I finished recapping the second chassis and fired it up with an 8XP4 test CRT and substitute yoke again

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Noticed a few problems right away.
* No dial lamps and eye tube barely glowing.
* Good FM radio reception, but no sound for TV stations
* No sync
* Power resistor subbing for focus coil cold
* Horizontal periodically "freaks out"

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I quickly solved a few issues. One dial lamp was burned out and the other two had dirty contacts.

The focus control has a large "dead" area. Setting it in a good area gets the sync working better and the brightness/contrast improves. However, the power resistor I'm subbing for the focus coil is now roasting.

Eye tube is totally dark.

Horizontal still freaks out periodically and no sound for TV reception. I'm thinking the sound alignment may be way off.

Also discovered the tuner mounting screws very all very loose and reception cuts out as the fine tuning is adjusted. I bet it's very dirty inside. it will be unmounted,cleaned and lubed.

I'll clean all the controls and tube sockets and swap in some fresh tubes.

Not the worst first power up, but still many issues to resolve.

Need to swap the horizontal yoke leads too.
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Sure enough the focus rheostat is bad :( It has a Dumont poart # and is likely the original. Should be 1250 not 7.8M !
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The break is somewhere near the scratch mark..
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So with the tap on one side of the break, all the power goes through the 1450 ohm power resistor, on the other side the focus coil which explains why it was sometimes cold and others blazing hot. Either way, the set is being starved of power for the tapped bleeder resistors.
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I'll have to track down a replacement. I'll see if I can dig up a couple 680 ohm power resistors in the meantime as a substitute.
Also tested all the tubes and found a bad 6AB4 and 6J6 in the tuner and two bad 6AU6 in the IF.

The set must have been in a heavy smokers home. All that goo protected the chassis nicely.
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Also discovered this dangling coil near the tuner. That doesn't look good :no:
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Bob,

Surplus Sales of Nebraska has a 1K 25 Watt wirewound pot that's a good substitute for the focus control, and it's only $8:

http://www.surplussales.com/Potentio...sRheost-4.html

Good luck,

Clark

Thanks for the tip. Now I'm looking for more goodies there to fill up my order :)

Order at least two of them. I ordered two to meet their minimum order and one of them measured over 3megs. They were both brand new in their vintage packaging too.

Sorry to hear you got a bad control. Mine just arrived also in their original packing including desiccant. Luckily, both test good :)
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Before I install it, I wanted to sort out the sync and sound issues.
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I put a scope to work and found the horizontal synchro waveform looked great, but the synch signal feeding it had a lot of video present.
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The sync circuit has new tubes, paper caps and bad resistors replaced. That left the micas. Sure enough, found some bad ones!
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That improved things, but still not quite right. Very touch horizontal with some bending.
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Continued checking micas and found a bad 270pF off the hor oscillator coil. Bingo!
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Continued checking micas into the sound circuit and found a bad 47pF on the audio output tube. Damn you micamold! I went ahead and replaced the two other micas caps in the sound IF just for good measure.

Finally, I have sound! Not the best but I expect I need to tweak the sound IF alignment after replacing those caps.
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I also replaced the very worn out eye tube and it's working great now:)
Looks like I'm in the home stretch on this project :)
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Looking good, Bob. Maybe I should check those mica caps in mine, particularly the ones in the sound IF.

I bought just one of the SSN rheostats and it was good. I get a kick out of opening 50+ year-old NOS components in the original packaging :yes:

It's nice that those 6AL7 eye tubes are so cheap (I think I paid about $4 a piece for the ones I got). A far cry from a 6E5!

-Clark

I got a kick of it too. They sure did pack a lot of desiccant in there. The owner of these chassis did me a big favor by including a full set of NOS replacement tubes when he dropped them off. No need to go hunting through my stash in the attic :)

That desiccant sure packs a kick, huh?:beer::drunk::D

Bob, where do you order your mica caps? Some places are incredible high and some have a very limited selection of values.

Mostly Mouser Electronics and Digi-Key. Yes, they are rather expensive but they have a good selection of values.

I replaced the candohm resistors with chassis mount power resistors. Although the originals in this chassis were OK, there were failures in the other two I've worked on. So I figure it's best to replace them.

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Also got the replacement focus control installed. Those big 100 ohm resistors barely even get warm.

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Did a little touch up on the alignment and the sync is even better now. Sound much better too. One of the slots in the thread shafts for the sound IF was spread apart like a screwdriver had been jammed in it. Seems likely someone had tried to improve the sound in the distant past so I wasn't surprised that it needed adjusting.

I also cleaned and lubed the tuner. I'm calling this one done which leaves fixing up the tuner on the other. I'll be glad to get these beasts off my workbench!
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Just seeing how good these sets look in your posts gives me hope that my own RA-113 will look this good when I'm finished with it.

Mine has sound through the tuner, picks up FM just fine, and has HV now. I can hear the vertical oscillator running. Still nothing on the screen, but my CRT tests very good, and is the DuMont original 17BP4.

Your Dumonts look great Bob! Gotta posts pictures of them back in the cabinets.

The owner just dropped off the chassis and he only has one cabinet back at his place. The 2nd chassis is a spare

Micas don't like having DC across them. Usually, in tuned circuits they are in parallel with a coil, thus almost no DC. But I've seen them go bad with as little as 50VDC across them.
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Nice work, Bob.

I've used those SSN variable wws in my DuMonts. They work fine.:smoke: