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Drake T-4 Line Circuit Modifications
by: Thomas Cathey - K1JJ
A few months ago I promised to post the modifications for the Drake T4X(B/C) when I completed them. Thanks to Chris, KD2XA for his actual mod suggestions and Guy, W1FRM for scanning and posting a readable schematic to work from. I am very pleased with the results!
I believe these mods can also be adapted to a TR3/4 as I understand the AM scheme is similar.
When the mods are done, the controlled carrier AM becomes a standard screen modulated AM constant carrier and the frequency response is from 5 Hz to about 6 kHz... and VERY clean. No transformers and phase distortion.
About one volt of hi-fi audio is injected into the pin 7 grid of the T4X 12AX7 audio preamp. Put an RCA jack on the back of the rig for this purpose.
You will make the following changes:
1) Tie the screen and plate together of the 6AU6 modulator tube.
2) Replace the 22 Meg resistor with a 100K at pin 1 of the 6AU6 modulator.
3) Replace the .005 coupling cap tied to the 22Meg on the 6AU6 grid with a 0.47 uF 200V.
4) Break the lead that runs from the plate of the 6AU6 to the switch/12BY7 screen and insert a 500K pot that has a 0.47 uF 200V capacitor in parallel across this pot. Connect one lead to the center pot pin and the other to an outside pin. Leave the other end pin of the pot unused. I used the 500K Vox Gain pot in the rig. Just disconnect the existing leads, tape them off, and add the new ones as described above. I mounted the 0.47 200V cap on the bottom of the chassis.
5) Unground the cathode of the 6AU6 modulator and insert a 330 ohm resistor with a 30 uF 15V capacitor in parallel across it to ground. The plus polarity of the cap goes to the cathode pin of the tube and the negative to ground.
6) Add a 2K resistor from the audio input RCA jack pin to ground.
7) If you want even more modulation and greater power output, replace the 6AU6 modulator tube with a 6AK6. Just plug it in, no other changed needed.
That's it! If you get it working you will be pleased too. Sweep it with an audio signal generator and check it out on a scope to be sure all works well. The sine wave should look flawless. The 500K pot is used to adjust the carrier level out of the rig. About 15-20 watts of carrier is about right to permit headroom, though anywhere from 5-25 watts can be adjusted with this pot to drive a linear.