Dumont RA-103
 

I am just starting to take a peak at the Dumont RA-103 Chatham. I just uploaded my reference photo's of the chassis to my Google drive. I've never shared photo's so I'm not sure if this will work correctly. Here is a test.

https://drive.google.com/file/d/1_Im...ew?usp=sharing, https://drive.google.com/file/d/1_Ko...ew?usp=sharing, https://drive.google.com/file/d/1_MV...ew?usp=sharing, https://drive.google.com/file/d/1_MY...ew?usp=sharing, https://drive.google.com/file/d/1_QD...ew?usp=sharing, https://drive.google.com/file/d/1_R-...ew?usp=sharing, https://drive.google.com/file/d/1_SR...ew?usp=sharing, https://drive.google.com/file/d/1_Ua...ew?usp=sharing, https://drive.google.com/file/d/1_Wf...ew?usp=sharing, https://drive.google.com/file/d/1_az...ew?usp=sharing, https://drive.google.com/file/d/1_dv...ew?usp=sharing, https://drive.google.com/file/d/1_ek...ew?usp=sharing

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

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.

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.:thmbsp:

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

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.

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Matt Davala

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?

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

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

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 ?

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

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

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!

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Matt Davala

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.

I thought that I might have a heater to cathode short as well. I'll have the chassis up on the bench(desk) later today but I think the voltage did vary last I checked. The brightness range does change from bright to very bright!

Thanks!
Matt Davala

Hi Everyone!

Thought I'd give an update on how the restoration is going! Very frustrating to say the least. There are so many out of tolerance resistors that I've had to swap out. Still no big improvement.. I think I'm starting to focus on things that aren't the real fault here.

My biggest issue I think is some incredibly high B+ in some circuits. I have an extra 30 volts on the cathode of the 5u4 rectifiers but I don't think thats something I can fix and I doubt that's the real issue. I managed to get the plate voltages in the horizontal osc sections within normal but V16 6SJ7(2nd sync clipper) plate voltage is 340 volts! Sams says 250V. 80 volts high. The screen voltage is a bit less. That explains my limited range of sync both vertical and horizontal. I have some sync so its partially working.

A more confusing issue is in my V7 6AL5 dc restoration. The cathode on pin 1 has 20 volts on it! It does vary with the brightness control but only from about 30 volts to 20. Sams says .4 with all controls set at minimum. There are only two resistors there R47 and R48 I replaced the 470K one but the 150K only tested 10 K higher. I doubt very much that it threw it off that much so I left it. The caps have already been replaced and are wired correctly. Pin 2 on the CRT should only have 1.1V but its right there at 20V. Which explains why the picture lacks contrast.

The contrast control only has a small range where the sync falls into place and there is a picture. The screen blanks out when the contrast control is less than a 1/4 and washes out again when set more than halfway. To get any sync its set about 1/3 from minimum. I have to imagine that the way the contrast control varies the gain the IF tubes are not being cutoff correctly.

I am really running out of ideas here. Tomorrow I will write down the plate and screen voltages and see if I can come to some kind of conclusion. I know I'm not far off from having this project complete as everything is nearly working..... Its just not quite there yet..

My B&K 1077B arrived yesterday. Broken.... Poor packing. The fuse holder on the back was broken off and the Horizontal output tube departed the socket and smashed around. Lots of glass shards to clean out inside. Now I am waiting for a new tube and fuse holder. Seems the CRT survived.

Thanks for reading! My brain feels mushy after so many resistance and voltage checks this afternoon. I should have saved myself time and wrote down the voltages on paper today instead I'll recheck them tomorrow.
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Matt Davala

Well I found my big "smoking gun" with my too high B+.. I feel real embarrassed to say but I should have check it earlier. R120, a big 25 watt 6K resistor on the 5V4 damper. Its a damper filter from cathode to plate and its open. I thought I checked it when I first had the HV box open as its very corroded at the terminals. Well I just checked it and its open.. Now to find a replacement.

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Matt Davala

Not to worry, Matt... You found it.

Things are on the final stretch! Got the damper and horz drive working well. Now the width control has no effect(width had never worked)! There is a mica cap right there on the flyback that I had previously changed out(Phil Nelson said in his restoration the mica cap was preventing him from changing width) I'm assuming my width coil is bad.

Neither SAMs or riders says the values for the width coil. There are a few replacement coils on EBAY but what do I go for? What MH am I looking for?

Thanks!
Matt Davala

The width coil has a very subtle effect and yours may very well be good. The width might change 5% from one extreme to the other. It's meant as a fine tune adjustment

I just need half an inch on either side of the screen to get full width. I've already swapped the 6BG6 that originally came with the set to one that gave me a more width. I think I have a 5V4 in my stash of tubes but haven't found one to swap yet.

