• This topic has 2 replies, 3 voices, and was last updated 4 weeks ago by LectronFan.
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    • #8152

      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 area 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 wire from the roof to a tree.

      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.

      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?


      • This topic was modified 4 weeks ago by mwpeters75.
      • This topic was modified 4 weeks ago by mwpeters75.
      • This topic was modified 4 weeks ago by mwpeters75.
    • #8166

      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!


    • #8206

      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.


      • This reply was modified 4 weeks ago by mwpeters75. Reason: Changed font and size styling for easier reading
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