[R-390] Correction To "Chambers Audio Derived AGC Mod"

[Dallas Lankford]

Hi Dan,

Sorry about the mistakes.

Your account of the CARRIER METER hanging rings a bell, but my memory just
isn't what it used to be.  Maybe it will come to me later.  Unless you did
the two diode mod yourself, my first guess would be that it was done wrong.
When I get a "new" R-390A, the first thing I do is restore it to original.
That has saved me a lot of grief in the past.

If the problem remains after restoring it to original, you need to swap out
the IF deck to determine if the problem is due to your IF deck or not.

The K5OP Audio Derived AGC probably won't work anyway for your R-390A
because from your description it appears that your SLOW AGC is the original
Miller effect circuit.  The attack time of the original SLOW AGC is
painfully slow, about 200 milliseconds according to my Hollow State
Newsletter article.  I'll stick my neck out on this one and say that the
K5OP ADAGC just doesn't move enough electrons to charge a SLOW Miller
circuit fast enough.

----- Original Message -----
From: "Dan Merz" <djmerz@3-cities.com>
To: "Dallas Lankford" <dallas@bayou.com>; "R-390 List"
<r-390@mailman.qth.net>
Sent: Friday, April 02, 2004 2:15 PM
Subject: Re: [R-390] Correction To "Chambers Audio Derived AGC Mod"


> Hi,  well,  I wondered why it didn't work and was pursuing the cause.  I
found the mentioned RS rectifier in my junk box,  still in the orig.
package,  probably 20 years old.  But I got no effect with your described
hookup - I'll try the corrected
> version shortly.
>
> But aside from that I found another quirk ??  in my 390a.  I've never been
happy with agc on the set - it has the two diode mod in the agc circuit.  I
began to wonder if the C551 cap was bad and tried the suggested test of
switching between fast,
> med and slow setting to see if the indicated carrier level changed as an
indication of leakage..  In doing this I noticed the quirk?? - when I
switched from slow ( which has B+ on one side of C551) to med or fast ,  the
carrier meter pins to the
> upscale direction then drifts back to a level that is about 20 db higher.
It stays at this level until I turn the rf gain down and back up,  then it
goes back to the lower level,  about 20 db down from where it went went it
drifted down from the
> meter pinned state.  While all this is going on,  I put a VTVM on the agc
line and don't see this happening with the agc voltage.  It didn't reflect
this quirk,  except it did go far upscale negatively when I switched from
slow to med but returned
> to a level that was the same as before the switch.  It there some kind of
latching of the i.f. tube on the other side of the carrier meter,  or is
this likely a bad carrier meter.  It seems to do it regardless of what the
signal carrier strength is
> and shows no quirks when I move it up and down with the rf gain control.
When I switch from fast to med to slow,  in that order,  the carrier level
stays the same.  Only when I switch back up to med or fast does the quirk
occur,  Dan.