[Elecraft] Amplifier Relay Keying Circuit

[Patrick Skerrett]

Hello folks,

I'm trying to build a simple relay circuit to key a Yaesu FL2100B 
amplifier. The amplifier uses negative keying voltage, so I need to have 
some sort of interface in between the K3 and the amp (Per the manual, 
the K3 supports +200v but not negative voltage).

If I put an ohm meter on the K3's key out jack, and hook the positive 
lead to the center conductor, and the negative lead on the shield, I 
measure an open when the K3 is not keyed up, and measure a short when 
the K3 is transmitting. So far so good..

I built a simple circuit that looks something like this:


                   Center conductor on K3 Key line
                           |
                           |
                           |
+12V from power supply ------             ----------------Relay 
coil                                               
                                         |
                                         |
                                  Shield on K3 Key line


-12v from power supply ------------------------------------Relay coil



When I DO NOT have the K3's key out plugged into this circuit, and short 
the RCA cable, the relay closes & the circuit works. Again, so far so 
good.....

However, when I try this plugged into the K3 and hit the transmit, the 
relay does not close.

Why would I be able to measure a short on the K3's key out when 
transmitting, however it will not bridge my 12v & close the relay circuit?

I hope I'm not doing something very stupid here :)

Thanks,

W9PDS