy]

This set uses parallel 6BG6s, probably the only tube available in 1953 when the set was designed. I've tried a bunch of NOS tubes with no luck.

[oldtvman]

What about using a capacitor sub box and dial in some different values for c133.

[kx250rider]

Is that the one from G. in Orange County, CA? If so, he might have some input on that problem. I remember him having that set all apart and chasing a yoke-related problem, if I remember correctly. He may have swapped the yoke, but I'm sure if he did, he'd have kept records and the original. Again, I might be off on that, but it's worth asking (assuming this is the Sparton from him).

Charles

[Penthode]

Originally Posted by Steve McVoy

This set uses parallel 6BG6s, probably the only tube available in 1953 when the set was designed. I've tried a bunch of NOS tubes with no luck.

The 6BG6 was a carry over to the tube base octalization process in the mid thirties. It started out as a UY (5 pin) based 807. By about 1951-52, the transition was on to newer and better power tubes. Your Sparton from 1954 is the latest set I can recall using a 6BG6! (I will research this further and would appreciate feedback from everyone on this). Incidently, it is one of my favorite tubes: see my avatar!

Nevertheless, the 6BG6 cathode current appears low and the voltage drive to the grid appears adequate. To get the cathode current up a bit, you may try increasing the screen voltage slightly by reducing the screen feed resistor value. If you increase the screen voltage slightly to raise the cathode current, I would expect you will get the width you require.

The low width may be inherent in the design and it seems with the considerable effort that has expended elsewhere to fix this problem, I would see this solution as a good bet.

Terry

[miniman82]

Originally Posted by Steve McVoy

This set uses parallel 6BG6s, probably the only tube available in 1953 when the set was designed.

The proto set had a 6CD6 dated the 17th week of 1951, parallel 6BG6's must be design preference. Much the same way my split chassis Philco used parallel 6BQ6GT's, when it could have used a single tube.

[Steve McVoy]

Yes, this set is from G in Orange County. There was evidence of experimentation in the horizontal section.

However, I've tried a new yoke (no change). Also tried increasing the screen grid voltage with minimal change in width.

As for the low width being an inherent problem with this set, that isn't the case. All 15 inch color sets use RCA components, and the sweep sections are very similar. Width has never been a problem with these sets.

I'm leaning toward the flyback as the culprit. When I get back in Ohio in a couple of weeks I'll try swapping it.

[dieseljeep]

Originally Posted by Steve McVoy

This set uses parallel 6BG6s, probably the only tube available in 1953 when the set was designed. I've tried a bunch of NOS tubes with no luck.

The first Westinghouse color set also used two 6BG6's in parallel. I thought it was strange, as the 6CD6 was out at that time.

[Steve McVoy]

I think both the Westinghouse and Sparton were designed earlier, since both had prototypes in 1953.

[Penthode]

Steve,
Any update on this? Did you manage to get the correct cathode current on the HO Tube(s)? What about the flyback transformer?

[Steve McVoy]

Just got back in town. Ill get back to it this week.

[Steve McVoy]

I messed around with it some more today. I rechecked all the components in the horizontal section and tried new tubes again. Everything is normal. I experimented with changing the screen grid resistor in the hor. output section and found that with the resistor shorted out I got about 16 v. on the cathode, indicating close to normal current, and width that is about 1/4 inch short on each side. Boost is still low at abut 560 volts, and actually dropped a couple of volts when I shorted the screen reisistor. Something is still wrong, but I'm sure I can get the small amount of additional width needed somehow.

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

I don't have a suitable replacement flyback to try.

Any ideas?

[miniman82]

Check the boost cap, if boost is low scanning width might be low as a result. I think it's loosing power from the horizontal section somewhere, even if the caps are new check them again. Make sure the cap doing boost duty is rated at least 1000 VDC.