Kenwood TS-520 SSB Transceiver Repair

​The Surprise

My father-in-law Ray, or "The Old Man from California" as he likes to refer to himself, surprised me a couple years ago by offering to give me the majority of his amateur radio equipment. Ray knew I was interested in amateur radio as I have built several regenerative AM vacuum tube receivers recently.
Ray was an avid Amateur Radio enthusiast in the 1970s and 1980s, acquiring many QSL cards, but in recent years he had let his amateur radio license expire and was content with just listening to the amateur radio bands. I think part of the problem was that he was tired of battling with the home owners association and their restrictive CCRs (Covenants, Conditions, and Restrictions) with regard to external antennas. Listening to the amateur radio bands only typically requires a modest internal antenna.
In the next couple months, my father-in-law shipped me the following from his collection:
- Kenwood TS-520 SSB Tranceiver
- Azden PCS-3000 2M FM Transceiver
- Ten-Tec Century 21 CW Transceiver
- Yaesu FRG-7700 All Mode Transceiver
- MFJ 949C Antenna Tuner
The Kenwood TS-520 was the most interesting piece of Amateur Radio equipment to me. It inspired me to study for my amateur radio license.

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​The Problem

I worked hard one summer to get my Technician and then my General Amateur Radio License. At the time, I just had a make-shift long wire antenna composed of several test leads connected together terminated to the antenna output jack on the TS-520 by a banana plug. With this simple antenna I was able to pick up transmissions mostly in the mid-west from my northern Ohio location. I was able to determine the origins of the transmissions by looking up the call signs of the senders.
Now I had my General Amateur Radio license and I was ready to tear up the High Frequency Airwaves, make many contacts, and send/receive many QSL cards. With the help of my father-in-law, I strung a G5RV antenna between a big Black Walnut tree and my house then installed a lightning arrestor at the end of the G5RV ladder line. I then looped the transmission line to make the balun and ran the other end to my Radio Shack in the basement. I attached the MFJ 949C antenna tuner between the TS-520 and the transmission line from my G5RV and grounded both the TS-520 and the antenna tuner to the main water line pipe that comes into our house. The main water line is conveniently located under the desk where my amateur equipment is located.
I figured I better transmit into the dummy load built into the antenna tuner first to see the output wattage of my TS-520 so I switched to the 40 Meter Band, changed the mode switch to CW and keyed the microphone. How disappointing, the cross-hair meter on the antenna tuner only indicated 10 Watts P to P. The TS-520 manual indicates it should output 160 Watts P to P!
Okay, so I read the manual and discovered the page on Transmitter Tuning, I am from the "point and click" generation an it never dawned on my that I would have to tune the transmitter circuit for resonance. I figured the rig would take care of it. I went ahead and followed the Transmitter Tuning procedure then I keyed the microphone again and now the transmitter power output was 50 Watts P to P through the dummy load. This was an improvement but still far short of the 160 Watt specification. Time to crack open this vintage TS-520 and see what has gone awry!

​Safety Warning!

Unplug the TS-520 from the power source before opening the cabinet. This rig can generate potentially life threatening voltages of over 800 Volts DC that is used to power the vacuum tubes in the final transmitter power output section of the transmitter.
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Step 1 Top Cover Removal

Remove the four top screws circled in red.

Remove the two screws circled in red on the side with the handle.

Remove the two screws circled on red on the side with the feet.

Remove the connector from the internal speaker.

​Great Tip!

​I use disposable salad bowls to hold all of the screws I remove during dis-assembly.

​Step 2 Bottom Cover Removal

Pull the plastic clips circled in red then remove the side cover.

Underneath the side cover is the Side Panel Controls.

Remove the two screws circled in red on the side with the handle.

Remove all screws circled in red from the bottom, you should now be able to pull the chassis free from the bottom cover.

​Step 3 Cathode Resistor Visual Inspection

Inspect the Cathode resistors (circled in red), installed on the bottom side of the Final Power Amplifier Board, for overheating and charring. Typically these are 10 ohm 1 Watt resistors. A previous owner replaced these with two 20 ohm resistors.

​Step 4 Cathode Resistor Resistance Check

All cathode resistors are in parallel and connect to ground. Resistance from the top of any Cathode resistor to ground should be 5 ohms. If over 5 ohms replace.

​Step 5 Cathode Resistor Replacement

Cathode resistors must be replaced if charred or the total resistance of all cathode resistors is over 5 ohms. I replaced with two 10 ohm 1 Watt Flame Proof Resistors.

​Cathode Resistor Replacement Warning!

Each Cathode resistor should be replaced with a 10 ohm 1 Watt resistor. Kenwood uses these resistors as fuses to protect the finals and power transformer if the rig is mistuned for any length of time. Replacing these resistors with larger wattage ones could allow more expensive parts to be damaged!
Do not replace the Cathode resistors with wire-wound types. This could introduce unwanted reactance into the transmitter circuit.

