CW Key to Radio Switch

There was no issue when I had but one radio capable of CW. My straight key and paddles were plugged into their respective ports on the back of my TS-590SG. In time I acquired an all mode 2 m rig, so a second straight key went onto the desk. Then the TS-520S and FT-101E followed over a twelve-month period. Each additional radio increased the number of keys on my desk, and a corresponding decrease of available desk space.

There is a strong correlation in my shack between effort required to get a radio operational and the amount of use that radio gets. Leaning over the back of my desk to switch a single key from radio to radio simply wasn’t viable, nor was a desk full of keys.

Enter the radio switcher.

It’s a simple affair, after all, it’s just a matter of switching the key’s tip between radio. We can wire it up as so.

That’s what I did, and it worked perfectly when switching between my TS-590SG and FT-290R2.

TS-590SG and FT-290R2: The model students.

But try as I might with the TS-520S and the FT-101E, every time I switched the key to either of them, it they entered a locked key-down state.

TS-520S and FT-101E: The non-compliant miscreants.

I did some digging. and upon further investigation learned that while modern radios use positive voltage switching, i.e. the jack tip sits at 5 V, which is pulled to ground when the key closes, older valve radios used negative voltage, or grid block switching at -50 to -150 V on the tip.

Now if I was using a straight key directly to the radio, there should be no problem. I am however using a keyer with transistor switching which caused an issue. I wasn’t entirely sure why it was an issue until I found this explanation:

The NPN transistor in reverse direction can be viewed as Emitter-base Zener diode with the breakdown voltage larger than 5V plus “ordinary” PN diode between base and collector. [Source: Stack Exchange]

Ah-huh! Putting an absolute 50 to 150 volts across a ~5 V quasi zener diode would indeed allow current to flow.

The solution was to upgrade the circuit to switch both tip and sleave, with some inputs reversed to cater for vintage radios. The switched lines remove the reverse voltage condition at the keyer’s transistor. It’s a bit of a hack really, as I need to remember which sockets are for older radios. This would be better addressed by adding some smarts in the keyer itself to seamlessly handle a reverse voltage condition. That’s for another day, however.