Dumont RA-103

[Davala]

Originally Posted by Tom9589

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

[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

[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.

--
Matt Davala

[kvflyer]

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

[Davala]

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

[bandersen]

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

[Davala]

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

[Tim Tress]

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

[Davala]

Originally Posted by Tim Tress

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

[Davala]

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!
--
Matt Davala

[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..
--
Matt Davala

[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

[Tim Tress]

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.

[Davala]

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

[kvflyer]

Originally Posted by Davala

Been enjoying the Dumont. ...
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!