[ARC5] BC-375 on 40 Meters? You Bet!

[David Stinson]

I love it when people tell me I can't do something,
like:  "The BC-375 won't work on 40 meters."
Oh yeah?
Just had a good QSO on 7160 KC and
will be joining the net there this weekend.
The rig drifts down about a KC after key-up,
then settles, but what of that?
As a friend used to say;
"Whaddayawant?  Egg in ya beer?"
If a person is using an AM detector
as The Almighty obviously intended
for these nets, 1 KC's not a problem.
And it sounds as good as it does on 75 meters.

There are three keys to getting a 375
to settle down on 40:

First- Make sure you have good tubes.
I had a stash of about 20 untested 211As.
Once I got the rig going, I tested each one
in the 375.  I now have 12 good ones
(and two less HV fuses).
Most had gone gassy.  Some had open fils.
Two had no emission.  One of those got fixed
by re-heating the fil pins with a frame iron.
The other I tried to "rejuvenate" and it worked,
at first.  Then I accidently popped the grid with
B+ while the fil was "cooking" and now the tube
has incurable grid emission.... rats!

Second-
A good ground plane under the whole rig
and a good ground connection.
Thank you, Mike Hanz, for waking me up to this.
You can see my SCR-287, which is undergoing refurbishment,
by looking-up AB5S at QRZ.com.  On each of the three shelves
(PE-73 dynamotor and power supply out of frame on the bottom)
and up the back of the wooden rack is a pattern of aluminum
"duct" tape you can get at any "home despot"store.
Each shelf has a cross-hatch of four runs which
go from edge-to-edge and are spaced to make full contact
with the shock mounts, dynamotor and supplies.   Where the tape
crosses, either a portion is "bent under" to make contact or
a small piece is folded and placed across the two runs
to make a contact point.  Another small piece of the tape
over the "fold" secures this down and makes a good contact.
The shelf patterns are then tape-connected to the vertical.
All is painted-over except for the places where the shock mounts
and other pieces need a ground contact.  An ohmeter from the top
of the transmitter to the bottom shelf reads a few ohms, and that's
just fine.  The station ground hooks to the rack ground plane
and you're good.

Third:
Careful neutralization.   This is tedious, but not as "scary" as it 
sounds.
If you have a service monitor that can show you FM deviation,
you're in "high cotton."   If not, you need a good receiver with
a narrow filter (2.8 KC SSB will do) and a bucket of patience.
Set your monitor receiver to SSB (yes... SSB).

Warm-up the rig for 15-30 mins.
Tune up the rig into a dummy load in "VOICE."
Unscrew and remove the tuning chart.
Under it, you'll find an adjustment wheel and a locking screw.
Unlock the wheel with a quarter-turn on the screw.
The wheel is your neutralizing control.  It will have a mark on
it that matches one on the face of the tuning unit.  Unless something
is wrong with your tuning unit, the "sweet spot" that will stop
the FMing is going to be within a few degrees of that marked point.
Use a "locking bar" key to put the rig in transmit; you're going to 
need
both hands.   Let the rig settle. It should only drift down about a KC
before settling.  If a lot more, you have a problem in the tuning 
drawer.
Set your monitor receiver off zero-beat so you can hear the carrier at
a few hundred cycles.  Say "FOOOOOOORE" into the mike while
listening to the low beat note.  If you have a tone generator to use
for modulation, so much the better.  If the rig is FMing,
you will hear the beat note go "ragged" under your modulation.
Raspy.  Tweak the wheel a degree or two.
Tune the MO until you get the same
beat note again (the neutralization control will pull the MO)
and repeat the above.  Do this until the beat note
 sounds more "solid" when you modulate the rig.
Then set the receiver on AM and carefully, gently tweak,
retuning the MO each time, until your audio sounds best.
We're talking small movements here.
I use the tip of a jewelers screwdriver with my hand
resting on the tuning unit.
The upside- once this is done, it will "stick" until you change tubes.

Don't let it scare ya.... go for it.

73 Dave S.