[ARC5] Lew McCoy's Q-Multiplier for BC-45x series

[Tom Bridgers]

Have been having a lot of fun lately getting a BC-454 to work.  But before I
describe the McCoy Q-multiplier, let me say that right from the start this
particular radio was interesting because of its obvious mods.  For one
thing, when I looked under the hood, a 50L6 stared back at me from where a
12A6 should be ... and instead of where a dynamotor sits, there sat a small
black box, of about the same size, which enclosed a transformerless power
supply.  Adding 1+1 and getting 2, I guessed the filaments were hooked into
the 120 VAC power, and they were.  But did this also mean that one side of
the power line was hooked into the chassis ground?  Yep; it sure was.  Not a
good thing!  So I rewired the filaments to be in parallel (for 12 volts),
replaced the 50L6 with a 12A6, replaced the 50L6 cathode resistor (of 100
ohms) with one of 1500 ohms for the 12A6.  Then I hooked the radio up to a
separate
power supply and it played fine.  Except that I noticed one of the 3-section
.05 caps beginning to get hot.  So I replaced it with individual .05 caps,
and the radio played ... and without anything under the chassis heating up.

At first I didn't know what the extra potentiometer (beyond the usual pot.
for gain control) was for; it was mounted on the side of the radio
using a bracket .  When the radio was on, and I turned the potentiometer in
one
direction, it would create a howling fit of oscillations.   But if the pot.
was left in certain positions, the radio played fine.

I didn't have a clue how to properly use this pot. until I ran across the
March 1960
issue of QST.  And there it was on page 46:  a Lew McCoy Q-mulitplier mod
for a BC-454.  Moreover the circuitry was simplicity itself.   The 2nd IF is
changed into a
Q-Multiplier by adding only 1 pot. and 1 cap!  And that is exactly the mod
that had been done to my receiver.

The circuit change consists of lifting V6's cathode resistor from chassis
ground and inserting a 5000 ohm pot. between the resistor end just lifted
and chassis ground.  The connection from Pin 3 of V6 to pin 5 is removed
from pin 5 and connected to chassis ground.  A "gimmick" capacitor is made
from two short lengths (each about 3/4 inch long) of insulated wire.  One
piece is connected to pin 8; the other piece of wire is connected to pin 4.
Now the unconnected ends of the two wires are left open; thus when
positioned in parallel next to each other,  the two wires serve as a
coupling capacitor between the grid and plate circuits of the 2nd IF stage.

Once I learned how to use the McCoy Q-multiplier, the improvement in the
receiver's selectivity was and is impressive.  Although getting there (to
the
improvement in selectivity) can be a tad tedious as you go back-and-forth
adjusting the pot and BFO --with the oscillations blaring in your ears --
until you
carefully and finally reach the IF "sweet pot" .  It took me several
attempts and readjustments to find the right settings.  And frankly at times
it was frustrating.  But once you find the IF sweet spot, you get
"almost" single signal reception with SSB ... and with CW, you can reach
single signal reception.  Remarkable, really, considering how little change
is required in the circuitry.

Was wondering if any of you are using a McCoy Q-multiplier mod, or other
similar Q-multiplier mod, on your ARC-5 or BC-45x series of rx's?    And if
so, what has
been your experience?  Also, are you using the original McCoy circuit, or
has the circuit been updated with improvements?
thanks,
-tom