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.

Since the amplifier provides high gain, the wiring layout and constructional details of Fig. 9 should be adhered to, to prevent instability. Both of the transformers are contained in Vinkor adjustable pot cores. To wind T1 use 40 s.w.g. enamel covered wire. Slip a couple of inches of fine sleeving over the start to protect the leadout, then wind on seven hundred and fifty turns. Put on another piece of sleeving over the finish leadout and wrap a layer of cellotape round the winding.

Put on two more windings of three hundred and seventy-five turns each and identify the starts and finishes with different coloured sleeving. Wind a layer or two of plastic electrical tape around the completed winding then very carefully assemble the bobbin in the ferrite core. Ensure that nothing gets trapped between the two halves of the core, preventing them mating.

The main amplifier, seen in Fig. 8, serves to increase the level of the precession voltages. A ferrite cored transformer, T1, with the primary centre tapped, is tuned to the required frequency by C4 and C5. The first stage comprising TR5 has a tuned collector load resonant at the same precession frequency.

The output from the secondary of T2 feeds the d.c. coupled amplifier TR6-TR7. This acts, in effect, as a pre-amplifier to the meter circuit, the input for this being taken from M1.