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​Step 6 Testing after Cathode Resistor Replacement

Once the Cathode resistors were replaced, it was time to bench test the TS-520 by transmitting into a Dummy Load to see if this resolved the problem. Be careful when powering up the rig with its cover removed. Do not touch any of the exposed circuitry in the chassis while it is plugged in.
In order to do an apples to apples comparison, I once again set the TS-520 to the 40M band then performed the Transmitter Tuning procedure in the manual. I then set the mode switch to CW and keyed the microphone button while watching the cross needles on my MFJ 949C set in the Dummy Load position. The power output was an improvement, it increased from 40 Watts P to P to 70 Watts P to P. This was a great improvement and I was excited but it still was not up the 160 Watts P to P specified in the manual.
Time to remove power from the rig and continue to troubleshoot issues in the transmitter power amplifier area!

​Step 7 Final Power Amplifier Cover Removal

Remove the three screws, circled in red, from the side of the Final Power Amplifier Cover.

Remove the three screws, circled in red, from the top of the Final Power Amplifier Cover. Pull cover straight up and out.

​Step 8 Discharge Final Power Amplifier Capacitors

Before proceeding, touch a test lead between the metal chassis and the metal top of the coil that sits between 6146B vacuum tubes to discharge any remaining high voltage!

​Step 9 Vacuum Tube Testing

Remove the Anode cap then carefully pull the vacuum tubes, circled in red, up an out of thier sockets.

Remove the driver vacuum tube, circled in red.

Use a suitable tester to test each tube. Replace if tube is defective or shorted. 6146B tubes do not have to be replaced in matching pairs. If you replace a 6146B tube, you will have to following the "Transmitter Neutralization" section in the manual.

Step 10 Testing After Vacuum Tube Replacement

If you have to replace the driver or final tubes you should retest by transmitting into a dummy load to see if this resolved the problem. Be careful when powering up the rig with its cover removed. Do not touch any of the exposed circuitry in the chassis while it is plugged in.
When retesting, do an apples to apples comparison. First perform the Transmitter Tuning procedure in the manual, then set the Mode switch to CW and the Band switch to 40M. Key the microphone key and measure the transmitters output wattage.
All of the vacuum tubes tested normal in my TS-520 so I am going to remove power from rig and proceed to step 11.
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​Step 11 Final Power Amplifier LC replacement

Many TS-520 repair sites recommend replacing the components circled in red even if they test good. C1 .001 mfd 3KV, C2 .0047mfd 1.4KV, L1 5uH 500ma RF Choke. This is my next troubleshooting step as these parts are relatively cheap.

I recommend removing the rear cooling fan for easier access to the C1, C2 and L1.

Comparison of the original components to the new ones. The blue capacitors and the small encapsulated inductor are the new components.

Picture of new LC components soldered in place. I had to use three capacitors in parallel to achieve the same capacitance and working voltage rating for C1.

​Step 11 Testing after Final Power Amplifier LC replacement

Once the capacitors and inductors were replaced in the Final Power Amplifier, it was time to bench test the TS-520 by transmitting into a dummy load to see if this resolved the problem. Be careful when powering up the rig with its cover removed. Do not touch any of the exposed circuitry in the chassis while it is plugged in.
In order to do an apples to apples comparison, I once again set the TS-520 to the 40M band then performed the Transmitter Tuning procedure in the manual. I then set the Mode switch to CW and keyed the microphone button while watching the cross needles on my MFJ 949C set to the dummy load position.
SUCCESS!
The power output increased from 70 Watts P to P to 160 Watts P to P which was specified in the manual. A combination of replacing the Cathode resistors then replacing capacitors and an inductor in the final transmitter amplifier resolved the problem.
Only one problem, when I changed frequency bands the power output decreased again. Upon re-reading the manual it dawned on me that I have to perform the Transmitter Tuning procedure each time I change frequency bands in order to bring the Final Power Amplifier into resonance. 160 Watts P to P was achieved on all frequency bands each time after the Transmitter Tuning procedure was performed from the manual.

​My TS-520 rig at Full Power Output!

​My Radio Shack ready for some serious CQ DX!

The recently repaired TS-520 is the centerpiece of the Radio Shack.

​Conclusion

As you can see, I was able to diagnose and repair the low transmitter power output problem of my Kenwood TS-520, at the minimal cost of a handful of passive electronic components, and a few weekday evenings.
I hope this hub encourages you to revisit a TS-520 that you have sitting on a shelf or in a closet collecting dust, due to a low transmitter power output problem, and bring it back to life!

​Useful Kenwood TS-520 Links!

Kenwood Amateur Radio Hybrid Pages

​Great resource for repairing Kenwood Hybrids such as the TS-520. Also a great source for hard to find TS-520 parts!

Kenwood Hybrid Restoration & Repair

Kenwood Hybrid Restoration & Repair, Models TS830S, TS530S, TS820, TS820S, TS520, TS520S, TS520D, TS520SE, TS520SP

KG6HAF : Free Downloadable Kenwood TS-520 TS-520S Series Manuals

Downloadable manuals and info for Kenwood TS-520S, Manuals for DG-5, Manuals for AT-200, AT-230 Manuals for VFO-520S