[Hallicrafters] SX-111 AVC mods?

[Mark Shaum]

----- Original Message -----
From: "Mike Everette" <radiocompass at yahoo.com>

> Maybe the best approach to this is just to undo the
> changes, but I'd like to ask if anyone knows of a
> published article someplace dealing with such mods?
>

My QST search shows no articles specifically aimed at the SX-111 agc,
but I did perform a modification to my early 111 to give it a faster
attack time and slower decay time.  It only involved adding one
resistor, diode and cap to the existing circuitry and required no
modification to existing wiring, so it's easy to remove.  I have a
similar simple modification in one of my R-390A's.  AGC is very
subjective, I like my 111 with the changes.  If folks are interested I
can post the details next time I have the 111 out of the cabinet.  While
I can visualize the circuit in my mind's eye, I can't recall the exact
component values used.  Reminder to self, ALWAYS document any changes,
temporary or otherwise, and place the documentation either in an
envelope taped to the inside of the cabinet or stored with the manual.

Before starting any removal of existing circuitry, I'd suggest some
basic troubleshooting first with a signal injector or generator.  Start
at the AF stages and work back.  A VTVM check of the voltage at the
control grid of one of the tubes controlled by AGC will tell you if
there is anything seriously out of whack there.  Zero volts just means
there is no AGC applied and things should be running at full gain.

Don't forget the obvious.  When I fired up my SX-111 a few weeks ago for
the first time in a couple years it also was dead until I remembered I
had to place a jumper across the standby/muting screw terminals on the
rear panel!

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Mark Shaum K9TR
email: k9tr at dtnspeed.net
http://www.qsl.net/k9tr
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