[Premium-Rx] RA6790/GM AGC oscillation

[Nick England]

Thanks for the helpful suggestions.
Finally traced this problem down to a bad 4042 latch that controls the
AGC-related CMOS switches. One output pin was following the input, another
output was open (acting as an antenna), and another was stuck high. No
wonder the poor AGC loop was confused.

Now to figure out my frequency LCD panel with the inverted segments.

FWIW this is one of the RA6790/GM units with 5 filters from an eBay seller
in Virginia. Found a few bad tantalums and chips but otherwise an OK
gamble. I made an offer and paid $600 for it. YMMV.
Cheers
Nick

On Sat, Apr 17, 2021 at 5:37 PM Nick England <navy.radio at gmail.com> wrote:

> 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
>
-- 
Nick England K4NYW
www.navy-radio.com