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 Post subject: AM loop antenna for Fisher AM-80 or 80-T
PostPosted: Aug Thu 02, 2012 7:42 pm 
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I was wondering if anyone has or knows what the dimensions and number of turns that was used in the AM loop antenna for these Fisher receivers?

It looks as if they were a low number of turns much like the two-turn loops as used by Philco just before WWII.

Does anyone have one or know what exactly they look like?

http://www.fisherconsoles.com/non%20con ... manual.pdf

Thanks!
-Steve

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 Post subject: Re: AM loop antenna for Fisher AM-80 or 80-T
PostPosted: Oct Fri 02, 2015 2:47 am 
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About 47uH, 15 pF cable capacitance.

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 Post subject: Re: AM loop antenna for Fisher AM-80 or 80-T
PostPosted: Oct Fri 02, 2015 4:02 pm 
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Just in case someone stumbles across this a future search:

The AM-80 is probably one of the best AM only tube tuners. Variable IF bandwidth, tuned RF stage, and two IF stages. I just had one on the bench, and I was very impressed with the selectivity settings and its sensitivity.

The loop antenna becomes part of the RF input tuned circuit when it is used.

The tuned secondary winding of the tuned RF input transformer has a fixed value 47uH added in series when the radio is configured for a wire antenna. The wire antenna is link coupled to the transformer secondary.

When the loop antenna is connected, it replaces the 47uH fixed choke, and becomes part of the tuned circuit, and is in series with the secondary winding on the RF input transformer.

There is a 15 pF mica cap across the 47 uH inductor, which compensates for the interconnecting cable length between the rear jack and loop. The lead in cable must be kept short and of low capacitance cable.

The loop inductance needs to be trimmed to match the radio--if you are going to homebrew one it will take a bit of playing around. If the loop is correct, adjusting the slug in the RF input transformer should peak at the same point for both the external wire antenna, and for the loop at the low end of the dial. If the alignment changes considerably, the loop inductance should be adjusted as needed. I started with a 48 uH loop about 10 inches in diameter... and ended up removing a turn or two before I was satisfied with the results.

I'd be tempted to use a long ferrite bar, with a loop winding that could be slid on the bar to adjust the inductance precisely.

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 Post subject: Re: AM loop antenna for Fisher AM-80 or 80-T
PostPosted: Oct Sat 03, 2015 2:45 am 
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Location: Charleston, W.Va.
Thanks Steve for starting this thread, and thanks Peter for your response. I had been planning to start a thread on this same subject myself.

I was fortunate to find a Fisher AM-80 a couple of months ago, excellent cosmetic condition in a beautiful Fisher blonde wood cabinet. All original inside, no evidence of previous service work, and performs well in initial testing although I need to go completely through it before putting it into permanent service. I plan that for a wintertime project after the weather turns bad, which it seems to be doing already down here in Virginia!

Fisher AM-80 indeed seems to be one of the best AM-only tube tuners ever built, at least in the post-WWII era. I had been looking for one for a long time before I found the one which I presently own. I passed on a couple of them over the years which had been "hacked" until I found this nice original one. But it seems hard to find much information about them. I do have an original Fisher service manual for it which, as far as I know, is the only service data available. AM-80 was never covered by SAMS Photofacts.

I wonder what the original Fisher loop antenna for these looked liked? I mean in physical appearance. I assume it must have been something physically attractive, designed to be used alongside, on top of, or behind the AM-80 wood cabinet. Anyone know of a photo of one? Either a catalog photo or a photo of one currently existing?

On the other hand, using a ferrite bar with loop winding as Peter suggests might be a better and easier solution. Of course the AM-80 also works great in configuration for a short wire antenna.

When I get started on the restoration of my AM-80 I will post further information, and will very likely have some questions. In the meantime if Peter or anyone else with previous experience on these can offer any advice I would very much like to hear it.

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 Post subject: Re: AM loop antenna for Fisher AM-80 or 80-T
PostPosted: Oct Sat 03, 2015 3:49 pm 
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They are pretty simple to restore. I replaced the electrolytic filter caps; but ended up needing to replace the tubes because quite a few were becoming microphonic or noisy. Other than testing the resistors, cleaning the controls, some lube, and an alignment, they are usually good to go.

The only thing I didn't like was the undampened signal level meter, it would bobble several times before settling down.

I understand the loop came with the tuner, but I haven't found a photo or information; my advice is based on reverse engineering the antenna circuit and customizing a loop to work with the set. It is amazing sensitive; and the variable IF selectivity works like a charm. I may look for one for my own use.

Pete

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