Salvaged RF Ammeter

Years ago, a 0.75A 'thermocouple heating element' was salvaged from a meterless BC-442 Antenna Relay Unit. Only recently was it taken out, to build an RF Ammeter for low-power rigs.

Salvaged RF Ammeter
A junk-box 50Ω, 1mA FSD  moving coil meter, with a bakelite housing, was also put to good use. It has graduations only upto 50% of its scale.

An aluminium baking tray was reworked to accept a couple of SO-239 sockets and the meter.

The unit was tested after completion of assembly and wiring.

Thermocouple RF Ammeter - Schematic
Salvaged RF Ammeter - Inside view
A homebrew CW rig was loaded to get a reading of 0.75A on a Weston 1.5A RF Ammeter. The Weston was then replaced by the unit to be tested.

The meter pointer stood at exactly 50% of full scale, indicating that the graduated portion of the scale covered the full range of the thermocouple. Notwithstanding the non-linear output of the thermocouple and the unmarked meter scale, staying within the graduated portion of the scale would avoid damage to the element!