XBM80-2 An Experimental 80m CW Transceiver G3XBM

I’ve been a little too busy to fire up the soldering iron and build anything, but I’ve been pondering putting together one of Roger’s G3XBM radios. I ordered a couple of high impedance crystal earphones just to be ready, but in the mean time, I’m studying the circuit diagrams fairly closely:

XBM80-2 An Experimental 80m CW Transceiver G3XBM.


The radio is simple enough that I should be able to understand it completely, but I must admit that I don’t understand the principle of regeneration very well, so staring at radio, it’s not particularly obvious to me how T1 serves as both mixer and oscillator. I also am not certain I see how the oscillator doesn’t radiate when the radio is in receive mode. I’ll ponder it some more, and maybe fire up LTSpice over the weekend to test my understanding.

6 thoughts on “XBM80-2 An Experimental 80m CW Transceiver G3XBM”

  1. I think during receive mode, when the key is not pressed, the rf signal being received by the antenna is mixed with the local oscillator due to nonlinear properties of the transistor. the mixed signal will be present at the emmiter of t1 and would be filtered out as it reached the base of t2 the transistor t1 current is very small and very small amount of rf is being radiated, but when the key is pressed large amount of current suddenly flows from the emitter t1 and is then radiated at its collector

  2. I have been using this circuit (with some minor modifications) for reception.

    My understanding is the following:

    This is a week oscillator CW receiver.
    Any week oscillator, as in this case, can be disturbed by a week interference of
    similar frequency; creating a beat-frequency/amplitude.
    In this circuit, T1 is the week oscillator, T2 senses the disturbance (amplitude/beat-frequency).
    The disturbance (signal) is fed from the collector of T1 into its base via the pn junction of the transistor(, which is usually 10 pF).

    Where is the regeneration ?
    It is in the process of adding the incomming signal into the week oscillator, thus getting amplified over and over again in the oscillating process.

  3. It receives by opening S1. Remember old timey rigs you had to switch between Rec and Xmit mode. You could also combine S1 and key Switch into one DPST switch.

  4. Correction. It receives by closing S1 and killing the oscillator.

    [It receives by opening S1]

    Remember old timey rigs you had to switch between Rec and Xmit mode.

    Disregard the combo switch idea.

    [You could probably combine S1 and key Switch into one DPST switch.]

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