Low frequency converters type RA37 and RA137, designed for use with Racal RA17 and RA117 receivers, accept an input in the range 0 to 1 MHz and give a corresponding output in the range 2 MHz to 3 MHz. The RF output signal is applied to the RA17/117 interpolation receiver directly, bypassing the Wadley front end. The 1MHz reference signal from the RA17, RA117 or MA350 is frequency doubled and filtered to provide the 2MHz heterodyning signal for the conversion. The converters have an internal mains transformer for valve heaters and lamps, but the HT supply is obtained from the main receiver. When the adapter is selected via its front-panel 'Operation' switch, part of the HT supply to the RA17/117 is disconnected (disabling the Wadley loop) and applied to the adapter.
To install the adapter, break the link HT1-HT2 on the back of the RA17/117 and connect the terminals to the corresponding terminals on the adapter. Connect the Adapter AVC terminal to the main receiver AVC or AGC terminal (lower barrier strip). Take a coaxial lead from the RA17, RA117, or MA350 1MHz output and connect it to the adapter 1MHz input. Likewise connect the adapter RF output to the socket on the main receiver converter connection panel marked 'R.F.'. Note that the adapter HT ground return is via the RF cabling, chassis bonding, and the mains earth; and it may be unsafe to switch on unless all connections are properly made.
The LF adapter can be used with any HF receiver, with a coaxial RF input, capable of tuning from 2 MHz to 3 MHz; but must be provided with +220 V DC on terminal HT1, an accurate 1 MHz reference signal of ≥2V RMS, and a gain-control (AVC) voltage in the range 0 V to -25 V.
RA37 Detailed Technical Information.
RA137 Detailed Technical Information.