Dumont RA-109 questions if anyone has one

[kramden66]

I was wondering mine doesn't have a back , the antenna connection is just an rca phono plug , does the back have terminals that plug into this or is this just the way it is ? and is it 75 ohm or 300 ohm ?

Also the 1.75 @ 60~ 850 volts capacitor , that is 1.75 uf ? since it is a big transmitter oil filled type its probably still good ? i will check it , if not good whats a good way to replace it ?

mike

[ggregg]

Mike,
They do have a back. It hooks into some little hingy things on the top panel and then screws in. Meant to be flipped up but it doesn't stay up very good. I believe the antenna connection is 72 ohm so a 75 ohm balun works fine. I left that big cap in mine also.

[Tim]

The antenna connection is 72 ohm as mentioned. Those tuners were designed inside also to accept a 300 ohm connection and there was a service bulletin issued showing how to connect the twinlead if desired.

[ggregg]

I found an RCA to PL59 adaptor somewhere. Probably Rat Shack. Maybe they still have them.

[kramden66]

Thanks
i have the ra-110 and it has the metal flip up back , not a match to the ra-109 and i don't recall what input it has , i though there were screw terminals so if so its probably 300 ohm , so i will just hook the modulator up and un it to that input with an adapter , i might have one , if not rat shack should.

35 tubes to run this monster , incredible , with that many tubes i'll expect 3d lol

mike

[Jeffhs]

Many older (late 1940s-early '50s) large-screen TVs used 30 or more tubes. Remember, commercial television was just getting started at that time. I don't know if there were any such things as multi-function tubes in those days, so it may have been that the manufacturers were forced to use one tube for each function of the TV. I think the very early RCA color sets (CTC3, 4, 5, et al.) had at least 30 tubes; some sets may have had more, as did the early color sets from manufacturers such as Arvin, Westinghouse, et al. The tube counts didn't start decreasing until multi-section tubes were developed.

[Tubejunke]

Originally Posted by Jeffhs

Many older (late 1940s-early '50s) large-screen TVs used 30 or more tubes. Remember, commercial television was just getting started at that time. I don't know if there were any such things as multi-function tubes in those days, so it may have been that the manufacturers were forced to use one tube for each function of the TV. The tube counts didn't start decreasing until multi-section tubes were developed.

This is true and good information for newer folks in the old TV hobby. I just wanted to ad that the DuMonts like that RA-109 were packed with every bell and whistle that the once great company's engineers could dream up. Muntz TVs are proof that you can make a basic TV set with minimal circuitry. Probably would be the best restoration candidate for any beginner, although I have never run into one.

I did however have an RA-109 DuMont years ago and it was really amazing the quality and intriqisy of the build, both electronic and aesthetic. The power transformer probably weighs 20 pounds. They were more or less for the wealthy as an RA-109 was about $1K I think. I personally would like to own a "roundie" table model of that era.

[cbenham]

Originally Posted by kramden66

Also the 1.75 @ 60~ 850 volts capacitor , that is 1.75 uf ? since it is a big transmitter oil filled type its probably still good ? i will check it , if not good whats a good way to replace it ?

mike

This resonating capacitor is part of the power transformer circuit. The power transformer is made by Sola and is a constant voltage ferroresonant transformer. The capacitor is selected at the Sola factory to work with *THIS* transformer.
If you have normal B+ it is working.
Attempting to replace the cap won't make it work any better. If it's bad you'll know it because there will be no B+ at all. Here is some information on how these transformers work...
http://www.powerqualityworld.com/201...oresonant.html

Cliff

[Kevin Kuehn]

Interesting. So what happens if you attempt to bring one of these supplies up slowly on a variac? Seems like it would attempt to draw lots of current at lower than normal line voltage?

[kramden66]

I wouldn't replace or attempt to replace it unless it was bad , if i did i would find one simular , thats why i asked if it is 1.75 uf

mike

[Tubejunke]

Just a footnote on multi-section tubes. I don't like them personally. They may have saved space, but to me they just complicate things when it comes to troubleshooting. Also, I think that the technology existed in the time period this thread is based on, but they were pretty much developmental and slowly became more prevalent around the late 50s and throughout the 60s, till the end in the mid 70s.