Dumont RA-113 (B3) restorations

[Radiotronman]

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.

[bandersen]

Sorry to hear you got a bad control. Mine just arrived also in their original packing including desiccant. Luckily, both test good


Before I install it, I wanted to sort out the sync and sound issues.


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.


The sync circuit has new tubes, paper caps and bad resistors replaced. That left the micas. Sure enough, found some bad ones!


That improved things, but still not quite right. Very touch horizontal with some bending.


Continued checking micas and found a bad 270pF off the hor oscillator coil. Bingo!


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.


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

[IsthmusTV]

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

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

[bandersen]

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

[Electronic M]

Originally Posted by bandersen

I got a kick of it too. They sure did pack a lot of desiccant in there.

That desiccant sure packs a kick, huh?

[Radiotronman]

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

[bandersen]

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

[bandersen]

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.



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



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!

[holmesuser01]

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.

[Radiotronman]

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

[bandersen]

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

[wa2ise]

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.

[M3-SRT8]

Nice work, Bob.

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