I want to use this simple circuit as a buffer/slave relay to key
my RF amplifier from the Flex 6300. I have used this many time before on
various rigs with success, but not the 6300.
The relay will key when just the
tip of the RCA connector is inserted into the key jack on the 6300 on receive.
This circuit works
fine on my Elecraft K2.
The TX jack is preferred and must be enabled in the menu under Transmit.
It's good you're using a separate 12 volt supply.
A more accepted practice if you must have an outboard relay for a dry pair is to introduce an NPN saturated transistor switch to buffer the current draw on your TX port. Very simple to build. For a transistor switch pretty much anyone will work.
Curious what amplifier you are driving, many modern amps work perfectly with an RCA m-m cable from the 6300 to the amplifier. My Acom 1000 is one that does.
When I had my L4B and TL922a I too built a transistor switch driven relay.
Here is a good primer on adding a transistor to switch the relay coil.
You probably want PNP, so it turns on when the rig's TX output shorts to ground;
But you might also want a capacitor to debounce the circuit, just be aware that this
may add some delay in keying an amp, so you want to add a much smaller capacitor
than they suggest in the below circuit, and you want to measure that switching delay
with an oscilloscope and add some keying delay on your rig so you don't "Hot Switch"
Again, if you find the specifications for how much current in milliamps that your Flex 6300
can switch on the TX out port, and if your relay coil is less, you are fine without anything
extra. For me, just for added idiot proofing, I added an external key circuit like you want to,
so I could key on multiple things like the amp, a transmit sign, etc.