The Family DuMont

[CaryLee]

Originally Posted by earlyfilm

Cary, if that first picture is not a composite picture with the CRT image and the cabinet from two different exposures, that is a good picture tube. Back when this set was made, most people watched TV in a semi darkened room! Most 19AP4's had a gray faceplate that absorbed a bunch of light to allow the set to produce a darker black in a lighted room and hide reflections. Even when new, that tube would not be nearly as bright as a modern tube.

I'm using factory numbers for the R113 and I am confused as to what your are talking about. Did you change C-245 the 270 µµf mica capacitor that goes between pin 4 of the 6SN7 and pin F of the horizontal transformer Z-210?

If so, C-245 will affect horizontal sync, but I don't see any way this capacitor could affect vertical retrace in your picture or the brightness (provided it did not change the width of the image.)

Have you replaced the 6SN7 that changed the picture when thumped? This usually is the sign of a shorted tube. If it now is in the sync circuit, it could well be the cause of our problem.

If C-245 is the part that you changed, you need to recheck all your changes for a wiring error, wrong value component or an over heated pin on the transformer while soldering, or a cold soldered joint.

Once you get the set back to working, for either retrace or lack of brightness on a set without traditional sweep blanking, the first thing I would check is the DC restorer.

My first check in the DC restoration circuit would be to measure the 1 meg resistor on cathode and check the voltages on that tube and also try a substitute 6AL5.

Where your retrace only shows at the top of the screen, usually this is caused by a wave shape error on the vertical sweep spike, although I've seen a gassy CRT do this. (The spike is the part of the sweep where the beam moves back to the top and if it slows down near the top, it often will be visible.)
Jas.

You may have a point about me expecting more from the CRT than it was designed to deliver. I don't have any point of reference to compare to since I never have ever seen this set working before. I'm probably expecting too much from 1950! However, that image of the picture is with the brightness and contrast almost all the way up. Towards the middle of adjustment, it would fade to black. And I will say that the photograph does make the picture appear slightly brighter than it is in real life. But I did take that photo this afternoon, with daylight in the room.

According to Phil's write up of his RA 113, he experienced a dark picture because he wasn't getting the high voltage required. He states he was only getting about 4000 volts, when the tube was supposed to be getting around 12000, I may have misunderstood what he wrote, but he found the waveform associated with C-245 incorrect, which seemed to be associated with the voltage deficiency. I'm not real knowledgeable in this, so I was duplicating what he did.

http://www.antiqueradio.org/DuMontRA-113Television.htm

It did seem to work somewhat. I don't have any way at hand to check high voltage, but I do have some old electrical test gear out in the shed that I'm going to dig out tomorrow. I think on of the testers will test high voltage, but I don't remember how high.

You hit the nail on the head about the C-245 mica affecting horizontal sync. I found that little "extra" adjustment on the chassis, right next to the 6BG6G tube. Once I made the adjustment and got the picture back, it did seem to be a bit brighter. But I could be seeing things, and it was getting dark outside by then.

I don't have any spare 6SN7 tubes yet, nor 6AL5's. I plan on getting a stock together of extra tubes for the set, but haven't had time to to locate and order them all yet. I'll make sure to put 6AL5's on the list of "must haves" too.

This sure is a learning experience! You will probably get a kick out of my next post!

[earlyfilm]

Originally Posted by CaryLee

. . . . However, that image of the picture is with the brightness and contrast almost all the way up. Towards the middle of adjustment, it would fade to black. And I will say that the photograph does make the picture appear slightly brighter than it is in real life. But I did take that photo this afternoon, with daylight in the room.

You just casually mentioned what probably is the best clue yet on your brightness issue.

If the image blooms (it suddenly gets bigger as it dims out) before it goes out, look something restricting the HV current to the picture tube. This is the normal symptom of sudden high voltage drop. The most usual suspects are the 1X2 tubes. And no, you cannot test one 1X2 by switching it with the other one as they work together to form a voltage doubler. *

If he image does not bloom, and the image stays close to the same size, look for the problem in either the video output, DC restorer, brightness control or picture tube circuit or the CRT itself. (The latter is not too likely.)

* If you had a scope, you could confirm that the waveforms and signal level going into the grid of the horizontal output are correct. You really need a known accurate HV meter before you go messing with the HV adjustments.