Dumont RA113 "Burlingame" restoration

[ggregg]

I bought one of those somewhere not that long ago, maybe three years ago when I got my RA-109. Check your local ma and pa hardware store.

[bandersen]

I muddled my way through the alignment procedure over the last couple nights and things are looking better. I still get double peak sound on the dial, but it's louder and clearer. So is the picture.

The response curves for two of the stages were complete garbage. Way off frequency and the wrong shape. I was puzzled by a little jumper wire on the bottom of one can feeding from a lug into a little tube. That wire had been clipped off the IF can to the left.


Eventually, I found a section on this in the service info. I'm guessing some service tech encountered this problem and made an attempt to fix them.


I was able to adjust the wire and slugs in the right side coil and get a reasonable response curve. For the other I tacked on a 2pF (the closest I had to 1.5pF on hand) mica cap and got a semblance of the proper response.

I'll track down a couple proper 1.5pF caps and realign. So far I only found a ceramic C0G cap at Mouse for about $1.75 each.

Evan as is, the picture is pretty darn good.


I do have some horizontal jitter though. I still need to get a scope on the horizontal lock coil and go through the adjustment procedure. Hopefully that will clear it up.

[Phil Nelson]

My RA-113 had an intermittent horizontal blip similar to that.





The source was a bad mica coupling cap. Details at the section titled, "Curing the Horizontal Blip:"

http://antiqueradio.org/DuMontRA-113...HorizontalBlip

There may be other causes for horizontal hijinks, of course

Your picture looks very good already.

Regards,

Phil Nelson

[Electronic M]

Those pics of horizontal issues with the guy's face distorted out towards the ladies sort of remind me of the way the eyes bug out on the wolf characters when attractive women are around in some old Tex Avery cartoons....

[bandersen]

Thanks for the tip Phil, I'll check that resistor out. While on your website, I also read up on the operation of the 6AL7 eye tube and I must say mine does not behave that way at all!

One side is fixed at about 3/4 but the other side operates in the opposite way it should. When no station is tuned in, the bottoms of the two bars are level. When a station is tuned in, the left side bar is at it minimum The 3.3K resistor in the socket has drifted up to about 4.3K but I don't think this would account for the odd behavior.

[bandersen]

Good call Phil - that 180pF mica cap was leaky as a sieve. All I had handy was a 200pF, but it seems to be working fine. That makes me wonder if others might be bad as well

[Phil Nelson]

I turned on my RA-113 to observe the indicator and it does behave as described in the manual. When the tuning is centered, with best picture and sound, the bars are more level than not.

The schematic shows a 1 meg resistor (and a couple of paper caps) connected to pin 4 of the 6AL7 indicator tube. Might be worth checking that resistor, too.

Phil Nelson

[bandersen]

Thanks, I'll take a closer look at that indicator and the tuner. Something is just not quite right with the tuning action.

[Kamakiri]

Don't want to change the focus of this thread, but I sure wish I had this much luck on mine. First power up today after the recap. Full static audio, no reception on anything, just an incomplete white line on the screen. More work for another day

[bandersen]

I don't know about luck. I've been working on it daily for about a month

I've found that sets like this one that were kept by the original owners and used for years and years are the worst to restore. Aside from numerous repairs, parts start to wear out. If I can't sort out the tuner issue soon, I may just leave it as is.

[Kevin Kuehn]

I've got a hunch the tracking is off between the individual sections of your inductuner. I don't see how the local oscillator being off would cause that kind of double tuning. The local oscillator only beats against whatever frequency the tuner is in resonance with. The local being off would cause everything to be shifted up or down on the dial.

[bandersen]

I received this comment on YT that I think explains why I'm receiving the output of my converter box at two points. I do not have this issue when tuning the commercial LPTV channel 6 broadcast.

That leaves a couple issues. The audio tuning in at two point very close together (one just at the edge of good video and one with bad video) and the odd eye tube indicator. Same issue when tuning in FM radio.

I've aligned the audio several times and it looks perfect to me. I'm going to go through and test the few mica caps in the audio IF and maybe go through the alignment one more time.

That buzz and poor reception of your converter box when you tune low is when you are tuning the lower sideband of the signal. An analog signal is fed into the video modulator with the video signal at baseband and the audio signal at 4.5 MHz. What comes out is an AM signal, with consumer equipment this is sent straight to the set. With Broadcast or cable equipment all by 0.75 MHz of the lower sideband is filtered out. With digital the signal is fed into an 8 Level modulator and again all but 0.75 MHz of the lower sideband is filtered out. This is called vestigial sideband. VSB as it is known is used to allow some redundant information (to make receiver filtering easier) and the use of the carrier (to eliminate the need for a touchy BFO or product detector) to receive what is almost a single sideband signal. If brodcasting used true AM the signal would be 10 MHz wide with VSB the signal is 5.75 MHz with a .25 MHz guard band prior to the lower sideband.

[Kevin Kuehn]

You could try an older DVD/VHS machine with built in rf modulation.

[IsthmusTV]

I guess I really did get lucky with mine. The IF alignment seems pretty good, as does the alignment of the IndcTuner. (Or is it an InpuTuner?) I think Phil's speculation is correct. That since it still has a strong original 17AP4, it was probably pushed aside decades ago when it quit for some other reason and not run into the ground.

[bandersen]

It looks to me like the 6AL7 is driven by one plate of the 6T8 discriminator through a low pass filter. So it's an indication of the audio carrier strength ? Since my audio isn't tuning as expected, I guess it follows the indicator would behave oddly.