The RA1792
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The A4 board …/cont

Basically it is possible to have an RA1792 that for all intents and purposes is working perfectly … as a radio, but yet fails the BITE in more that one place. There are two ‘layers’ of BITE … continuous and user-invoked. Certain BITE functions are monitored continuously, some at ‘switch on’ and others come into play when invoked by the user. For instance the various supply voltages throughout the receiver are checked at switch on … and if one is out of spec. the dreaded ‘fault’ flag appears in the RH display.

Cleverly, the extent of BITE is dependent on the options fitted. BITE can be invoked by pressing and holding the REM key, keying in 00 then releasing REM. If BITE encounters a fault the sequence is halted and the failed test number is displayed in the LH display. BITE can be invoked to start at any applicable test simply by keying in that particular number whilst holding in the REM key. How does this impinge on the A4 board? Well, it’s all about the DAC which is located on the A4 board. Where the BITE is checking voltage levels, the output of the DAC is used as the reference. On the odd occasion it is possible to have the DAC set within limits and still find yourself going round in circles trying to find a fault in a seemingly fully working radio, only to find that a minuscule tweak of the DAC clears the fault. Another ‘fault’ which stalled my work for some time was a BITE failure relating to the 1st LO. In this case the board was a MK3 and the RX was working perfectly. The failed test implied that the varactor voltage was out of spec. It very definitely was NOT and the relevant paragraph in the service manual was about as useful as a chocolate tea-pot!! I eventually tracked the problem down to a wrong value resistor on the LO board. This was a part of the board which was dedicated to the BITE, hence why the RX was working fine. Not only that, but I found another two MK3 LO boards with the same anomaly. The 4th board that I looked at complied with the schematic. I replaced the errant 1K resistor with a 22K as per the schematic and the RX sailed through the entire BITE sequence without a single failure.  It is possible that changing the 22K for 1K was for a pre-BITE system or for a board setting known as ‘Quick-Lock’ … but I can’t be sure.

The A4 board (with derived BW3) in my RA1792