[R-390] Wise's Barebones Ending the Moment of Silence

[David Wise]

R-390/R-390A AGC - Ending The Moment Of Silence

In a previous article I published a deluxe circuit which unfortunately
requires replacement of S107.  Here's a cheapie version that doesn't.

My recipe here is functionally equivalent
to Charles Steinmetz's.  His approach does away with the
capacitance multiplier; instead, SLOW is a 22uF cap and MED
is a 2uF cap, and each goes between the AGC line and ground.
Nothing tries to establish the right charge on a cap before
you switch it in.  Mine's the same except I use the multiplier.
SLOW is 2uF and MED is 0.15uF .

1. Add wire between S107.7 and S102.3 or 5 .
2. Cut wire at S107.8, insert 2uF or 2.2uF cap.
3. Remove wire at S107.9, add 0.15uF cap between 9 and 8.

These caps have to withstand B+, so 250V or more.
That's 5x voltage but 0.1x capacitance, so my parts are
smaller capacitance-voltage product: physically smaller,
and maybe a bit cheaper.

We've shorted out C551.  S107.7 is V506A grid.
The wire we cut off S107.8 is V506A plate.  That's all we need.
In SLOW, C190 (2uF) goes between grid and plate, same as before.
In MED, C191 (0.15uF) goes between grid and plate.  V506A
multiplies it by about 15 so to the AGC line it looks the
same as a 2uF cap to ground.

And that's that.  When you switch away from a cap, it retains
for some time whatever charge it had.  If you switch back
and the signal doesn't happen to be the same strength, the
system will take a moment to adjust, exactly as if the
station abruptly changed while you were listening.  This
will give you a Moment of Less/More Loudness, and
very short compared to the original Moment of Silence,
because less charge needs to be added or removed.

ASSEMBLY

Where to put our new caps?  How about the open space between S107
and the ANT TRIM shaft?  Is it enough?  The DigiKey catalog
lists six different 2.0 or 2.2/400 caps.  They're all less than
2" x 1-1/4" x 1-1/4".  If we elevate a cap this size to where it
doesn't quite graze the tallest nearby gear, it squeezes in under
the top cover (if used).  Let's mount it on a terminal strip.
Then C191 can go on the back side and hang upside-down in front
of the gear.  To avoid gluing or drilling the panel, we can mount
the terminal strip to a flat sheet-metal bracket anchored under
the terminal board mounting bracket.

PARTS LIST

Piece of 22ga sheet steel, 1" by 2-1/4".
Terminal strip, Radio Shack #274-688 (5-lug) or similar
0.15uF/400V capacitor, NTE MLR154K630 or similar*
2.0uF/400V capacitor, NTE MLR205K400 or similar**
A couple feet of wire

* It only needs to be 400V, but with NTE, the 630V model
has more convenient lead spacing.

** 2.2uF is fine too; there are six available from DigiKey,
they're all small enough, and several are cheaper.

TOOLS
Vise & hacksaw
Sandpaper
Drill
#1 Philips
#2 Philips
Dikes, stripper, soldering iron

Take your sheet metal, drill a 1/8" hole 1/2" from one end, and
sand the other half bright.  This is our mounting bracket.

Solder the terminal strip mounting lug to the bracket, exactly
1-1/2" from the hole, with the strip facing away from it.

Solder C190 (2uF) on the "normal" side, and C191 (0.15uF) on
the back.  Lug 1 is C190 and goes to S107.8, 2 is C191 and
goes to S107.9, 3 is unused, 4 is common to both caps and goes
to the wire you took off S107.8 .