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Kenwood TS-520 AM mod

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Author Topic: Kenwood TS-520 AM mod  (Read 14957 times)
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« on: June 14, 2005, 04:49:37 PM »

I pulled these from the 'net. If I can find the site I'll edit it in later.

TS-520 Increased sensitivity, Better audio and Amplitude Modulation

1. Increased sensitivity.

On the RF board, Q5 is the RF RX amp. Notice that G2 is tied to the AGC line.
Bend G2 up from the socket, solder a 10k resistor to it and connect the other end of the resistor to the 9v line on the front of the board. This will allow the FET to run wide open, and you will have a better S/N ratio. You will gain about two S units in signal and you may want to adjust your S meter to compensate. I did this mod almost a year ago and have had no problems even with signals that were 40db over!

2. Better audio.

A few capacitors should be changed to accept a wider audio bandwidth.
These will only increase the BASS response and not the treble as it is limited by the IF filter. Change the following on the generator board;

C3 (1F) to 10F
C4 (4.7F) to 47F
C5 (1F) to 4.7F
C8 (.047F) to 4.7F
C9 (1F) to 10F
C11 (4.7F) to 47F
C10 (10F) to 22F

This will make the Tx audio flat. Make sure that the Local Oscillator is adjusted correctly or else you will have the bass attenuated by the IF SSB filter.

Changed caps on the AF board.

C14 (.47F) to 4.7F
C13 (.047F) to .47F
C1 (1F) to 4.7F

Since the audio is limited to 4 KHz anyway, don't go bonkers with tweeters. You may want to play with a few caps inside the speaker to roll off some of the very highs which will only be transistor noise anyway.

Along with changing the caps for flat audio, install two jacks for audio on the back of the 520. RCA female jacks that are used to connect to an equalizer.
Unwrap the wire on the generator board marked MAO. This is the output of the Microphone Amplifier. Connect some microphone coax from this terminal to your RCA socket on the back of the rig marked output. Remember to ground both ends of the cable so you won't have hum/RF problems. Now connect the other RCA socket via mic cable to the wire that you unwrapped from the MA O post.
Make up a jumper cable for those times you don't want to use the EQ.
I recommend an old Radio Shack 5 band EQ. I have no RF problems at all and I run 500w PEP with the amplifier sitting next to the Kenwood. Stay away from EQ's that use all IC's. I find regular transistors are better for handling RF.

3. Amplitude Modulation

This radio can talk AM in a pinch. You will be limited to SSB reception though. That's OK if you don't mind playing with the RIT to get rid of the beat tones.
I decided it was degrading the specs when I put in a regular AM detector so I pulled it out. Here is how you make it talk.
Get a DPDT relay (12v) and connect it inline with the fixed channel selector switch, so that when you select crystal 4 the relay energizes.
Now lets take the relay in two sections. The first is connecting the output of the mic amp into a matching transformer via this relay. You want it connected so that the MAO is on the wiper and the relay NC1 is connected to the Balanced Modulator. Connect the other relay terminal NO1 to the low impedance side of the transformer. Connect the other lead of this winding to ground. So when you select fix channel 4, the relay energizes, and disconnects the mic from the balanced modulator and connects it through the transformer to ground.

Ok now the second part of the AM mod.
Take the HIGH impedance winding of the transformer and connect one leg to ground. Take the other leg and connect it to a 47uF cap. (Neg lead) Connect the positive side of the cap to an 820 ohm resistor.
Connect the resistor to the 9v terminal (on the RF board or most any other board) Connect the junction of the resistor/capacitor to the NO2 terminal of the relay.
Connect the wiper of the relay to S on Q3 on the RF board. I found R17 to have its exposed wire connected to S on Q3. Simply scratch the paint away on R17 and solder to the resistor.

Since this design modulates after the IF, you will have full fidelity and bandwidth of your audio. If you use an EQ, don't be excessive with the highs or you will have enemies from the splash you create. Also, there is no limiter here. If you have access to one, install it in the EQ loop.

73 de Ed/KB1HYS
Happiness is Hot Tubes, Cold 807's, and warm room filling AM Sound.
 "I've spent three quarters of my life trying to figure out how to do a $50 job for $.50, the rest I spent trying to come up with the $0.50" - D. Gingery
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