Width & Focus in DuMont RA-103

[jeyurkon]

Maybe it's just the angle of the photo, but to me it looks like a bent gun just like in the 12LP4. Maybe the rebuilder didn't have the proper gun. The Emerson 665B that I picked up recently has a 12LP4 with the single field ion trap in the same exact location that yours is. That seemed strange to me, but now yours is the second set that I've seen one placed that way.

Can you compare the gun to the photo of the one that I posted in this thread?

http://www.videokarma.org/showthread.php?t=246075

They look the same to me but your photo is at a different angle.

If it is a 12LP4 gun with a single field ion trap that may explain the angle of the focusing coil.

John

[jr_tech]

So shouldn't a 12LP4 gun (slashed field ion trap) really have a two pole magnet?

jr

[jeyurkon]

Originally Posted by jr_tech

So shouldn't a 12LP4 gun (slashed field ion trap) really have a two pole magnet?

jr

Yes, but I have two sets that came with only single field traps with their 12LP4. One adjustable single field magnet I have came with instructions saying that it could be used in place of a double field, but that it wouldn't work as well.

I'm not sure if the sets I have were that way originally or if someone replaced them.

John

[Phil Nelson]

Here's a view of the gun. It does resemble the one shown in the other thread that you referenced, but I don't know guns from Shinola. The slanted gap shown in your photo is also present in mine, just at a different angle.

I also took three photos of the set minus the ion trap. The picture is very dim, almost invisible in normal room light. In these pics, the brightness control is turned up all the way.

In the first photo, the focus coil is perpendicular to the deflection yoke, as per the manual. Notice the dark crescent in upper right.

In the second, I canted the focus coil in the same direction as seen earlier. Not quite as cockeyed as shown there, but almost. The picture is centered in the screen and I can get sorta-good (not great) focus by turning the focus control all the way clockwise.

The third is the same as Number Two, but with contrast increased for a better picture.

The picture isn't quite as blurry as seen here. My camera has a hard time finding anything to focus on in a dark room.

I assume the picture would brighten up a lot if I put the ion trap back on. All of which is interesting, but doesn't get me any closer to solving either of these problems.

There are still some things to investigate. The resistors that Eric mentioned earlier. I can check more voltages in likely places. The manual also mentions a CRT bias adjustment which I haven't checked.

Phil

[jr_tech]

Is the gun not straight in the neck ? ... perhaps it is an "offset" slashed field style, which should use a single ring magnet over the gap.

jr

[jeyurkon]

Originally Posted by jr_tech

Is the gun not straight in the neck ? ... perhaps it is an "offset" slashed field style, which should use a single ring magnet over the gap.

jr

That's a good point. Maybe someone can explain why it looks the way it does, but it appears wrong to me. I'd expect the anode end of the gun to be straight and the cathode through G2 to be angled. It looks like only the end of the gun is angled. The beam would exit at an angle.

Of course I've never seen a 12JP4 in person.

John

[jeyurkon]

Originally Posted by Phil Nelson

The width coil was not taken out of circuit. I temporarily unsoldered one of its leads at the flyback and tested it, as well as the 5-6 connections on the flyback. Neither is open.

I will look at the WW resistors next.

Look at how far off-kilter the focus coil has been cranked. Kosher? (If not, why would they give you adjusters with that much leeway?) It looks like none of this stuff, including the CRT, has been touched in ages.

Phil

That's odd. Unless I'm brain dead, which is a distinct possibility, adjusting the core of the coil out should reduce the width in spite of anything else going on. Although come to think of it, adjusting it out should cause the flux in the core of the flyback to be reduced as the current in 5-6 bucks it. This would reduce the horizontal sweep but also the HV and the effects could cancel each other.

I must be brain dead. They wouldn't design something that can't work would they?

If you get bored you might measure the HV at two setting of the width coil.

John

[Phil Nelson]

If y'all are talking about the largest gap shown in my gun photo, both of the pieces on either side of the gap are angled, if that means anything. It's not easy to see in the photo.

Meanwhile, I put the trap back on and fiddled some more. With the magnet rotated to the opposite side of the neck (opposite from the earlier photo), the picture is bright, even with the focus coil made perpendicular as it should be. (Maybe I unknowingly reversed it the other way, too. I didn't mark anything before, and the trap looks identical both ways.)

The trap has to be all the way back to the tube base. Any farther forward, and there's no picture.

Anyhow, now the coil isn't cockeyed and I can get good focus closer to the midpoint of the focus control's travel. Maybe I'll call this good enough.

An additional nit I didn't mention before is that there is slight horizontal "trailing" in high contrast lines. If you look at the very first picture in this thread, it's especially obvious in the E in PAUSE. Horizontal lock is very stable, and moving the horiz hold control doesn't affect this.

I tried the width adjustment again, but still no joy there.

