Admiral 20B1 Combo restoration

[SwizzyMan]

This set is really confusing me. The power supply is no longer smoking but I STILL can't get a raster on the CRT i have about 6 to 7 KV but the CRT is weak. But I can't even get a very dim raster. This is really puzzling and I am not sure where to start with diagnosing and fixing this problem. All filter caps stay cool (I will replace them when I can get a raster). All tubes have been replaced except for the horiz out tube that is somewhat weak and is only a quarter way through the good zone and fails the trans-conductance on my hickok 600a. Any ideas where to start?

[bandersen]

That's definitely enough HV to produce a raster. Try moving the ion trap magnet around. If it's the typically dual blue/black Admiral magnet, the black magnet goes towards the base of the CRT.

Also, check the voltages on G1, G2 and the cathode of the CRT. Could also be an issue with the focus coil. Check it's resistance.

[Phil Nelson]

You can rotate the ion trap magnet as well as push it forward or back along the neck.

There should be a definite sweet spot where the CRT achieves maximum brightness with no shadows around the screen edges.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[holmesuser01]

I'm restoring a DuMont RA-113. It was totally dead when I got it. Someone had burned up the power transformer when they plugged it in. I have replaced most of the electrolytics, and some of the other caps. So far, I've now got sound, and FM radio! I've also got no image on the screen. My CRT checks fine, and I now have 12kv at the anode after replacing the HV capacitors. I can hear the vertical running. I'm not seeing any signal from the video output tube, yet. It's a challenge that I like. I'll get it going, yet!!

[SwizzyMan]

Good news!! Got a raster and picture finally! Was an adjustment of the ion trap. The picture is lacking vertical height,lack of focus,and the vertical and horizontal holds are pretty bad and barely lock at all and dont lock for long. I also noticed that a small little portion of the h out tube plate is glowing red. It is concentrated in the middle of the plate on two sides and is a small dot. Picture is nice and bright. I did a full recap of all the paper caps in the TV and some off value resistors. I have not replaced the filter caps on the TV but none of the caps are getting warm but one does get a bit hotter than room temp which is part of the vertical sweep. Any suggestions on where to go from here? I would assume restuffing the filter caps would be a good idea.

[Kamakiri]

Wow. I don't even put power to the chassis of a set of this vintage without doing the filter caps......

[holmesuser01]

I don't like the red-plating horizontal output.

[SwizzyMan]

Most likely the filter caps not powering it on again till they are replaced.

[holmesuser01]

Hope there's room for the new capacitors. There's plenty of room under the hood on the DuMont, so I used regular radial lead capacitors and anchored them in place.

[Phil Nelson]

For the record, feeling caps to see if they're hot is not a scientific (or reliable) method of testing whether they are good. If you have ever watched a power transformer burn up before your eyes -- ask me how I know this -- you will be very careful about powering up with 50-year old or 60-year old electrolytic caps in your power supply.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Kamakiri]

Originally Posted by holmesuser01

Hope there's room for the new capacitors. There's plenty of room under the hood on the DuMont, so I used regular radial lead capacitors and anchored them in place.

There's a ton of room to route caps under the chassis of this set.

SwizzyMan, you do good work, but there are no shortcuts on something like this. On a 1940s/early 50s set, you *have* to start by recapping the whole thing, then work from there. Even if for some reason the old wax caps and cans are good, the chances of the set being a reliable performer are close to zero....let alone the damage that might be caused.

At this point in the set's life, you are doing something the engineers never designed the set to do....operate after 60 plus years. You have to throw the diagnosis and repair paradigm out the window until after all the caps of every style are replaced (except micas, which are fine about 90% of the time). I usually replace all resistors with a value of 5 watts and over, as well, before the first power up.

[SwizzyMan]

Will be sure to restuff those filter caps Saturday and then continue troubleshooting my current raster imperfections. I do hope you all knew that I had no intention to ever leave those filter caps in. But now that I know there is hope I can continue. I think I am just going to restuff the electrolytics instead of route them under the chassis, I have a particular fascination of sawing the filter caps off and seeing how bad they have deteriorated over the years. Also thanks for the advice kamakiri!

[Kamakiri]

You can restuff them if you want, as many people do. Just bear in mind the sizes of the caps you're getting.....they all have to fit inside that tube.

When you cut apart the can, you're going to find that the wire terminals are attached to a thin ribbon of aluminum which won't take solder. You'll need to drill holes in the phenolic wafer and feed the wires down through the chassis. Doing so successfully is an art form.

Myself, I'm a mechanic . I do the filters by snipping the wire terminals off the can one at a time, and use the can ground to anchor the new cap in place (most of the mounting tabs already have holes in them....some unused). I then trim and de-solder the cut terminal, which leaves a ring that I can slide right over the terminal wire of the new cap...then solder in place. Naturally, I use all axials for this.

With practice, the repair is neat and clean, and doesn't disturb too much underneath, facilitating future diagnoses if you ever need to.

[SwizzyMan]

Routed the caps under the chassis after I ditched the restuffing idea. Have full vertical and horizontal deflection now. But I cant get the horizontal or vertical to lock. I can get both pretty close to locking but I just cant get it to stay. Any ideas on where to start since I have recapped the chassis and replaced a few resistors.

[Electronic M]

check resistors in the synch separator.