The RA1792
Page 2

One failing of the RA1792 is its lack of input preselector and its tendency to react badly to very strong signals. However it does come with a low-pass filter.


Right …

Two incarnations of the receive pre-amp/low-pass filter unit. The one at the top is an ‘older’ version where the links for bypassing the amplifier are wires as opposed to plug-in links as in the later version below. There are also a few subtle component placement changes.


Whether active or not, this unit does afford the receiver a great deal of protection in that it contains both a fuse and a spark gap. The input can withstand a sustained e.m.f of 50V.


Gain of the unit (Incl. filter) is approximately 10dB.


Perhaps the fact that the active stage can be bypassed is an admission that the first mixer can be overloaded.  

To date I have encountered two versions of the 1st mixer board. The silk-screen on the one on the left indicates that early versions of this board carried additional circuitry; possibly a pre-mixer amplification stage. The 1st mixer board converts the input RF signal to the 1st IF of 40.455MHz. The LO signal from the 1st LO synthesiser is LP filtered before being fed to the FET Switching Mixer by the balanced drive amplifier. The RF input signal is passed to the input of the mixer by another LP filter. The resulting output is then fed through a 40.455MHz roofing filter which has a 3dB bandwidth of 16KHz. The output of the filter is then amplified by a single grounded-gate FET amplifier whose gain is controlled via AGC.

The Pre-Amp/LP Filter
The First Mixer