POSITIVE SWITCHING OF AN EXTERNAL AMPLIFIER
I get just as much satisfaction from modification as I do when I build something. This project details what I did to three Yaesu radios but there is no reason why it can't be employed with any radio you can get the screws out of.
A STUPID IDEA
A while ago, I purchased a couple of Yaesu radios that used a 2 pin microphone plug and socket for the power supply. This was/is a stupid idea because it is possible to make contact even though the plug doesn't fully engage. The idiot diode had been replaced on all of these radios. I would have thought the people at Yaesu would not have been so stupid and should have used anything else instead of the rubbish they supplied.
The real problem with this system of 2 pins only occurs when the power is plugged in without an antenna or with an ungrounded system such as a dummy load. If the antenna is plugged in first and the contacts are made round the wrong way, the fuse in the power cord blows but there is no reverse voltage applied to the radio.
Instead I replaced them all with 3 pin plugs and sockets. There is no way to make a mistake with 3 pin plugs. The third pin I have used to key an amplifier. There has been enough room inside each radio to add a transistor and a couple of resistors. In all cases there has also been suitable circuitry inside each amplifier to accomadate this system.
MODIFICATIONS TO RADIO
The following parts were added inside the radio. Usually there is a Tx8 or T8V (or Tx13 in one case) signal inside the radio, depending on the brand, which is switched on during transmit mode. This simply turns the transistor on and pulls the external connection to ground when transmitting. The second 10k resistor simply pulls the open collector output up and gives RF chokes or capacitors something to work against. Many amplifiers have such a key input and often this extra output on the power pin can be connected directly to it. If so, the AMP should be set to FM or FAST keying no matter what mode is used.
There is usually enough room inside the radio around the power socket to add these few bits directly to the socket but they can go anywhere really.
MODIFICATIONS TO SOME AMPLIFIERS
There was actually only one amplifier I needed to modify. All the rest had pull to ground keys anyway. The only thing you have to make sure of is that the delay is set to SHORT or FM operation regardless of the mode you actually use. Because this is a positive key, that is, the amplifier is activated while you hold the button down, SSB delays are unecessary.
The following is a typical circuit of an RF detector in an amplifier. When RF exists on the input socket, a small amount is passed through the input 1pF capacitor which is rectified and used to turn on the first transistor. This pulls the base of the second transistor to ground turning it off and allows the third to turn on driving the power relay.
All that was needed was to remove the two wires from the FM/SSB switch and connect them together and then to the outside world. When this new signal is pulled to ground, the amp turns on. The rest of the circuit can be left in place. The positive action of this key turns the amplifier on quickly while the FAST switch position turns it off as soon as you let go of the button. The two resistors charging the base capacitor of the first transistor are further helped by the reisistor inside the radio.
The final circuit is shown above. If I ever sell the amp, I can return it to original condition in a few minutes because I have used the same wires without other modification. The ferrite bead probably doesn't do much but I put one there just in case.
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