[Premium-Rx] RA6790/GM AGC oscillation

[robinson at tuberadio.com]

Hi Nick,

Have you sorted this out yet?

In closed loop systems,
I usually break the loop at a convenient point,
then manually control the input some how
and observe where the problem is.

In your case
break the DC loop somewhere
and use a bench power supply to supply the AGC.

Alternatively
break the IF loop somewhere
and use a signal generator to supply a signal.

Twiddle these up and down
within their limits
and find which stage is faulty.

Not so easy.

Regards
Ray

> 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
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