[TenTec] Computer Interface Question

[Mark Erbaugh]

I built what I think is a completely isolated interface between my computer
and my Pegasus accy connection.  I use 1:1 isolation transformers on both
audio out and audio in, I use a pot to drop the audio in to mic levels, and
I use an opto-isolator to interface the serial port RTS line to the PTT
circuitry.

At first It thought things were working well. When monitoring signals using
DigiPan, I saw no evidence of 60 Hz hum. One quick on-air test and I got a
'good looking signal' report. Well, tonight I was doing some fine tuning and
listening to my Pegasus on another receiver. I discovered some very audible
60 Hz hum on my tx signal when the input signal is very low. I suspect that
it was there during my brief on-air test, but masked by the PSK tones.
Further investigation reveals that the hum all but goes away if I turn off
my computer monitor.  I was able to greatly reduce the hum with the monitor
on by moving the interface box around.

The interface is built in a plastic box. The computer case is grounded to
the shack ground as is the Pegasus, but the interface box (plastic) is not.
I tried un-grounding the computer case, but that had no effect. I also
placed torriods on the monitor video cable and the cables to/from the
interface box with no effect.

It appears that somehow my monitor (a cheap Pixie 17" color montor) is
coupling with my interface. Would a metal box provide better shielding?
Other suggestions?

73,
Mark, N8ME


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