Dumont RA113 "Burlingame" restoration

[robert1]

Originally Posted by bandersen

I finished replacing the leaky mica caps and out of tolerance resistors including the one in the eye tube base then redid the audio alignment. Bingo!

The eye tube now responds like it should, I only get good sound at one distinct point on the dial per station and it coincides with a sharp picture.

Time to start putting this set back together

As i mentioned sometime back on your You-Tube video, Bob, i learned a long time ago to just go ahead & replace all them postage stamp (Mica) capacitors because they can & will fail. after many years of lack of use, moisture & humidity can get inside of them & contaminate the Silver Oxide (or whatever electrolytic material) they use for a dialectric. it amounts to the same thing as Silver Oxide Migration that is common to I.F. cans. back then, the technology just was not there to manufacture a perfectly sealed capacitor. those Micamold capacitors were not really much better then paper capacitors, especially in Humid envirements.

[bandersen]

Overall, I've had pretty good luck with them still being good. If they weren't so expensive I might be inclined to replace them more often.

I thought I traced the source of that squealing to this wire arcing to this resistor, but nope. After replacing it and touching up the solder joints in that area I can still get it to squeal if I position that HOT lead just right. Even so, it's easy enough to secure it such that there's no squeal so I'm calling it good enough unless someone has a suggestion.


After a long session of fighting with the trap magnet, focus coil and yoke, I got the picture centered and focus fairly well. Just in time to catch on of those butchered "Sixth Sense" Night Gallery episodes. They're actually not bad when seen in their original 50 minute run time.



There's a faint, bright vertical bar on the far right you can just make out in this shot. I've seen them on the left before, but not the right

I'll get a pattern generator hooked up and see if it goes away if I adjust the drive and linearity controls.

[IsthmusTV]

It looks great, Bob. I think you can call this one done. I doubt the owner would even notice the vertical band on the right side (I also have never seen one on the right side of the picture). But hopefully the drive control will take care of it.

As for the squealing in the HV cage, mine also has electrical tape wrapped around the HOT plate lead. I haven't heard squealing from mine but I wonder if this was an issue with this set

I ordered a bunch of Twilight Zone DVDs that I plan to watch on the DuMont over the holidays.

-Clark

[bandersen]

After sleeping on it, it occurred to me that the plate lead is in series with that 68 ohm resistor so there's no potential difference to cause any arcing between them.

What I think is more likely is that the wire is stressing the phenolic board and flyback.

The CRT isn't quite as bright as I expected considering it tests like new and is aluminized. Perhaps that's because the HV is only 12kV for the original 17AP4 while a 17BP4 typically sees 14kV.

[robert1]

Originally Posted by bandersen

After sleeping on it, it occurred to me that the plate lead is in series with that 68 ohm resistor so there's no potential difference to cause any arcing between them.

What I think is more likely is that the wire is stressing the phenolic board and flyback.

there could also be the possibillity that where the wire enters the cage (without the tape wrap) that there may be a "capacitance" issue, where the wires insulation is acting like a dialectric, especially where the wire itself is touching the metel on the HV cage, hence, the reason for the tape wrap.
that added capacitance can throw the waveform off, causing the squeal.

[holmesuser01]

Originally Posted by bandersen

After sleeping on it, it occurred to me that the plate lead is in series with that 68 ohm resistor so there's no potential difference to cause any arcing between them.

What I think is more likely is that the wire is stressing the phenolic board and flyback.

The CRT isn't quite as bright as I expected considering it tests like new and is aluminized. Perhaps that's because the HV is only 12kV for the original 17AP4 while a 17BP4 typically sees 14kV.

Your CRT is aluminized? I don't think you should need an ion trap, then.

[old_coot88]

Re. the low brightness, have you tried tweaking the brightness range a bit? Ie., shifting resistance(s) a bit to nudge G1 slightly more positive relative to the cathode?
Hard to be more specific without seeing the schematic.

[bandersen]

Originally Posted by robert1

there could also be the possibillity that where the wire enters the cage (without the tape wrap) that there may be a "capacitance" issue, where the wires insulation is acting like a dialectric, especially where the wire itself is touching the metel on the HV cage, hence, the reason for the tape wrap.
that added capacitance can throw the waveform off, causing the squeal.

There's a large hole cut out of the metal cage where the wire enters and it's supported by a piece on phenolic insulator. The squealing seems to come and go depending on the angle and pressure of the wire inside the box. It will be clearer in the next video I post.

Originally Posted by holmesuser01

Your CRT is aluminized? I don't think you should need an ion trap, then.

Yes, it's an aluminized 17BP4B. The probably used a bent gun to avoid confusion because the 17BP4, 17BP4A and 17BP4C are not aluminized.