I'll have to pull the chassis out again to check the width coil as I installed it to watch some TV! The set draws the correct amount of current at 117 volts on the variac. about 2.73 amps.... 300 watts... It will keep the room its in warm during the winter! 28 tubes.... Wow.

I know there are two remaining issues with the set but it provides a very watchable picture. Its logged well over 10 hours with me so far. The Dumont Chatham looks great and was one that I've wanted to have since I was in high school... I'm now 38.... It was about time to finally get one.

Thanks!
Matt Davala

Make sure that the screen resistor for the 6BG6 hasn't gone up in value.

I will double check those resistors. I'll be pulling the chassis out tonight from the cabinet. Dumont was wierd that they used a 22k and 27K resistor in parallel for the 6BG6 screen. Many larger resistors in this set are set up in parallel I assume to dissipate more heat but usually they are the same value resistor.

Thanks!
Matt D

I just changed the sharing permissions on my google drive... whoops! This should work now

https://drive.google.com/file/d/1fdU...ew?usp=sharing, https://drive.google.com/file/d/1fex...ew?usp=sharing, https://drive.google.com/file/d/1fvV...ew?usp=sharing

Just a couple pictures of the current state of things. linearity is off, drive is off and the phasing is off. The picture should start about half an inch to the left as the raster is there when you drop the signal to the TV or turn up the brightness.

The picture I've shown is with the brightness control at its lowest setting. Also the CRT bias is as low as it goes. Signal is from a VCR so not the best I should have hooked up.

I'll be tinkering starting tomorrow!
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Matt Davala

Well I replaced the 6BG6 screen grid resistors. The 22K had drifted up to near 35K and the 27k in parallel didn't drift at all.

Now the set draws way to much current. At 117V its now drawing 2.88 amps. Width is only controllable with the drive control. Any higher than I get those vertical lines on the right side of the screen.

The width coil did ohm out fine but there is 0 change to the width on the screen. Same with the linearity control, it has no affect.

Brightness is still too bright and only gets a tad brighter before blooming. I'm getting 30 volts now the cathode on the DC restoration half of the 6AL5 vs 20 volts before. Should only be about a volt.

What the heck is going on? Voltages are just so high in the set and I'm running out of ideas. The sync tube V16 6SJ7 its plate is still 310 volts. Should be 250, but my B+ off the rectifiers is 30 volts high. That still leaves an extra 30 volts that shouldn't be there. Its been recapped and those resistors check out. The contrast control previously had to be set about 1/3 to get sync and a watchable image. Now it needs to be set 2/3 up and has almost no range before loosing sync. I really need to figure out where I'm getting extra voltage from because its screwing up the bias on many tubes I'm sure.

If I could only get the set to draw the current I was getting before replacing those screen resistors on the HOT I'd run the set as is but with it drawing this amount I know its the horizontal output drawing that extra current. Resistance check on that 6bg6 screen is 12K and about 278 volts with the drive set about halfway.

This set just keeps having problems and its getting very frustrating.

Wish me luck..
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Matt Davala

I finally got back to working on the Dumont. I had to get the horizontal output section working properly as something was way off after replacing the HOT screen grid resistors. The set was drawing much higher current and the drive control didn't seem to be doing what I thought it should do.

I found R113(Sams) a 680K resistor went nearly open at around 3 megs. Looking at the circuit I don't really understand how the set was working with that resistor so far out of tolerance. With it replaced I have plenty of width and the drive control works as it should. The horizontal output tube seems to be running a heck of a lot cooler. The set is drawing 10 less watts! Yikes..

I'm discovering a few resistors that are slightly out of the 10% tolerance. A couple oddball values that I don't have. I'll have to see if I can cobble up anything I have to give me the correct values.

I decided by ear to tune up the audio IF and detector stages as the TV audio was thin sounding. Heck did I wake the sound up! So much better and louder.

So...... The Dumont is pretty much done except for one major problem that's plagued this set since I started working on it. Excessive B+ in nearly all the circuits(except sweep)

Today I checked the 6AG5 1st video IF plate and screen voltages...... 285 to 290 volts. Holy hell, they should be 120volts! The sams did say that their readings were done with the contrast at 100%. At 100% contrast it was still 200 volts. 80 volts high.
The sync tube plate is 327 volts. 80 volts high. The sync tubes screen was 60 volts, only 10 volts high. The 1st audio IF tube, 6AU6, its plate was about 329. . nearly 80 volts high.

