The Kenwood TS-520’s Noise Blanker board, ID# X54-1080-10,
can be installed in a Heathkit SB-303 receiver quite easily. The
Noise Blanker board operates directly off of the SB-303’s 15VDC
supply. Therefore, you MUST ensure the SB-303 voltage adjust
is made, as outlined in the SB-303 assembly manual, BEFORE
you add the TS-520 Noise Blanker board to your SB-303
receiver.
It is advisable to have the complete assembly manual for the
SB-303 including the full schematic before attempting to
perform the Noise Blanker installation and audio wiring
changes. While I have made every effort to make it as easy as
possible, your electronic knowledge, skills, and ability to read
and understand a schematic plus your ability to properly and
safely drill mounting holes for the Noise Blanker board in your
SB-303 are a must! If, for any reason, you do not feel
comfortable installing the TS-520 Noise Blanker board and
making the necessary wiring changes to your SB-303 receiver,
do NOT repeat DO NOT attempt to perform the changes to your
SB-303. I will not acceptable responsibility for nor will I be held
accountable for your lack of skills and knowledge when adding
the TS-520 Noise Blanker board to and making wiring changes
to your SB-303 receiver!!!!
The SB-303’s RF gain control Pull/Push audio switch wiring is
changed to control activating/deactivating the Noise Blanker.
This is very easy to do. The audio wiring is changed from it’s
original configuration and the headphone socket is changes
from a non-switching socket to a switching socket. If you don’t
know what a switching socket is and how it works then you
better learn because I’m not going to explain it to you!!!
Remove the receiver from its cabinet. With the front panel
facing you, look down behind the front panel at the RF Gain
control. There are 2 wires attached to the switch, a DARK BLUE
wire and a WHITE wire. The DARK BLUE wire is connected to
the rear panel “SPKR” socket. The WHITE wire connects to the
back plane’s “AUDIO OUT” terminal.
Looking at the back plane’s AUDIO OUT, there are 2 WHITE
wires, one connects to the rear panel’s ANTI VOX socket and
the other connects to the RF Gain Pull/Push switch. Disconnect
both WHITE wires from the AUDIO OUT terminal then used an
ohm meter to determine which WHITE wire connects to the
rear panel’s ANTI VOX socket. You CANNOT determine which
WHITE wire connects to the rear panels ANTI VOX socket
without disconnecting both WHITE wires then use an Ohm
Meter. The WHITE wire that connects to the rear panel’s ANTI
VOX socket is reconnected to the backplane’s AUDIO OUT
terminal and soldered. Use just enough heat to solder the wire
to the terminal. The other WHITE wire is left unconnected, it
will be connected later to the Noise Limiter’s “NBS” terminal.
The DARK BLUE wire that connects to the rear panel’s SPKR
socket terminal is disconnected from the SPKR socket terminal
and reconnected to and soldered to any nearby socket ground
terminal.
Next, replace the original non-switching headphone socket with
a 1/4” mono switching socket. These sockets are available on
eBay, All Electronics, or Mouser. They were also used in the
HW-100, HW-101, and SB-100 line of as well as the SB-300 and
SB-301 receiver’s headphone socket. I pulled mine from an HW100 parts radio.
The original headphone socket has a single “WHITE” wire
connected. Disconnect this wire as close to the socket terminal
as possible then tape the end and push it out of the way, it will
not be used. Remove the original headphone socket and install
the replacement ¼” switch socket. Look closely at the switch
socket and you will see the “TIP” terminal and a thin strip
terminal, these act as a “switch” when the headphone plug in
inserted or removed. The “Thin strip” terminal is connected to
the “SPKR” terminal. The “TIP” is connected to the Backplane’s
“AUDIO OUT” terminal using shielded cable. I used stereo
interconnecting cable they work fine for this application. Audio
is always present at the TIP terminal.
Next, use an ohm meter, connect one lead to the chassis and
one lead to the lose WHITE wire taken from the AUDIO OUT
back plane terminal. Pull the RF Gain control knob OUT the ohm
meter should indicate “infinity”. Push the RF gain control IN and
the ohm meter should be zero ohms (short). An analog ohm
meter is best for this test, easy to see the meter movement
instead of reading numbers in a DVM.
Next, remove the RG-174 jumper from the receiver’s Mixer
board’s OUT socket. Remove the other end of this coax cable
that connects to the crystal filter select wafer switch and
ground tab below the chassis. You won’t use this but keep it if
you plan on removing the Noise Blanker later on.
Next, make a soft cardboard template of the Noise Blanker
board, tracing the outline of the board and all 4 mounting
holes. Cut the temple and clear the mounting holes on the
template. Place the template on the outside of the left side
panel, making sure the placement of the board on the inside
left side panel will not put the Noise Blanker transformers T1,
T2, and T3 equal to or below the Preselector shaft.
