Dumont RA-103 - Videokarma.org TV - Video

Davala · #1 09-12-2020, 01:59 PM

Davala · #2 09-12-2020, 02:08 PM

I hope that was an ok way to share photo's. Someone sure has been here before. A bunch of replaced resistors and the focus resistor/rheostat was replaced.

I really was excited to see someone use speaker wire down at the bottom to connect up a couple coils. That shows true dedication to do things the correct way. No, I was pretty upset when I saw the two speaker wire connections. What other shortcuts did the previous repair person do?

That focus pot looks pretty interesting. Micropot? I was able to rotate it more than one revolution so I'm not sure what's up with that. Is this not just a regular rheostat?

I should post a picture of the set powered up but since I'm about to recap I'd rather not worry about charged caps. It only has half a raster and draws a tad more current than its supposed to so its best for me just to replace all those leaky caps. 55 replacement caps from JustRadios... This will take a few days for sure..

Regards,
Matt Davala

Electronic M · #3 09-12-2020, 02:22 PM

That speaker wire is factory. Don't touch it! If you look carefully it actually does not directly connect the 2 coils. One wire of the zip cord goes to one coil and the other wire goes to the other. It forms a very low pF capacitance that couples the 2 stages. Radio men called it a 'gimmick' capacitor.

I've seen this in multiple RA-103s over the years. If you remove it you may have to realign your IF.

There were a few production runs of the RA103 and some used a small capacitor and others used the zip cord gimmick.

Davala · #4 09-12-2020, 02:33 PM

Thank you for the quick response. I didn't change it out yet! The speaker wire sure looked like a cheap fix someone did, but I can see now how the two wires aren't connected. Funny how you can overlook things when just glancing at it.

Right now I'm just looking at replacing the electrolytics. Not going to restuff them.

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Matt Davala

Electronic M · #5 09-12-2020, 06:56 PM

Quote:

Originally Posted by Davala

Thank you for the quick response. I didn't change it out yet! The speaker wire sure looked like a cheap fix someone did, but I can see now how the two wires aren't connected. Funny how you can overlook things when just glancing at it.

Right now I'm just looking at replacing the electrolytics. Not going to restuff them.

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Matt Davala

Your welcome. Good luck on the restoration.

Tom9589 · #6 09-12-2020, 08:55 PM

Does anybody have an explanation for putting the discriminator, V11 (6AL5), under the chassis?

Davala · #7 09-15-2020, 02:20 AM

Well I'm 3/4 through recapping. I went through the electrolytics, horizontal section, vertical, part of the sync, and the audio caps. I finally have a full raster. The old cap was shorted on the cathode bypass on the vertical output.

On the second powerup I failed to ramp up the variac and a resistor smoked in the audio. The time delay relay is open and I have the B+ bridged until I have a replacement relay from Tim. I think the resistor that smoked was already on its was out as there looked like it was taking a carbon track journey! It was R53(Sams photofact) I check its resistance and it was only about 40 ohms. Clearly it was shorting out with the carbon track.

I also noticed I lacked voltage on the plate of the video output and discovered L26 peaking coil was open. On taking it out one of the leads simply detached from the body. I had continuity in the coil so I had to work at resetting the lead into the body and getting the coil to stick to the lead. It is hair thin..... I got it working and now have plate voltage on the video output tube.

Now my biggest issue is with excessive voltage on a few tubes.... After troubleshooting to get good focus, the focus pot is near 0 ohms to get good focus. Mind you its a 1k pot! That puts the plate voltage on V9 and V10(and I'm sure other tubes on that B+ line) 60 volts HIGH!!! The picture tube brightness is at full and unable to dim since that B+ line supplies the cathode of the CRT.

I know that getting good focus is dependent on load through the B+ since the focus coil is in series with the B+. I have to think that something isn't drawing enough current to load the coil(and to lower my crazy high B+)So instead of going down the troubleshooting of that I will finish recapping the rest of the set. I don't assume that replacing the remaining caps will bring down the voltage and increase the current... won't leaky paper caps tend to bring down voltage instead of causing it to be 60V high?

I'll get back to recapping Wednesday and then troubleshoot whatever else shows up.. I am happy that some things are working. Its tough trying to tell my wife that certain things are working on the set but then she asks is the tv "working".... Noooot really... Saying that I have a raster and noise through the speaker doesn't cut it as a working set. I am happy that some of the important items are working though.

