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 Post subject: Re: Dumont RA-103 magic eye tube
PostPosted: Jan Sat 12, 2019 7:19 am 
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Joined: Apr Mon 16, 2012 5:31 am
Posts: 143
I would like to add that the 6AL7 can be used to measure the same as a 6U5 for the AM band. I believe in that instance, both bars would change size to indicate maximum negative afc voltage when tuned to a station. If you find it works on the FM band but not the AM band, I suggest there may be something wrong with the switching or circuitry.

You will find the sources of 6AL7 control separated. Ry chasing back to the AM detector and AGC to find the problem.

 Post subject: Re: Dumont RA-103 magic eye tube
PostPosted: Jan Sun 13, 2019 9:51 pm 

Joined: Oct Mon 12, 2015 4:44 am
Posts: 405
The bars on the 6AL7 vacuum tube as I mentioned do work well on the FM Band, although I don't fully know how to read them other than to recognize the bars when fully green and wide present themselves when a station is obviously getting the strongest signal. The top and bottom bars can be different with the bottom bar generally being the fullest as the top bar can be smaller in length.

On the AM Band there is some movement when the station signal is strong It does assist in getting the tuning just right.
The right hand portion of the bottom bar has a brighter green glow than the rest of the bar when a signal is strong on FM.
The bars are changing more than I realized on the AM Band, but in a minimal way as compared to the bars when on the FM Band.

The green bars are missing when the radio is set to phono.

Not many people (not even some of the radio technicians I know and use) are even familiar with this "Magic Green Eye", so this information you have provided has been most helpful.

 Post subject: Re: Dumont RA-103 magic eye tube
PostPosted: Jan Mon 14, 2019 12:39 am 

Joined: Aug Wed 04, 2010 5:40 pm
Posts: 355
Location: Tyngsborough, Massachusetts
In the 1948 Beitman's Most Often Needed Television Servicing Information, on page 139 there is a description and sketch of how to use the 6AL7 tuning indicator. The entry is under the Stromberg Carlson TV-12, which uses the Dumont RA-103 chassis.

They show the right hand bar to be fixed and the left hand bar to shorten and lengthen on the bottom edge. They note to tune through the lengthening and shortening, and that the correct tuning is when the two bars are of equal length.

I have a TV-12, and that is indeed how the tuning indicator works on mine.


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