[Milsurplus] Decent AM Audio with the RAK Receiver.

[David Stinson]

Several people have expressed an interest in
getting better AM audio from the RAK receiver
without destructive modifications.  The culprit is
a low pass audio filter between the Detector and
First Audio stages:
 
https://photos.app.goo.gl/bxSwRhnheg5iWmE29

This filter, L-107, severely attenuates highs-
40dB down for 1600 Hz and above.  It's easy to
disconnect and bypass this filter.  The selectable
audio filters will still function for CW.  With
the filter bypassed and a properly-matched, a
quality speaker or headphones, AM signals sound
acceptable.  Not "HiFi," but good communications
audio.

Remove the receiver from the case.  Use your
engine block crane or a strong nephew to put the
receiver on the bench on its right side:

https://photos.app.goo.gl/cGSsvBUTc2T2Y2CR6

Be careful tilting the rig or placing it "bottoms
up" as one can break the power terminal strip or
crush the coil cans, among other nastiness that
will bite the careless.

Under the receiver bottom, remove the 87 screws
holding the cover over the band switch/RF section.
Someone I won't name (but I shave his face every
day) had the sense to have a holder for all those
screws handy, and the not-sense to set it too
close to the edge of the bench.  I really enjoyed
crawling on that nasty floor to find the last
screw and washer- oh, yes.

Bottom of the chassis, at the inside top near the
rear, you'll find C122 (large brown mica mounted
between a board of three resistors and a small
oil-filled cap).  There is a white wire connected
to the inboard side of this cap.  Unsolder it and
move the end out of the way.  Solder a nice long
piece of insulated hook-up wire to this point
(orange wire in the following photos):

https://photos.app.goo.gl/a89jc56QPZwjpvcr8

Route the wire through the bandswitch area toward
the side of the switch enclosure nearest the front
panel- make sure it's clear of the switch
mechanicals.  Route the wire out one of the holes
on this side of the enclosure:

https://photos.app.goo.gl/hGchYX6CV2KGsHLb9

Underside of the rig, at the extreme upper left
corner next to the front panel, there is a hole
for passing wires to the chassis top.  Route your
wire through this hole:

https://photos.app.goo.gl/UnT7bbqJeJm5jTxt8

Top of the chassis, mounted on the inside of the
front panel near the 6D6 1st Audio and 41 Audio PA
is a terminal board with some resistors.  At the
top of this board is a terminal with a white wire
and a blue wire.  Disconnect the blue wire and
move out of the way (yes; I did put heat shrink on
these removed wires).  Solder your new lead to
this terminal:

https://photos.app.goo.gl/WpeiMoAf4LuAQkyt5

That's it.  Your RAK should now play with decent
AM audio and the switchable audio filters are
still functional:

https://photos.app.goo.gl/575SjtvwZxoKzX9X6

--------------------
P.S.  
On the "tip" about not using a standard headphone
plug:
I said "three circuit" when I should have said
"two circuit."  It's commonly called a "quarter
inch Stereo headphone plug," which has a tip, a
ring and a sleeve.  Audio channels are the tip and
ring, the sleeve is ground:

https://photos.app.goo.gl/uBx5S47vhZSp25jU9

Here's the audio output circuit:

https://photos.app.goo.gl/WRH2cVyawhxPWntW9

As you can see, the audio output is balanced with
a center-tap to ground.  Terminals 7 and 8 are
audio out to an external controller.  The other
connection is for the headphone jack on the front
panel.  This is a "stereo," two-circuit jack:

https://photos.app.goo.gl/r7QEduCGzJzoHehaA

If you insert a common "single circuit" plug into
this jack, its sleeve will short half the audio
output transformer's secondary to ground.  Speaker
or headphones should be connected "tip to ring"
without connecting to the sleeve (ground) or, if
you wish to use half the secondary winding,
connect tip-to-sleeve or ring-to-sleeve and leave
the other circuit open.

GL OM ES 73 DE Dave AB5S