Output for the K2
I have had several requests to provide a fixed
from the K2. After looking at the
circuits that others have designed, I found that they used single ended
input and were usually incorporated with some other circuits that were
useful to me. I already have my
external Digital Modes Switchbox built to automate digital
transmissions, but I
still lacked the fixed audio that would allow me to position the K2
anywhere without disturbing the gain settings on my soundcard.
I choose a LM386N-3 Audio Amplifier IC for its
inputs, high available gain, small packaging, and low external parts
count. The –3 version works well from
the 8 volt supply in the K2. It seems
not widely known that the LM386 works just fine into a high impedance
well as low impedance. The amplifier is
directly specified for voltage gain (Av) which then gives rise to the
available once the load resistance is known.
The power available increases as the resistance decreases (P = V2/R)
but the voltage is independent of the load.
After building a couple of prototypes successfully
perfboard, I talked with Tom Hammond N0SS at Dayton 2004 and Tom
offered to lay out a small PC board for this application.
Tom did more than just do the PC board
layout, he made up a nice documentation sheet with the schematic and
list, provided some sample boards for evaluation, and provided the
layout to FAR circuits for final board production – then he even
shear the panels of boards for me too, I owe Tom a lot for his help on
project – without his assistance, the etched PC board would not have
I no longer have kits nor boards for this project. The circuit is
simple enough that it can be
easily handwired on a small piece of perfboard, or even built Manhatten
This Fixed Audio Output should serve the needs of
want fixed audio for digital modes as well as those who would like to
tape recorder or other audio device.
The following pages provide instructions for
mounting my full kit.
The schematic, parts list and board layout are
(compliments of Tom Hammond).
Insert the components into the circuit
solder. Clip all leads as close to the
board surface as possible (use flush cutters if available)
The plastic locking bar on the top,
of the board
mounted (male) connector is not needed in this application and should
removed to reduce the force required to remove the mating female
connector. Clip this bar off with
diagonal cutters. The keying function
of the connector is retained even though the locking bar is removed.
Drill a 15/64 inch hole in the rear
for the RCA jack. This hole should be
centered no more than
5/16 below the upper edge of the panel and ¾ inch from the left edge
key jack). The clearances are close –
use care and be certain the jack will clear the 40 meter bandpass
inductor located nearby.
De-burr the hole for the jack.
Mount the RCA phone jack with its
lug. Orient the solder lug toward the left
Cut 2 - 1 inch squares from double
Place one square of the foam tape on
solder side of
the PC board and remove the protective paper
Place a second layer of foam tape over
a double layer.
See the KPA100 Mounting Note at the end
document if you are mounting this board in a K2/100.
Position the PC board on the left side
above relay K13 and as close to the RF board as possible without
interference. Press the board firmly to
be certain the foam tape adheres securely.
flat cable into a 4-conductor cable and a 2-conductor cable.
Mark one edge of each cable all along
identify conductor #1. A permanent
“Sharpie” marking pen does an excellent job.
At one end of each cable, strip 1/8
and tin the wires.
Solder each conductor to a Molex female
terminal. Be careful not to get excess
solder onto the outside of the terminal. Only
a small amount of solder is required.
Insert the terminals into the housings.
Place the keying tab of the housing up and
with the cable to be inserted on the left side, the marked edge of the
will be inserted into the edge away from you.
If you are uncertain of the cable
with the markings on the PC board. The
marked conductor #1 of the 2 conductor cable will mate with the AF OUT
conductor #1 of the 4 conductor cable will mate with the DC IN pin.
You will have to separate the conductors
for about ½ inch
back to insert the terminals into the housing.
- Cut the 2
conductor cable to a length of 7 inches (including the housing). Separate the conductors about ½ inch, strip
1/8 inch and tin the wires.
Solder the marked conductor #1 of the 2
to the center terminal of the RCA jack.
Solder the remaining conductor to the
solder lug of the
Cut the 4 conductor cable to 10 ¾
Separate the 4 conductors into two
for a distance
of 2 5/8 inches.
Cut 2 3/8 inches from the pair that is unmarked
(conductors 3 and 4). Strip 1/8 inch of
insulation from each conductor and tin.
Remove the Control Board from the K2
solder side up and the edge containing the pins that connect to the RF
closest to you.
Locate P2 pin 5 (+8 volts) and solder
conductor #1 to it.
Locate P2 pin 7 (ground) and solder
conductor #2 to it.
Locate P3 pin 1 and solder conductor #3
Locate P3 pin 2 and solder conductor #4
Where the cable exits the left edge of
Board, bend the cable sharply so it exits toward the component side of
board. Place a piece of electrical tape
across the cable near the edge of the board to keep the cable in place.
Replace the control board in the K2.
The side to side fit will be a bit tighter
than before because of the thickness of the cable, so use appropriate
care. Be certain the added cable exits
close to the RF board to allow clearance for the KDSP2 option if
installed. You may put a few creased
folds in the cable so it remains in place and will plug into the new
Plug both cables into the fixed audio
board. Position the 2 conductor cable to
jack close to the RF board to allow clearance for the KPA100 or
Turn the K2 on and tune to a suitable
Set the 10k potentiometer on the fixed
audio board for
a level suitable to drive your external audio equipment.
Up to 3 volts p-p (1 volt rms) with a 50
microvolt signal applied to the antenna input is the normal output
Connections to the control board
Routing and securing cable onto the control board
Installed Fixed Audio Board showing cable routing
the RF Board.
Installing between the Front Panel
Board and the Control Board
I have added a PDF document which shows how I mounted the board between
the K2 Front Panel and Control Board, and is now my recommended
mounting method even though the components must be laid flat. If
you have level controls in your external device, the variable pot can
be replaced by a couple of fixed resistors. Since this mounting
may use short connecting leads, the connectors (the most expensive
parts) may be eliminated since all but the output wires are contained
entirely on the K2 Control Board. Right click this link to
download or left click to view here FP-CB mounting
NOTES for mounting in a K2/100
The side-panel mounting position becomes quite
the K2/100 with the upgraded shield and its speaker shield. It depends on how the speaker shield cap was
To determine if you can use the side panel
assemble the K2/100 and remove the left side panel.
Test fit the fixed audio board
to see if it projects above the plane where the inner surface of
side panel will be (use a straightedge from top to bottom). You may have to position RF Board C60 a bit
away from the edge, and place the Fixed Audio board so it overlaps the
Measure the position VERY carefully and place the
the side panel, or you might be successful if you remove the paper from
foam tape and lay the side panel in place (move it straight downward
side movement) – if the board sticks to the panel it will be in the
place, if it doesn’t stick, you will have to measure the position
translate the mounting position onto the side panel.
Measure to the nearest 1/32 inch (1 mm) and “measure twice,
Should you find inadequate room to mount it on the
panel, you can use an alternate mounting location near the center of
board. Remove the KPA100 and locate the
IOC chip – it is the one in a socket located to the left rear of the
board. Remove only a portion of the
paper from the foam tape and stick the Fixed Audio Board onto the top