The circuit for the meter amplifier is given in Fig. 10. Here VR2 acts as a sensitivity control in feeding the complementary pair TR10 and TR11. The meter circuit can be used in conjunction with, or to replace the headset.

The meter needle follows the amplitude of the amplifier output which varies at a rate equal to the difference between the two input frequencies. It is particularly useful with very low difference frequencies.

Constructional details of this module are given in Fig. 11.


The inductor L3 is contained in an adjustable pot core, the type being the LA2301, the same as used in the main amplifier. To achieve the specified inductance, 1,076 turns of 40 s.w.g. enamel covered wire should be wound on the bobbin. With the winding complete the free ends should be cleaned and the bobbin assembled in the cup cores. The mating surfaces of the cores should be fixed together with an adhesive such as Araldite, and the whole stuck to the Veroboard.