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    • #8152
      Guenther
      Participant

        Don’t know if anybody used the AM Broadcast tuning blocks.

        Since years I was quite angry about the (missing) scale on the tuning blocks #8036 + #8074. Turning to the right: low frequencies, turning left: high frequencies. The radio frequencies have to be (in that order) from 1620 … 531 kHz/cps – but not with the available built-in coil (#8074) or the ferrite antenna (#8037)! Never – no match!
        I did some testing and calculations, and so the following overview may help a little:

        My markings 16 and 5.3 stand for the usual reception limits on the worldwide produced pocket radios – everybody had seen this and means the AM BC reception range 1600 … 530 kHz/cps.

        The formula for L/C circuits (in the middle) calculate the required values for L (coil inductivity) or C (capacitor) – or like here: the resonance frequency.
        But this only valid for L/C circuits without an external antenna, e.g. a long antenna wire from the roof to a tree (please use isolators at the ends!).

        The components (variable capacitor = tuning cap.) and the used coils (without ferrite core in #8074, ferrite antenna in #8037) have tolerances, see values above.

        In my graphic the black arrows show the actual range for tuning (probably the second harmonic: <400 kcps + 531 kcps in the left picture); after minor changes I “moved” it to the reception area marked with the red arrow.

        The tests with my AM modulator started at 531 kHz/cps, had a middle position of 1003 kHz/cps and a higher one at about 1200…1350 kHz/cps.

        If anybody has a good reception of a radio station at about 1600 kHz/cps he may check before he is thinking of one of the following changes:

        This is the well known experiment 50 from the good old Egger / Braun sets.
        The ferrite antenna (with these values) need a variable capacitor of ~ 150 pF, but the used one in #8036 has ~ 250 pF.
        So a serial capacitor of ~ 330 pF (available as Lectron #2313 ) will solve this problem!

        I tried with values up to 390 pF, depending in the actual value of the variable capacitor – see in the photo above without cover (please note the old tuning knob – you can see it on a picture in the first Egger / Braun manuals).

        The formula is: (C1 * C2) / (C1 + C2), where C1 is the maximum value of the variable capacitor and C2 is the value of the serial capacitor, all values in pF (pico Farad).
        The built-in coil in Tuning block #8074 has too few turns so I disassembled it and put some additional turns (about 20 turns may be enough) on it – please note: same direction!

        So I had ~ 160 µH before and about 220 µH now. With the above shown frequency formula I should have had  360 µH with this variable capacitor to receive the whole BC band, but – as I mentioned before – the required external antenna for this tuning block is a – technical – part of the L/C circuit !

        Here the open tuning block with the extended coil:


        Any other suggestions?
        Best,
        G.

        • This topic was modified 4 years, 4 months ago by Michael.
        • This topic was modified 4 years, 2 months ago by Guenther.
        • This topic was modified 3 years, 2 months ago by LectronFan. Reason: Making the text readable
      • #8166
        Michael
        Keymaster

          Hi Günther,

          Great contribution, thank you.  

          You are way out of my league (you know so much more than I do) I have no suggestions to offer.  Just admiration!

          Best,
          Michael

        • #8206
          LectronFan
          Moderator

            Hello all,

            Thank you Guenther for your solution to the AM coil matching.

            I also noticed a shifting in the tuning when building this circuit.

            By the way, to my surprise, the same circuit build with the NPN transistors performed better ! Something to do with the gain of the first transistor.
            As an experiment a few weeks ago, I used a higher gain transistor MPS-A18 and distant stations popped in loud and clear.
            I would love to build a transistor block with this high gain transistor.

            Just need to order an empty block one of these days.

            Greetings

            • This reply was modified 4 years, 4 months ago by Michael. Reason: Changed font and size styling for easier reading
          • #8525
            Guenther
            Participant

              I just found another old tuning block with a cylindric coil:

              In this case the problem could be solved with a ~ 10 mm piece of an old coil carrier with a (shortened) HF ferrite that fits exactly in the Lectron coil and can be adjusted with a small screwdriver to the required value:

              Best,

              G.

               

            • #8526
              LectronFan
              Moderator

                Hî,

                Thanks for the tip ! Unfortunately the tuning circuit which was included in my set has only a rectangle coil form.

                What also surprised me was the fact that the bar antenna has no litze  wire, only solid copper wire. The same wire was also used in a legacy block.

                I thought that the litze  wire has better qualities concerning stray capacitances in HF circuits.

                The litze wire is used in the tuning block.

                many greetings

              • #8531
                Guenther
                Participant

                  The properties of the “HF Litze” (high frequency stranded wire) are indeed better than the CuL version – but this wasn’t used in the Lectron world. I think that is was easier to produce and for less money to order.

                  Here is a photo of several versions with CuL (1…3 left) and 2 homebrew versions made with “litze” to the right, which can be ordered still today – from time to time – see samples below:

                  Why not make the coils with this material – if available – by yourself? Helpful is a L/C/R-Meter to check the µH values. As a rough thumb rule: ~ 120 turns for the vari-cap, 10 turns for the coupling coil. All turns on a – moveable –  cardboard tube for better adjustment.

                  Best,

                  G.

                  • This reply was modified 4 years, 2 months ago by Guenther.
                • #8533
                  Guenther
                  Participant

                    To solve the above problems with the rectangle coil (from a small transformer) I tried with some ferrite pieces from my coil collection. I had a separate coil from a disassembled block and simulated the groundplate with a piece of iron and the tuning block bottom with a piece of polystyrol:

                    Moving the ferrite into the right position and fix it with some pieces of a wooden match or the like and glue it – ready!

                    Best,

                    G.

                  • #8539
                    LectronFan
                    Moderator

                      Hello Guenther,

                      This is a great find !

                      Never thought there could be an easy way to fix this coil.

                      i will try the same with my coil. I just have to check for a correct ferrite rod to fit inside.

                      Many greetings

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