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

[Dan Merz]

Dallas,  I changed the K5OP audio agc  gadget to the corrected config and it
seems to help a bit on the reduction of distortion on initial attack.  But that
aspect wasn't too bad without it with the diode mod in place.  I did the diode
mod myself and I'm pretty confident that it's installed the way it should be.
My story is a little more complicated than  I stated since I'm using a 390 i.f.
chassis in my 390a radio but I've convinced myself that this is not the cause of
the quirk that I see.  I'll probably wait to see what characteristics you
observe with your unmodified i.f. chassis as far as carrier meter hang goes
since my 390a i.f. chassis also has the diode mod in it and I'd rather not undo
the mod at this point and stick it in to see its characteristic.  I would be
interested in the observation of anyone else on absence or presence of this
quirk of meter hang and whether or not the carrier meter is pinned in a set with
or without the diode mod when switching from slow to med on agc.   Dan.

Dallas Lankford wrote:

> 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.
>
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