Phil

[cbenham]

Originally Posted by Phil Nelson

I tried the width adjustment again, but still no joy there. Phil

This note and your early comment about the focus coming in momentarily then getting blurry again as well as the higher voltage than shown in the schematic has me wondering if by chance an electrolytic you installed is wired wrong?

This has happened to me. I mis-wired a cap in the power supply and while the set worked, the picture was too wide and the focus control would only get 'close' to correct at the end of rotation of the control.

Cliff

[old_coot88]

Originally Posted by cbenham

This note and your early comment about the focus coming in momentarily then getting blurry again as well as the higher voltage than shown in the schematic has me wondering if by chance an electrolytic you installed is wired wrong?

This has happened to me. I mis-wired a cap in the power supply and while the set worked, the picture was too wide and the focus control would only get 'close' to correct at the end of rotation of the control.

Good point! Hum (ripple) in the focus coil could definitely cause the problem. Phil, have you measured for ripple across the coil?

On the other (excess width) problem, there's two 22K, 2W resistors in parallel feeding the screen grid of the HO tube. Have you measured those? Carbon comp 2W resistors that run hot have a habit of going low in value. That's because the phenolic casing carbonizes, throwing low resistance across the actual element. Bill(oc)

[old_coot88]

P.S.
Phil, just for the heck of it, assuming those two resistors check good, try disconnecting one and see what happens. The width will drop, probably too much. But it may give an indication of where the G2 voltage wants to be to give you the proper width.

[jeyurkon]

Originally Posted by Phil Nelson

If y'all are talking about the largest gap shown in my gun photo, both of the pieces on either side of the gap are angled, if that means anything. It's not easy to see in the photo.

Meanwhile, I put the trap back on and fiddled some more. With the magnet rotated to the opposite side of the neck (opposite from the earlier photo), the picture is bright, even with the focus coil made perpendicular as it should be. (Maybe I unknowingly reversed it the other way, too. I didn't mark anything before, and the trap looks identical both ways.)

The trap has to be all the way back to the tube base. Any farther forward, and there's no picture.

Anyhow, now the coil isn't cockeyed and I can get good focus closer to the midpoint of the focus control's travel. Maybe I'll call this good enough.

An additional nit I didn't mention before is that there is slight horizontal "trailing" in high contrast lines. If you look at the very first picture in this thread, it's especially obvious in the E in PAUSE. Horizontal lock is very stable, and moving the horiz hold control doesn't affect this.

I tried the width adjustment again, but still no joy there.

Phil

Angled as in cut at an angle or the gun pointing at an angle or... It sounds like whatever it is that it needs a trap. This might help explain the adjustment. You have a single field trap.

https://www.msu.edu/~yurkon/Document...adjustment.pdf

I noticed the trailing earlier but I thought it was a tape source causing it. Those kind of problems I hate having to solve. It might be a DC restorer issue. Do they give adjustment info for that section?

John

[Phil Nelson]

Thanks, lots of good stuff to think about.

If I'm reading the article correctly, it says my single-magnet iron trap is in the right spot, "well behind the flags."

With that and the focus coil both adjusted, the focus control works within the middle of its range. You can adjust so the picture is sharp, then too sharp & grainy, and if you keep going, it will eventually blur again. Which seems normal to me.

Re the trailing, a little while ago I switched from the DVD player to rabbit ears (receiving signal from my agile modulator across the house). Maybe that player just overloads this AGC-less set. With rabbit ears, I can see a tiny amount of trailing if I look hard, but I doubt anyone else would notice it. The picture seems generally better & easier to adjust in other ways, too. Riders doesn't have any adjustment info for DC restoration.

Re the width, I checked voltages on the 6BG6 tube. Pins 3 and 5 are right on the money. Pin 8 is a little low, 224v rather than 250v. I checked the resistance of the parallel 2W resistors (R247/R301) in place, getting 14.3K (would the ideal would be 12-something K ?). I don't have any 2W resistors in the house, but it will be easy enough to order replacements and see what happens if I replace those two.

Wish I could post a photo showing the improved picture, but my danged auto-everything camera can't focus the TV image well enough to show the difference. Maybe the bulbed face of the CRT is tricking it.

Anyhow, thx for all of the good advice.

Phil

[old_coot88]

Looks like the ol' gal is shaping up nicely, Phil. Re. the paralleled G2 resistors, they are OK, and if anything, have gone a bit high in value. That's the direction you'd want to go for reducing sweep. oc

[Phil Nelson]

Looking back at a couple of these photos, now I am wondering whether this CRT was rebuilt.

The sticker on the CRT bell is hand-lettered 12JP4, which made me think "rebuild." But the stamped name looks like it could be Hytron. Would a rebuilding shop use a factory sticker? Or something more like "Joe's Rebuilt Picture Tubes?"

The picture of the gun inside the neck doesn't show any welds or seams in the glass. When you rebuild a tube, wouldn't you leave some kind of visible seam in that area?

It's a very strong tube either way. Just curious.

Phil