Elenco Model AM/FM-108CK Superhet Radio: antenna coil resistance readings appear incorrect

Discussion in 'The Projects Forum' started by swansok4, Aug 11, 2015.

  1. swansok4

    Thread Starter New Member

    Aug 11, 2015
    1
    0
    I bought the Elenco Model AM/FM-108CK Superhet Radio off Amazon about a month ago and am slowly working my way through building the AM radio. Until section 4 I managed to get all of the resistance and voltage readings on my circuit to match those provided in the user manual.

    My antenna coil provided by Elenco is of the four wire variety. It has white, black, red, and green wires. The instructions say to check the resistance between the wires before hooking anything up. I checked the resistance between the red and green wires as described on page 31 I get 2.2 ohms instead of the 1-1.5 ohms Elenco says you should get.
    Q: Does it matter that the antenna wire resistances do not match those listed in the user manual? If yes, is there anything I can do about it?

    I continued to build the circuit and tested the voltage between test point 7 and test point 15 (with TP6 and Q7 shorted) as escribed in the static measurements section on page 34. The voltage here should read 1.6V according to elenco. I get 0V. Also my 9V battery is reading 9.5V instead of the 8.5V listed by Elenco. I checked R31, R32, R33 and Q7 as instructed and still cant get the voltage readings to match.
    Q: Could this be related to the incorrect antenna wire resistance reading?
    Q: Any ideas why the battery voltage is showing to high?


    Here is a link to the user manual.
    http://www.elenco.com/admin_data/pdffiles/AMFM-108CK_low-res.pdf

    Thanks for any help you can give.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Reading ~1Ω on any Ohmmeter is highly unreliable...
     
  3. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    After shorting the OSC coil, the equilvalent circuit is as attached...

    elenco radio.PNG
    The voltage at the base of Q7 is about 1.6V which is correct as indicated in your book. If R33 is shorted, you'll get about 0.7V and if B-E of Q7 is shorted, you'll get 0.4V. So if you're getting 0V, I presume R32 is shorted on the PCB with a blot of lead.

    Allen
     
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