B300P PTT input
The B300P has an automatic transmit switching using its RF input signal, a tiny part of which is detected in order to polarize a transistor switching the relay. A Time Delay is available for the SSB mode due to variations of RF level in this mode which will cause instability in the switching of the relay if it was not present.
With a ptt input, it is then possible to switch it directly from a transceiver with a ptt output and thus get rid of the delay.
The schematic diagram
The B300P has experienced several versions with differences, particularly in terms of transistors used for switches. I had to modify a 1987 version to add a PTT input, a polarization circuit, a low pass filter and a ventilation output. The diagram opposite therefore takes up all these modifications but it is the cleanest scheme I have to offer you.
The components concerning the proposed modification are in yellow. There is only one transistor (2N2907), a resistor (100K), a capacitor (10NF) and a stereo jack socket.
On the one hand, it is a question of inhibiting the delay when a jack plug is inserted in the socket and on the other hand to add the 2N2907 in parallel to the existing switching transistor.
No matter that there is only one transistor (BC237 most often) or two as here (BC237 + 2N2222) depending on the version we have.
The jack socket
We could put the jack socket screwed directly on the back of the cover. But you will notice that the latter must slid on its entire length before being able to extract it completely. This will require a significant length of wires between the socket and the printed circuit.
I preferred to design a small printed circuit fixed via spacers on the aluminum of the heat sink. A simple hole in the lid comes opposite the jack so that it passes through once the cover completely closed.
Set up
The proposed printed circuit has three outputs :
- A ground that can also be ignored if the spacers used for fixing are metallic.
- A "- C2" output on which the AM/SSB switch will be connected (instead of its own ground).
- A "col" output which must be connected to the collector of the switching transistor of the main relay (the large). It can be a BC237, a 2N2222, it is easily spotted: its collector is connected to the relais coil.
The photos are there to illustrate the method but it was a old circuit with only jack socket.
In use
The transmitter must have a PTT output which will be connected to this input via a mono male jack 3.5 plug (or stereo, whatever). Upon insertion, the amplifier will be able to switch to transmit state at the same time as the SSB transmitter and delay will be deactivated.