Next, mark the 4 mounting holes on the outside left panel. I use
DuckTape to hold the template against the outside left panel
then used a metal scribe to trace the mounting holes on the
outside of the left side panel. Mark the center of each
mounting hole so the drill bit will NOT walk outside of the
center of the mounting holes when drilled. Place the receiver
on it’s right side, rotated the Preselector so the capacitor plate
were fully meshed, and lay a cloth over the variable capacitor
to keep any metal shavings from failing into the capacitor. Drill
the mounting holes using a 3/8” drill bit. Make sure the drill bit
does NOT walk out of the center of the mounting hole while
drilling. Clean the shavings from the holes then use a vacuum
cleaner to remove all metal shavings. Turn the receiver upside
down and gently shake the receiver to allow any shavings to fall
out. Inspect the receiver “thoroughly” to ensure all shavings are
removed!!
Next, mount the noise blanker on the inside left side panel with
the board’s 6 pins closest to the front panel, using 3/8” metal
threaded female standoffs and eight 3/8” 6-32 machine screws.
The board should fit snug against the side panel and the Noise
Blanker T1, T2, and T3 transformers should not be blocked by
the Preselector shaft. There should be no shorts between the
mounting holes and the surrounding points on the Noise
Blanker board when installed. CHECK your work
THOROUGHLY!!!! If you failed to check for and remove any
shorts then you could end up ruining your receiver AND the
noise blanker board!!
Next, make up 2 RG-174 coax cable jumpers, one about 4 to 6
inches long and the other 8 to 9 inches long. The actual lengths
are determined to ensure the cables are not stretched tight
when installed. The short length will be used to connect the
receiver’s Mixer board OUT socket to the Noise Blanker IN and
GND terminals. An RCA plug is connected to one end of the
short RG-174 cable and plugged into the Mixer board’s OUT
socket. The other end of the short coax cable is connected to
the Noise Blanker’s “IN” and “GND” terminals.
The longer coax cable is connected from the Noise Blanker
“OUT” and “GND” terminals to the crystal filter select wafer
switch and ground tabs below the chassis.
Next connect a wire from the Noise Blanker “B” terminal to the
front panel FUNCTION switch wafer terminal with the
“ORANGE” wire (+15VDC).
Next, connect a wire from the Noise Blanker’s “NBS” terminal
to the loose WHITE wire removed from the AUDIO OUT back
plane terminal. Use heat shrink tubing over the WHITE wire
connection to prevent grounding to the chassis.
This completes the installation of the Noise Blanker. Check your
wiring to be sure there are no lose connections and no shorts.
Pull the RF Gain control OUT (disabling the Noise Blanker).
Connect a speaker to the receiver’s rear SPKR socket.
Power up the receiver, you should hear audio in the speaker.
Plug a set of headphones into the rear headphone socket, you
should hear audio in the headphones, not from the speaker. If
you do then you have rewired the audio correctly. If not, you’ve
made a mistake, there’s a bad solder connection, a broken
wire, or your wiring is wrong. Turn the receiver OFF and check
the wiring.
Next, turn the receiver’s calibrator ON, 100Khz or 25Khz, it
doesn’t matter. Adjust the tuning knob for a beat note heard in
the speaker and a peak indication on the S meter. Adjust the
Preselector for a peak S meter indication.
Next, adjust T1 thru T5 on the Noise Blanker for a peak in the S
meter. You should not have to adjust T1 thru T5 very much,
only a ¼ of a turn will result in a peak in the S meter. DO NOT
adjust the Noise Blanker’s T6 and T7 transformers!!!!!
Next, without touching the LMO or the Preselector, adjust the
receiver’s 3 IF transformers , T501, T502, and T503 on the
receiver’s IF board for a peak in the S meter. Repeat this
adjustment to ensure you have a maximum S meter indication.
This completes the noise blanker installation in your SB-303.
The Noise Blanker is powered ON all the time the SB-303 is
powered on. The only part of the Noise Blanker that’s turned
OFF and ON by the RF Gain control PULL/PUSH switch is the
Noise Blanker board’s noise gate stages.
If you want a copy of the TS-520 schematic that shows the
schematic of the Noise Blanker, go to
www.w5rkl.com/kenwood-hybrid-operator-and-servicemanuals/
and download the TS-520 service manual, it’s free.
Bob Mansker, KE5FTF, may have some 520 Noise Blanker
boards. Contact Bob at:
Ke5ftf at mail.com
for details on the availability of 520 Noise Blankerboards.
73
Mike W5RKL