[Premium-Rx] Fw: IF converters for the RA6790/RA1792

[John Graham]

While on the subject of Racal IF converters, one of my RA6790s came with an A14 card that appears to be a programmable IF converter.

It accepts a 455kHz input and 20MHz from the A8 card, and its output goes to the IF OUT connector on the rear panel.
It has a 1496 mixer and the division ratio for the LO input to the mixer is set by DIP switches controlling a divider chain.
There is silk-screening on the board for a second [unpopulated] signal path, presumably for ISB use.

There's no mention of it in the 6790 documentation, but the 6793 manual mentions such a converter as an option with an output range of "15 to 100kHz in 5kHz steps", but without giving any details.

The board was faulty with [of course!] some shorted tantalum caps. Unfortunately a couple of resistors are so discoloured as a result that I can't tell what value they are supposed to be.

Does anyone have any information on this board?
A circuit would be nice, but just a photo to identify the resistor values would be a start.

BTW, I discovered that this board requires the /2 revision of the A4 IF board, with the 100R resistor in the 5V feed to the A14[/A5] card replaced by a link [the 6793 has a 10uH inductor instead]. Needless to say, the IF board it had been attached to had none of the above, but a 10R resistor!
With the standard 100R resistor, you'll get about 2.5V instead of 5V, which doesn't help matters.

thanks,

John G.