[Premium-Rx] RA6790/GM AGC oscillation

[Nick England]

I'm troubleshooting a Racal RA6790/GM whose AGC line breaks into
oscillation if the signal is above a certain point. It's all a big loop
(RF/IF>AGC>RF/IF) so I'm having trouble isolating the problem. If I am in
MAN mode and have the manual IF gain control down low enough, all is well,
but at some point the oscillation starts.

I thought I'd be able to use MANUAL mode to break the loop, but the
IF-derived AGC voltage is still added to voltage from the manual IF Gain
front panel control. Why is this right? OK if you are in manual+threshold
mode it should be, but I would have thought that in manual mode, there
should be no signal-derived AGC.

Referring to Sheet 5 of the A4 schematics, there is a gate (U11b) from the
peak signal detector to the input of AGC amp U14a. This peak detector
signal gets added to the manual voltage (sourced via gate U11c, amp14c and
gate U12b). I'd have thought that gate U11b would get turned off when in
MAN mode. But it doesn't and the manual makes no note of when U11b is
turned off (U11b gate control voltage is from a microprocessor loaded latch
U13).

The AGC Circuit manual section 4.3.4.3 makes no mention of U11b.

Hints and suggestions greatly appreciated. Thanks.

Nick England K4NYW
www.navy-radio.com