T4XC AM Mod

I saw this modification on the web somewhere but don’t remember exactly where. Google on T4XC AM Mod and see what you get.

The mod is a simple matter of replacing the 220 ohm cathode resistor on the 6AU6 am modulator with a 1K pot. Then the 1K pot is adjusted to give 20 to 30 watts carrier output with the transmitter in AM mode and keyed on the air via the microphone.

Why do I need this? My thought exactly. I had always known that the T4XC had an AM function but why bother with it when it has such a good SSB feature?

I probably still will not bother with the AM function but this mod makes the AM function much more functional. You see the transmitter goes to ‘controlled carrier’ mode when doing AM. Carrier output is only about 10 watts or so when unmodulated in AM mode (dead mike). Power out increases when you talk up the mike but for those periods of silence between sentences the signal appears to disappear as the power level goes to QRP.

So, by increasing the bias resistor value from 220 ohms we can get more carrier output with a dead mike and the signal does not disappear as much. Carrier output increases to 25 to 30 watts. You could probably get more but it makes it hard on the finals.

The mod has you installing a 1K pot in one of the extra phono jack holes on the rear apron of the transmitter. After I figured out my pot would not fit such a small hole, I used a small PCB mount trim pot soldered to heavy wire extensions from pins 2 and 7 of the 6AU6 socket.

While working on the socket trying to remove the wires and one resistor lead off pin 2, I ended up breaking off the solder lug part of the tube socket pin. While it is not impossible to replace the socket, it is a major undertaking that I was not interested in undertaking. I ended up removing a pin from a surplus 9 pin socket and inserting it in place of the broken pin. I pushed the new pin down into the socket with the 6AU6. It worked and the wires and resistor were far easier to remove off the broken pin now that it was out of the radio.

Once the pot is installed, you tune up the transmitter for max power out in the tune position, switch it to AM making sure the SSB switch is in the XLSB position, and monitor your power out while adjusting the new pot. I set mine to output about 30 watts. Have not tried it on the air yet but it sounds okay on the receiver. Then it sounded okay before the mod too.

One last thing about the mod. You don’t need to leave the pot installed. Measure the resistance of the pot after the adjustment and replace the pot with a fixed resistor of that value. Note that the original 220 ohm cathode resistor is left with one end remaining soldered to pin 7 just in case someone wants to remove the mod and return the transmitter to stock condition. I figure if it is a mod worth doing, it is a mod worth keeping. Still, my T4XC now has an extra unused 220 ohm resistor hanging off pin 7 of the 6AU6 socket.

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