Width & Focus in DuMont RA-103 - Videokarma.org TV - Video

Phil Nelson · #1 10-26-2009, 09:29 PM

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

jr_tech · #2 10-26-2009, 09:34 PM

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

Phil Nelson · #3 10-26-2009, 10:25 PM

I just checked. It is 9.5 KV.

Phil

old_coot88 · #4 10-26-2009, 11:39 PM

Quote:

Originally Posted by Phil Nelson

...
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.

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)

jeyurkon · #5 10-27-2009, 12:18 AM

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

John

***Eric H*** · #6 10-27-2009, 01:14 AM

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

Don Lindsly · #7 10-27-2009, 10:29 AM

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.

jeyurkon · #8 10-27-2009, 10:55 AM

Quote:

Originally Posted by Don Lindsly

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

Phil Nelson · #9 10-27-2009, 12:48 PM

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

***Eric H*** · #10 10-27-2009, 01:22 PM

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.

jr_tech · #11 10-27-2009, 01:51 PM

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

Phil Nelson · #12 10-27-2009, 02:03 PM

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

***Eric H*** · #13 10-27-2009, 02:06 PM

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.

Don Lindsly · #14 10-27-2009, 02:09 PM

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

Phil Nelson · #15 10-27-2009, 02:39 PM

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