Need repair advise for a Heathkit SB-101

Discussion in 'Radio Circuits, Repair & Performance' started by KG8ML, Sep 9, 2020.

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  1. KG8ML

    KG8ML Ham Member QRZ Page

    My old SB-101 has developed a problem. When I put the frequency control into locked normal position, I no longer receive any signals, nor do I have any RF output. When in the locked AUX position, the audio goes up a bit in volume, and I do get output at a fixed frequency (14.515 for example on 20 meters), until the radio has been on for a while, then no output. Could the LMO be bad?
  2. W5RKL

    W5RKL Ham Member QRZ Page

    There are 3 frequency control switch positions on the SB-101 Freq control switch:

    1. LOCK AUX
    2. LMO

    Position "1" uses a single crystal plugged into the single crystal socket on the Band Pass board. That crystal replaces the
    LMO, providing FIX crystal control on one RX and TX frequency based on the frequency of the crystal. The crystal
    replaces the LMO (LMO is disabled). The formula to calculate the crystal frequency is found in the SB-101 assembly
    manual on pages 98 and 99.

    Position "2" uses the LMO to control RX and TX on the same frequency.

    Position "3" allows for LMO to vary the receive frequencies and the single crystal control crystal frequency on the Band Pass board
    to control transmit.

    The 14.515Mhz transmit frequency with the Frequency Control switch set to LOCKED AUX, would indicate an approx crystal frequency
    of 4.985Mhz.

    If you set the Freq Control switch to LMO, do you regain RX and TX? You should if the LMO is working. Check the rear of the LMO.
    There's an RCA socket and a coax cable with an RCA plug inserted into the LMO RCA socket. If you have a frequency counter, pull the
    RCA plug from the rear of the LMO and plug the counter into the LMO RCA socket. Power up the SB-101 and look at the counter.
    The counter should display a frequency between 5.0Mhz and 5.5Mhz. If you see a frequency within that range then the LMO is
    working. If the LMO dial is set to zero the LMO output should be close to 5.5Mhz on ALL bands. If the LMO dial is tuned to 500
    then the LMO output frequency is 5.0Mhz on ALL bands.

    Mike W5RKL

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