The CRT appears to be a dumont original straight gun with a very noticeable ion burn in the center of the screen. This set must have been old enough where the bent gun and ion trap magnet weren't introduced yet.. Eventually I'll look at another tube that requires an ion trap. There are a couple 12 inch roundies on ebay. Got to get the TV in good running order before gambling on a good CRT.

Sorry for the length of my story of the Dumont.. Its a lot to take in and wrap my head around. I love the challenge though, as long as critical parts keep working.

--
Matt Davala

Davala · #8 09-16-2020, 08:12 PM

I got it all recapped. Still getting that additional 60 volts in the sound IF B+. Near 320 volts when it should be 260. Havent checked the video IF plate voltages yet to see if they are cranked up yet.

I get about 285 volts on V10 when the focus control is up near 1K and 320 volts when the pot is 0 ohms. Focus is sharper when the control is near 0.

No signal passing through anything of course since the voltages are so far off.

Resistance checks tomorrow.

Any ideas on anything specific to check?

Davala · #9 09-23-2020, 01:19 PM

https://drive.google.com/file/d/1bcH...ew?usp=sharing

This set has come a long way to get a picture. My video source is an RCA selectavision. Not the greatest in stability but its what I have in that room until my B&K 1077B comes later this week. So many ceramic and mica caps were bad. Also a bunch of resistors were off. I still think I have a heat related resistor going out of tolerance as the contrast looks great right after it powers up but within a couple minutes it starts to get way too bright in the dark areas.

Width is not working, but thanks to Phil Nelson's restoration there is a mica cap I have on order to replace the one in the High voltage box. Horizontal drive is off and I'm hoping that will be corrected by fixing that width and also a couple more ceramic caps in the Horizontal output section. The tube is ever so slightly red plating with a 1/4 turn on the drive control. At its minimum the tube red plates..... So I still think the HOT is drawing way too much current...

And my brightness cannot be dimmed. In the picture I sent in the link that is pretty much the dimmest it goes. Now that I know the circuits a bit more I see that the HV boost supplies the B+ that goes through the focus coil to the audio amp section as well as the cathode of the CRT. So if the width and drive are off I'm thinking that the boost is out of whack and there is too much B+ in that bus. Also the focus coil is near the lowest it goes to get good focus... All of that points to the HOT and damper circuits.

This set has been the most challenging and fun Tv's I've worked on. I can't wait to finish the last few remaining items!

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Matt Davala

bandersen · #10 09-25-2020, 11:00 AM

Looking good

I haven't worked on this chassis so don't have any advice but wanted you to know folks are following along.

bandersen · #11 09-25-2020, 12:16 PM

As for the lack of ion trap, I'm not sure why Dumont went without one in the 12JP4.
They were most definitely around by this time. Just about every other set used one going back to 1946. Aluminized CRTs were around as well.
Perhaps it had to do with patents or using up existing inventory ?

EdKozk2 · #12 09-26-2020, 12:42 AM

Hi Matt,
These are a few pictures of the relay substitute I found in the Crosley 9-407M, a RA-103 clone I recapped / repaired this past February. The ballast tube mounts on the side of the HV cage. The octal connector is plugged in under the horizontal output tube.
Ed

Boobtubeman · #13 09-28-2020, 11:46 PM

I have one of these with the 6AL5 underneath, anyone have a schematic so i can start studying?

Meanwhile, following the thread to brace myself for what i may face..

SR

Davala · #14 09-29-2020, 12:42 AM

EarlyTelevision has both the riders and the sams for the Dumont.

http://earlytelevision.org/tv_schema...ms_dumont.html

The riders has an in depth look on how the circuits work.

I got a bunch of mica caps from Justradios to hopefully finish out my RA-103. I'll get to it Wednesday. Please let it fix my high B+ and undimmable brightness!

--
Matt Davala

Tom9589 · #15 09-30-2020, 08:08 AM

Check the voltage on Pin 9 of the CRT while you vary the brightness control. If it doesn't vary, you might have a heater-to-cathode short in the CRT. An isolation transformer (looks like a CRT brightener) can cure that problem.