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38 116 Magnetic tuning problem - Printable Version

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38 116 Magnetic tuning problem - SteveJB - 09-02-2012

I tried to adjust the magnetic tuning on a 38 116, code 125 after a recap.

When tuned slightly to the right of a station and then switched on, it seems to pull the station in. But when I detune to the left of the station and turn it on, it gets worse. When tuned a station dead on and I turn the mag tuning on, the reception gets worse, sometimes picking up another station. I know it should not change at all when tuned into a strong station, but that isn't happening here. I adjusted the screws on the transformer as stated in the alignment instructions and it doesn't change performance. The larger 53b compensator makes no difference no matter where it is turned, but the other one, 53a, does make small changes to the station when turned and the mag tuning is on, but it doesn't peak as it should. It is fully clockwise and tight when it starts to peak but I can't get to go any further. I did try this with a signal generator at first, but gave up after failing to get it right. I thought I could adjust it by "ear" with the radio on a strong station, but that doesn't work either.

To get a peak, cap 53a (I think it is the secondary) is pegged fully clockwise, but not quite there. I am not sure how the primary 53b should work, the screw and nut just turn together and never tightens. Can it be stripped?

Does anyone know what i am doing wrong? Should the 53b compensator have no effect?

http://www.nostalgiaair.org/PagesByMode ... 013235.pdf

Thanks, Steve

RE: 38 116 Magnetic tuning problem update - SteveJB - 09-03-2012

I installed the radio and was playing it. After a while I noticed that one of the discriminator tubes had a dead filament so I changed it. At this point I haven't removed the chassis to try to readjust the mag tuning, but I am guessing this was the problem since it worked on one side of the dial.

I need a tube tester!

Steve

RE: 38 116 Magnetic tuning problem - Ron Ramirez - 09-03-2012

The alignment of the "Magnetic Tuning" (AFC) circuitry is very, very finicky, and the factory data must be followed to the letter, using a visual indicator such as an output meter or an oscilloscope. One slight error and the AFC circuit won't work properly.