The RA1792
Page 6
The A4 board

This is the final board in the RF path. Although its official name is Main IF/AF, this board does a lot more. It carries the selectable IF filters, the 2nd IF amplifier, Product Detector, FM Detector, AGC detector and distribution, AF amplification, AF & RF ‘meter’ driver and D/A … plus a host of  solid-state switching. Hence why it is easier to refer to it by its other designation … A4.

The board in the above photograph is not the final build standard. Note the two unpopulated IC locations on the left. These don’t feature on later boards. There are 27 different filter configurations that I am aware of. The configuration pictured above is applicable to Suffix GP and was peculiar to ‘Piccolo’ systems. Note the bottom two filters: the USB filter is 400Hz and the LSB filter is 800Hz.

If there are any configurations that can be considered ‘general purpose’ then these would be suffixes GA and GB.

The basic filter settings are 16KHz, 6KHz, 3KHz, 1KHz and 300Hz. The 16KHz setting bypasses the filters on the A4 board and relies solely on the Roofing Filter on the 1st Mixer board. The remaining 4 bandwidths are set by the filters on the A4 board. However the GA and GB configurations differ in the way the 3KHz bandwidth is created. Receivers with suffix GB carry a full set of symmetrical filters for 300Hz through 6KHz, whereas receivers with suffix GA carry one less; there is no symmetrical 3KHz filter. Instead the 3KHz bandwidth is derived by selecting the USB filter (2.7KHz or 3.2KHz) and offsetting the 1st LO and BFO by 1.7KHz.

I have found USB & LSB filters of 2.7KHz and 3.2KHz B/W. The bandwidth displayed in the RH display appears to be determined by the firmware. My own Suffix GA 1792 has 2.7KHz filters, yet the B/W is displayed as 3KHz. One of the 1792s that I have just rebuilt has 3.2KHz filters fitted and the display indicates 3.2KHz when BW3 is selected. I had to make sure that I didn’t fit 2.7KHz filters in that receiver! There may be other configurations that also employ a derived BW3 setting.

Essentially the RA1792 is a very straight-forward receiver and that is definitely true when it comes down to what it does. It is even easy to test, manually. However the majority of RA1792s come with BITE: That is Built In Test Equipment. Being ex-Racal myself, the concept of BITE is not alien. However, the BITE in the RA1792 can be both your best friend AND your worst nightmare!