Please help with 49-905 - Printable Version

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+--- Thread: Please help with 49-905 (/showthread.php?tid=10909)

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Please help with 49-905 - OZ4 - 11-08-2014

I almost have this thing working.

 There is little to no gain through the mixer stage. It occurred to me that the reason why the gain was so low is  from the antenna tank being out of tune.

 So I stopped the osc and coupled a 1000khz signal in the antenna and tuned the radio for peak through mixer stage. Next I let the osc run and measured it and it didn't agree.  ( I just bought a frequency counter)

 The peak of the antenna circuit is off (low) by 150khz and won't tune up. This is in comparison with the osc frequency minus the I.F.  I can't get either one to tune so they agree.


Of course the antenna loop coil has been mangled and is the inductor in this tank circuit.

Does anybody know what the inductance of this coil should be?  Or even how many turns there should be?

UPDATE
Sometimes I am my own worst enemy. After glueing the antenna coil back together I hooked it up with about 18 inches of speaker wire to extend it to the chassis. Well that totally messed it up. Once I put the antenna coil back into the case and hooked it directly to the chassis it tuned nicely. For what it's worth it measured 190uh with the chassis in the case.

Next I will see what I can do for an FM antenna.

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RE: Please help with 49-905 - mikethedruid - 11-11-2014

I have the same plug on my 48-482, which uses the line cord antenna for FM, which I have found works quite well. Here is how to make a duplicate.

First, you will need to find, or make, a plug that replicates the original. Cut a piece of 16 gauge stranded PVC covered hook up wire 19 3/4" long. The original color was white. Strip enough from one end so that you can solder it into the pin of the plug shown in the schematic.

Next, get a piece of fairly thin sheet brass from a hardware store. It should be thin enough so you can bend it fairly easily with a couple of pairs of long nose pliers. We are not making a name plate, LOL. Cut a piece of this sheet brass 1" x 1" square. If hard pressed I have a feeling a piece cut from a tin can would work OK too.

Strip and solder the other end of the wire to one corner of the brass square.

Wrap the brass square in a single layer of PVC electrical tape. Now, form the square into a tube around the line cord so it is wrapped around the line cord as closely as possible, then crimp it down so it is snug on the cord, but the cord can slide if tugged. Set it about 19" down the cord from the chassis. Give that a try. You will have to adjust the position of the brass square for maximum gain, but it should work OK.

Good luck !

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