Originally Posted by old_coot88

Re. the low brightness, have you tried tweaking the brightness range a bit? Ie., shifting resistance(s) a bit to nudge G1 slightly more positive relative to the cathode?
Hard to be more specific without seeing the schematic.

No, I haven't messed with that. If I turn the brightness up all the way now, the picture will get washed out. Here it is in a brightly lit room. I guess it's not that bad after all.

[bandersen]

I hooked up a pattern generator and fought with the settings some more. The linearity is better now and the bright line is gone from the right side. The have the width switch and coil set for minimum, but still just a bit too wide. Not that any would notice.

So I'm finally calling this one done


It's tough to get a good shot of the cabinet while the set is playing.

[Dangler]

Very, very nice. The owner should be tickled pink!

[Phil Nelson]

Excellent. I remember having a devil of a time getting the image focused and positioned without shadows, etc., on my RA-113. It would have been easier if I could have pushed the yoke & focus coil just a wee bit closer to the front. I theorized that my rebuilt 17BP4 had a slightly different profile to the CRT bell than the original CRT. It's hard to imagine they would have had to struggle like that in the factory.

Phil Nelson

[robert1]

Originally Posted by bandersen

There's a large hole cut out of the metal cage where the wire enters and it's supported by a piece on phenolic insulator. The squealing seems to come and go depending on the angle and pressure of the wire inside the box. It will be clearer in the next video I post.

Ok, Now it's clear to me. it appears that ether the hardware that holds the flybacks ferrite core together might be loosening up or has developed a crack in the core itself. you might want to check the two nuts that hold the core together to see if they are loose.

unfortunately, due to changes to You-Tube, i cannot make any comments on any post anymore because they screwed something up. even though i have a G+ account, the You-Tube Commenting does not work any more for me.

[Kevin Kuehn]

Originally Posted by robert1

unfortunately, due to changes to You-Tube, i cannot make any comments on any post anymore because they screwed something up. even though i have a G+ account, the You-Tube Commenting does not work any more for me.

I noticed something screwy was up too. They are currently trying to force me to change my username to my actual name, or I can't comment. Seems very strange. Suppose I could open a new account using my old username as my real name.

Great job on the DuMont, Bob.

Just out of curiosity - are you convinced that this set has a better picture than any other decent set of the period?

[bandersen]

Well, I've definitely seen better pictures on earlier sets, but I think it's better than the few other 1951-52 sets I've restored.

Here's the bandwidth of this set which peters out around 3.5 MHz (2nd bar from the right). Now to be fair I wasn't able to align it as well as I'd have liked due to a couple funky IF transformers.
This also illustrates how the width was too great before I squeezed it down.


Here's an RCA 630TS where you can distinguish lines out to 4.5 MHz

[HighHopes]

Hi Bob,

I just reviewed the Dumont RA-113 videos on YouTube. That was quite an experience. I suppose if a set is over-engineered, more things can fail.

On the sets I fiddled with years ago, there was always a coil that would adjust for the audio and video separation that allow one to sync up the tuning. These were sets from the early to mid sixties. Basically it shifted the band where the audio would be fetched from. One tuned the best video and then adjusted the coil for the best audio - no fuss, no muss.

While you were wrestling with the IF strips, I couldn't help but think there was something wrong with the tuner local oscillator alignment(s) which might account for the difficultly in tuning the best video and the best audio at the same spot.

I get confused with the setting of IF frequencies and always run back to AM radio alignment experiences to keep myself straight. In an AM radio, the alignment can be done without using exact frequencies. I seem to remember tuning the local oscillator for 540 KHz (tuning cap closed) on the dial. And then the oscillator trimmer cap for 1600 KHz (tuning cap open). It didn't really matter exactly what the IF frequency was, as long as it was peaked. The point is that things could be approximately on frequency and all was well.

Of course, with old TVs, there are two bands, audio and video which have to be calibrated together. Given the over-engineered nature of that Dumont set, I am wondering if there are two separate mixers, one for video and one for audio. I am wondering if the two slugs that were on the Dumont tuner were for this.

Is this a crazy idea? Two mixers?
Would they allow the IF frequencies to be off a bit as long as the IF strips were tuned accordingly.

Anyway, in all these musing I find myself wondering what exactly is critical in TV alignment, and what really has to be matched to what.

Also, because the audio is FM (wider than AM), maybe the audio IF needs to be stagger tuned as well. Would stagger tuning of the audio IF allow less critical station tuning with respect to audio. Would the bandwidth of the ratio detector accommodate this? I wonder how much of the detector slope is actually used.

Steve