[R-390] AGC

[Charles P. Steinmetz]

Dave wrote:

>It's been my understanding that people think the AGC in the R390A 
>was always not very good and reduction of the RF gain could compensate.

In a properly-working 390A, the AGC works fine for signals with full 
carrier (as it was intended to).  If there is a general issue with 
the AGC in a properly-working 390A, it is that, like pretty much all 
radios of the era, the AGC does not work very effectively on signals 
without full carrier (e.g., common SSB).

>I think there may be another problem going on.  As the filters age, 
>they have less resistance to ground through the deteriorating 
>foam.  Look at the AGC line in the schematic where it's connected to 
>R507 and that resistor is connected to the mechanical filters on the 
>output side which is RF grounded by C512.  If you have low 
>resistance in that circuit due to the filters, it certainly will 
>affect the AGC action by dragging the voltage down.  All it takes is 
>one of the filters to be non-compliant.

If the AGC is not working properly, the prime suspect is leakage to 
ground on the AGC buss.  This can be from leaky capacitors, spilled 
coffee, or, as Dave says, deteriorating filters.  The point is, find 
the leak and fix it.  So far, I have not seen an epidemic of leaky 
filters.  However, it is possible that the examples we have seen are 
a harbinger of things to come as the radios continue to age.  (Now 
don't you wish you'd bought a 390 instead of a 390A....)

Best regards,

Charles