Can someone chime in and let me know how I'm possibly getting such high B+ voltages? The TV works so I know every section is drawing current and the B+ should be near normal. I am getting 450 volts on cathode of the 5U4's, thats only an extra +20Vs to the B+.

With the IF sections having such high plate voltages to get a stable picture I have a super tiny range on the contrast control before loosing sync. Again I know the tubes cuttoffs are all incorrect.

No obvious burned parts and nothing smells too hot. Not sure what resistors to go and check, its just going to take time looking at all the resistors attached to the B+.
It has to be resistors, right? Its all recapped.
Thanks!
Matt Davala

From the pictures, it looks like someone has replaced the Candohms with individual resistors. Make SURE that they are all of the correct values, and watch out for any "bleeder" resistors going to ground which may be open.

Been enjoying the Dumont. The set has logged close to 40 hours since I've had it. Still can't get the brightness down. What you see in all the pictures is with the brightness at minimum. Linearity is off on the right side of the screen and I can't get to the control without removing the chassis. Once I order some power resistors I'll pull the chassis again. The set weighs near 90 pounds so its not easy to move around.

Here is the first picture I got on the screen almost two months ago

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

The set worked great with no interference on the screen when I had it upstairs under repair. After lugging it downstairs with my wife's help I get constant interference. Not as bad now with the DVD player connected but still pretty noticeable. I think its coming from inside the set as I've shut everything off I can think of downstairs including our Wifi and it the set still shows interference....

Some current photos.
https://drive.google.com/file/d/1jom...ew?usp=sharing
https://drive.google.com/file/d/1jrj...ew?usp=sharing
https://drive.google.com/file/d/1jxg...ew?usp=sharing
https://drive.google.com/file/d/1k-L...ew?usp=sharing
https://drive.google.com/file/d/1k-k...ew?usp=sharing
https://drive.google.com/file/d/1kFg...ew?usp=sharing
https://drive.google.com/file/d/1kI_...ew?usp=sharing
I sure hope I got the permissions set right this time for the googledrive link..

Thanks!
Matt Davala

Looks good Matt and you must have gotten the permissions correct, I can see the pictures!

I'm back revisiting the Dumont. It's taken a bit of time and motivation to get it out and on the "bench". I finally got around to testing the CRT and it pegged on the grid emmisions. Now I have an idea of why the brightness was so bright even when the control was at its minimum.

Theres a few 1000pf caps that didn't look right on my cap tester, the eye on the tester didn't open as fully as a new cap does. So that got me going down the rabbit hole of looking at some other areas. I remember last time I was testing the set I had excessivly high b+. Its highly likely that the grid emmisions on the CRT might have cause that.. But shouldnt that load down the B+ instead of raising it?

A big issue I stumbled apon was v6(3rd video IF). The grid(pin 1) connects to a 3900K resistor and two coils. The resistor goes to ground. The schematic shows the resistor going to a 6800mmf cap and then to ground as well as through the 100 ohm cathode resistor.

Why would my set be wired this way? The service notes don't mention a production change...

Thanks!
Matt Davala

I looked at the RA-103D schematic and how my set was wired matches. I must have a late production model that incorportates changes in the RA-103D model but do not have the 6x4 bias rectifiers. .

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Matt Davala

The CRT is definately the issue. I get about 15 seconds after the set kicks on of normal brightness control and then the CRT goes super bright. Its going to be a bear sourcing a good 12JP4.

This one really wasn't a winner tube anyway as it has a nice big ion burn in the center of the screen.

Is there any harm in running the set like this with high grid emmisions? Its not like I'm going to make the CRT worse....

Thanks!
Matt Davala

Not sure what you mean by grid emissions. Is there a G1 short? Have you tried to clear it?

You can sub a 12LP4, 12KP4, 12QP4 or 12RP4 but you'll have to change the HV connector.

I'm using my B&K 466 and on the G1 leakage test the needle pegs fully to the right.

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Matt Davala

I ended up trying to remove shorts at least 20 times. The G1 test would indicate good then randomly peg again. So I went between rejuvinate 1 and remove shorts over and over. I figured the tube is tired anyway.

Its been about 10 minutes now with the Dumont running and I now can actually go fully dark for the first time ever. Now I'm dreading moving the chassis back into the set in fear of jarring the CRT.

Thanks Bob for telling me to remove the G1 short. I completly misunderstood that test.
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Matt Davala

Back fitment also changes as the 12LP4 at least is a bit longer and bulges the back out on the doghouse (mine has an LP4 so I know first hand).... It'll still fit but provides no protection for the CRT neck. One good slam against the wall and it's done.