Probably the most common ‘option’ to be found in an RA1792 is the Independent Sideband (ISB) board pictured on the right. The input circuitry on this board is identical to that on the A4 board.
From the manual:
Selection of the position of LK1 on the Main IF/AF board allows filter FL1 (LSB) to be connected to the ISB IF/AF board, whilst leaving the other filters connected to the main circuitry. For modes other than ISB, only one filter is selected. When the ISB option is used, however, two filters are selected under software control, FL1 for LSB and FL2 for USB.
The photograph on the left is of the 100KHz IF option board.
This is mounted within a screened box which sits where the ISB board would normally fit. This essentially provides a third IF of 100KHz. The manual states that this replaces the 455KHz IF. This is ONLY true with respect the Main IF output at J2 on the 1792 rear panel. Even with the 100KHz option installed, the main A4 board in the RX is still responsible for the IF filter bandwidths and what comes out as resolved audio.
The option manual is also a bit misleading in that the installation guide assumes that the 1st LO in the RX is a MK3 board which takes its 1MHz reference from the 2nd LO board. In which case, since the 100KHz board also requires a 1MHz reference, the 1MHz from the 2nd LO is first routed to the 100KHz board then on to the MK3 1st LO. This is unnecessary when the 1st LO is a MK4 (reference is 20MHz). In this case no connection is required to J3 on the 100KHz board, as in the photograph on the left.
This board allows the RA1792 to be connected to various legacy auxiliary units whose input is 100KHz.