[Hallicrafters] SUCCESS!! SX-71 BCB performance now HOT!

[Philip Atchley]

Hello all
Those who have been on the list awhile have undoubtedly read my reports of
the extremely poor sensitivity of my refurbished  Hallicrafters SX-71
receiver.  This to the point that even semi local powerhouses at the low end
of the dial barely moved the S meter.  Conditions gradually got better
towards the top of the band.

I suspected that the antenna coil for the BCB was bad as one winding was
blackend and looked burnt, even though continuity was good through both
windings.  This set had a power line to chassis capacitor blown up and I
suspect the BCB coil took the brunt of the current.

Anway, Waaay back on the 3rd of May I ordered 'generic' BCB coils for both
the antenna and RF amplifier coils from Antique Electronic Supply.  These
were ordered via PRIORITY mail, were mailed on the 3rd and finally arrived
today.  All the way from Arizona to California 8^(

Well, I first installed the antenna coil and did a preliminary alignment.
WOW!  What a difference, especially on the low end of the dial.  I then
installed the RF amplifier coil and DOUBLE WOW!  Yee Haw!   I now have BCB
sensitivity.  Yes, even replacing the RF amplifier coil made a big
difference.

Here are some "On Air" tests of before and after the coils were changed and
the set peaked.  three stations were "benchmarked" before installation then
rechecked afterwards.

Frequency          Before              After
570kHz              S5                     S9+40
760kHz              S4                     S9+30
920kHz             <S1                    S9+10

One of the best things about these coils is that they are slug tuned so that
you can set up both the antenna and RF amplifier coils to track across the
band.  NO NEED to rock the oscillator frequency at 600kHz to get maximum
sensitivity, set the LO for exactly 600kHz on the dial and peak the coils.

An examination of the RF amplifier coils shows why chaning it made such a
BIG difference, even though the original was not bad.  The original MAY have
about a dozen turns of fine wire on the plate winding.  This on a air core
coil form doesn't give very much inductance at BCB band frequencies!   The
replacement has many turns on the plate winding.  That combined with the
ability to actually peak it makes all the difference in the world.

Installation notes:

1.  The new coils have 4 lugs just like the originals, BUT, they are not
pinned out the same.  Be sure to read the little paper packed with the coil.

2.  After removing the original coil you can drill out the original mounting
hole with a 5/16ths inch drill to "clip mount" the new coil, OR you can
suspend the new coil by the stiff bus wire these coils are wound with.  I
did the former.  Doing it the former way allows you to align the set after
modification from the top, with the bottom cover plate in place

3.  On both coils I had to "extend" one ground wire to reach where it needed
to go, all other wires reached ok.

4.  Be EXTREMELY careful with the new coils.  Though I'm careful in my work
I broke one fine wire on each coil in the process of connecting the heavy
bus wires to the lugs and soldering them.   An easy fix but frustrating when
you turn the radio on and it isn't doing what you think it should 8^(  Of
course all good techs "continuity check" all their wiring before firing a
rig up anyway. (don't they?)

5.  I did not re-install the 100 Ohm resistor that is across the plate
winding of the RF amplifier coil.  With the ability to peak the coil there
is no need to broadband this circuit and the receiver shows no overload when
tuned to a weak station 20kHz away from a local blockbuster station.  IF you
do get overload you can add it, decreasing the sensitivty of the set.

There is NO DOUBT in my mind that EVERY SX-71 receiver out there would
benefit from this mod.  I also suspect that this same "modification would
probably improve many other communictions receivers that have a "standard"
550-1600kHz BCB band.  As these coils are not tapped but rather discrete
primary/secondary windings they should be usable in any radio covering that
range.

Part numbers you ask?  I almost forgot that part of it.  They are available
from AES, here are the P/N's and prices.

Antenna Coil  P-C70-A           $7.75
RF Coil          P-C70-RF         $7.75

Shipping/handling                    $6.00

73 de Phil  KO6BB

REAL Radio is heard on Boatanchor Receivers.
REAL Music is played through  'Valve' Guitar amps!
k06bb at elite.net
Merced, Central California