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 HW- 100 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