In article , mikea
wrote:

Or it may be a 2-conductor plug on a radio which requires a 3-conductor
plug.

Mike-

I think you have it. I found the service manual at mods.dk. It shows a
two-circuit key jack with additional circuits switched when the key is
plugged in. I didn't get the operator manual, but it probably has more
information about keying.

When the TS-820 came out, it was a top-of-the-line rig, so it may very
well have a built-in keyer. With only one jack, it would have to
accommodate both a straight key and a paddle. I wonder if there is a
menu choice that disables the second circuit?

Fred