[RTTY] It hums but doesn't know the words

[Phil Sussman]

This is Phil, N8PS, and a devoted follower of
audio. I've got an odd problem and thought I'd
run it by you.

I have an SCS-PTC modem (5 pin DIN) connected
to an JRC JST-145 Transceiver (25 pin male D).

Everything works fine. There's no RF or ground
problems that I can determine. However, when I
turn ON the Henry 4K (across the room no less)
there's a big 60Hz hum induced into the TX audio
of the rig. (Linear not active, just the big
transformers running)

Input to rig is 600ohm Unbalanced.

I've tried any number of things, such as rewiring
the cable. Testing for ground loops. Whatever I
try, the problem gets worse. Double shielded wire,
opening one end (and then the other) of the shield.
Grounds are to a common point. PTC and rig on same
power supply. Tried two supplies -- got much worse.

Only two things work. Turning off linear or unplugging
the 25 pin connector from the rig. Unplugging at
the TNC instead of at the rig cuts hum a bit. No effect
removing power from the TNC - ungrounding the TNC chassis
or running the TNC on separate (floating) batteries.

Tried SHORTING the AF input to the rig. No change. Big
hum still transmitted. Disconnected all other wires
(except AF input and Ground) from 25pin (balanced 600ohm
output to TNC input and PTT) - no change - still big hum.

Looks like the linear is inducing 60Hz from the
transformers (two of 'em) in the linear into the TNC
cable. Tried moving cable, adding cable, moving TNC
60Hz still there. Changed cable, double shielded, etc.

Next tried an isolation transformer on the input. Hum
still there same magnitude.

Test: I had a 60w Bogan amp and left an unterminated
high impedance input open right on top of the linear.
Non a peep of hum in the Bogan.

Note: Does NOT appear to be any RF feedback as the rig
can be feeding (open wire feeder) or terminated in a
Bird 500w dummy load with no RF leaving.

Any hints? Sure has me stumped.

Would appreciate any suggestions.

Thanks ES 73,

de Phil - N8PS