Width & Focus in DuMont RA-103
 

These issues aren't necessarily related, but why not ask two questions for the price of one?

I have replaced all of the electrolytic and paper caps in my DuMont RA-103. Nice bright, stable picture, great audio. Two minor issues remain.

1. I can't reduce the width using the horizontal size adjuster. It seems to do nothing, in fact. (Horizontal hold and position do work.) If it overscans this much on the bench, I'll lose even more (1/2 inch on both sides) when it's in the cabinet behind the mask. I haven't fiddled with linearity because it looked correct when I put on a crosshatch pattern.

2. Focus ain't quite there. See photo. The focus control has an effect, but the picture should be sharper. When I'm adjusting, it reaches a point of maximum sharpness, then gets more blurry again.

Voltages look generally good, although I haven't checked everything. I have 329v on the 300v line leading to a leg of the focus control.

If I could fix only one problem, it would be the focus. It bugs me when an adjuster (width) doesn't work at all, tho'. Here is the schematic.

http://antiqueradio.org/art/temp/DuM...Schematic1.jpg
http://antiqueradio.org/art/temp/DuM...Schematic2.jpg

Thanks for any ideas.

Phil Nelson

Do you have a High Voltage probe? perhaps the two problems are related to this being too low?

I just checked. It is 9.5 KV.

Phil

Phil,
To reduce the width a little, one thing you might try is to drop the screen (G2) voltage on the HO tube slightly. The caveat is that the HV will drop a bit too. I dunno if this would be acceptable or not. Bill(oc)

An open L220? You could try momentarily shorting it and see if the width reduces.

John

How are the wire wound resistors, 285 286a & 286b? I think those are all in one unit?

Check to see if the width coil was cut out of the circuit earlier as a short cut to increase width. I would avoid shorting the width coil as it will overload the flyback.

That was why I suggested "momentarily." Or would that still be too much of an overload? I'd feel bad if my suggestion caused damage.

I suppose besides L220 being open, or cut out of the circuit as you suggested, L204 could be open between 5-6.

John

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

http://antiqueradio.org/art/temp/DuM...Deflection.jpg

I don't think it should be that far off square, the adjustments are used for centering aren't they?

I'd try adjusting it, preferably moving that one corner closer to the yoke.

I notice your Ion trap seems too far back, near the base of the tube, try flipping it over front to back and see if you can move it further forward.

After you flip it the magnet should wind up on the opposite side of the neck.

BTW, is this a Dumont 12JP4 tube? They don't use a trap at all. If it's been replaced with a 12LP4 or similar then it should have a trap but it might also explain why you can't get the focus right.

I can't find any spec for HV for the set but 9.5 KV sounds ballpark... The Max spec for the tube is 12KV, so I would not be surprised to find 10 to 11 either.

That whole yoke/ ion trap magnet setup looks strange! Almost looks like the trap magnet is mounted on the base cap.... Is the gun ok? no broken rods or anything like that?

jr

I assume this is a rebuild sticker, which says 12JP4. It's on the bell, halfway under the cushioned mounting strap.

It would be easy enough to take off the ion trap and see what happens.

When I first got the set, I feared it might have a bad video problem, since there was nothing on the screen but extremely faint, wandering mush. Small adjustments of the ion trap did nothing. At one point, I tried moving the trap around a lot more and a bright picture suddenly snapped in.

The trap was nowhere near working in its original position. Now the magnet is rotated roughly 180 degrees and it has been moved slightly front or back. Of course, a trap could have been nudged by accident in moving. The previous owner had the chassis in and out of the cabinet at least a couple of times.

I don't know who would put an ion trap on a CRT that doesn't use one. Maybe the same guy who hotwired the ginormous 10 mfd cap in the power supply.

Phil

http://antiqueradio.org/art/temp/DuM...ildSticker.jpg

That tube shouldn't use a trap, unless the rebuilder retrofitted it with a bent gun to prevent the Ion burn problem associated with the 12JP4.

I would try it without the trap, you might then have to reposition the focus coil to get the picture back on the screen.

The picture looks like a straight gun tube. If so, as Eric says, it should have no ion trap. That may be a source of the problem and cause the focus coil to be shifted to re-center the picture.

Make sure the yoke is pushed forward so it is up against the tube. Remove the ion trap and see what the picture looks like.

If it does not focus, note which end of the control gets closest, min or max.

Don

I temporarily removed the trap, and confirmed that it produces no picture at all with the current setup. Back on, it does make a bright picture, but only if pushed all the way back to the CRT base and rotated to the sweet spot.

I just looked back at the Riders manual, which says the focus coil should be perpendicular to the CRT axis and 1/8 inch behind the yoke. Looks like you guys might be onto something. Forehead slap. Maybe someday I'll learn to RTFM, as we used to say back when I was a technical writer.

Phil

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

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

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

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

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

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

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

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)

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.

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

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

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

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

But the gun does not appear to be a 12JP4...remember a 12JP4 does NOT require an ion trap, your gun appears to be a tilted offset/ slashed field type which requires a single ring magnet. Why would the factory put the wrong gun in a tube? I suspect that rebuilders often made substitutions when the proper gun was unavailable.

The re-neck seal could possibly be hiding under the yoke or focus coil.

Also, the first letter of the logo on the label does not look like an "H", but instead has a closed top... so I don't think it is "HyTron" but perhaps something designed to look similar.

jr

Any idea of which type it could be? I think this is a diagram of the tilted offset/slashed field that you're describing. It's from the 1969 RCA tube design book.

John

Perhaps 12QP4, RP4, UP4, ZP4 ...? I don't know,but all of these 12" types are shown in my old (don't see a date...early 60s?) GE tube manual, as requiring only a single magnet.

jr

I know a 12JP4 is shorter than a 12LP4 (maybe because it doesn't need a trap?), also a genuine Dumont 12JP4 is a lightweight tube with a thin somewhat rounded face, possibly Pyrex glass, and the envelope is shaped somewhat differently, also the anode connector is different having a small ball inside for the connector, the 12LP4 is heavy and has the usual thick flat face and just looks like a bigger 10BP4.

That said I have an unmarked tube that is supposed to be a 12JP4 but it looks like a 12LP4, it doesn't use a trap though and it has an Ion burn just like most of the non-trapped 12JP4's

If you Google the numbers you will find some PDF's with the data for both.

The ETF has a picture of a real 12JP4, if you look closely you can see it's more rounded and there's a bump where the neck meets the bell.

http://www.earlytelevision.org/postwar_crts.html

This tube has a little ball inside for the anode connector, and the face is slightly rounded. Without pulling the CRT forward, I can't see clearly whether there's a bump where the neck meets the bell. When you push the deflection yoke forward, the bell (or bump) actually touches the coil inside before it meets the rubber pads. There's a gap of maybe 1/4 inch between the bell and the pads.

The gun looks like the diagram. The ion trap doesn't work if placed right over the gap as in the diagram, however. The only spot where it's bright is pushed back against the tube base. Don't ask me what's going on here, but the CRT is as bright as any B/W tube I've seen.

The bulbed face drives me nuts when I'm trying to take a screen shot. The image looks sharp to my eye, but not as sharp in the photo. Maybe the camera's auto-focus can't decide what to focus on . . . .

Phil

Man! I've learned more about CRT/gun design, ion traps and rebuilding this week than in all my collecting years combined.

Good luck getting it humming along properly.