[Premium-Rx] RA6790/GM AGC oscillation

[Cecil]

I’d love to have a 6790 but it appears they have moved out of my price range.

Used to be able to buy one for around $1k in nice shape with filters. It’s now closer to $3k.

Cecil 
K5DL

Sent from my iPad

> On Apr 21, 2021, at 5:23 PM, Nick England via Premium-Rx <premium-rx at mailman.qth.net> wrote:
> 
> 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
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