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    Dynaco FM3 coil form

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    Toynbee

    Posts : 3
    Join date : 2018-03-04

    Dynaco FM3 coil form

    Post by Toynbee on Wed Mar 07, 2018 11:01 pm

    I have a Dynaco FM3 which I found years ago sitting on top of a post next to a dumpster at a nearby university where I found a lot of tube equipment thrown out and took home.  I usually use it to listen to a radio station from that university (WMUC) on Saturdays.  I align it now and then and recently opened it up to adjust the upper IF amp ferrite cores and the ferrite cores in the 19 kHz and 38 kHz transformers in the stereo demodulator.  I noticed that there's a plastic coil form that appears to be the impedance transformer connected between the antenna jack and RF amp (L1).  The secondary of L1 has one of the tuner capacitors and another variable capacitor in parallel with it (C1A and C1B).  In my FM3 there was no ferrite core in it, so I tried taking one out of a circuit board from an old tubed TV set and put it in L1 in my FM3.  I tried checking it with a Heathkit grid dip meter, which I tuned with a frequency counter to about 88-89 Mhz and then turned the radio and stereo on and adjusted the ferrite core and C1B for lowest noise (hiss/static) level.  I found a number of posts about people complaining about noise in their FM3s when in stereo, but not in mono.  Has anyone tried fixing this problem?  Does your FM3 have a ferrite core in L1? You can find the schematic on the internet.
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    Peter W.

    Posts : 759
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Dynaco FM3 coil form

    Post by Peter W. on Thu Mar 08, 2018 8:34 am

    For what it's worth, and over perhaps a dozen-and-a-half FM3s, I have never seen a ferrite core in that coil. Including my present two examples.

    Sorry for the brevity. No ferrite core - but the coil is adjustable from the screw. I have not found adjusting that coil has had much effect on hiss or other noise, if centered well, and when I do alignments I have a Ramsey FM-100B transmitter that allows me to send out mono and stereo signals, as well as put signal on only one channel at a time. So various adjustments become much easier.

    I snagged a pretty rough specimen recently (see prior post here) that will see a full refit shortly - there are pictures. These are very nice tuners with a little bit of care!


    Last edited by Peter W. on Thu Mar 08, 2018 12:43 pm; edited 1 time in total (Reason for editing : Too much brevity.)
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    Toynbee

    Posts : 3
    Join date : 2018-03-04

    Re: Dynaco FM3 coil form

    Post by Toynbee on Thu Mar 08, 2018 6:06 pm

    I didn't check the coil with the dip meter without the ferrite.  I found the noise level is lower with the ferrite and when its adjusted for lowest noise level. When the FM3 is tuned to about 88 MHz the L1, C1A, C1B tank circuit seems to be tuned about right according to the dip meter.  When the screw on the tuning capacitor is unscrewed (appears to be C1B) the noise level goes up.  Keeping it screwed down all the way gives the lowest noise level.

    Another thing I did a week earlier was replace C2 and C3 with mica capacitors and IIRC R7 and R8 with metal film capactors. R2 and R3 in the FM3 don't look like typical carbon composition resistors, so I figured they might be low noise resistors (metal film?). This seemed to help some and gave a more stable stereo image on one weak station. The FM3 works pretty good on most stations in the area.
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    Toynbee

    Posts : 3
    Join date : 2018-03-04

    Re: Dynaco FM3 coil form

    Post by Toynbee on Tue Mar 27, 2018 9:23 pm


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    Re: Dynaco FM3 coil form

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