[R-390] AGC

[Dennis Wade]

My $0.02:  I've had to remove from the circuit the 16, 8 and 4 kc filters
because they were showing a measurable resistance to ground, and were
dragging down the AGC line, regardless if they were the selected filter or
not.

                          Dennis


On Mon, May 27, 2013 at 1:13 AM, Charles P. Steinmetz <
charles_steinmetz at lavabit.com> wrote:

> 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
>
>
>
>
>
>
>
>
>
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-- 
"If they trust you, it is an extraordinary privilege, and you simply can't
abuse it."
      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.

-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA