Alan WA4SCA wrote:
Jack,

The first thing to check is that the interface is FULLY isolated.

I don't agree. If the problem was in the interface, it would work
equally badly with the desktop, laptop, or laptop on batteries. The
thing that Jack did to resolve the problem was to disconnect the power
supply from his laptop. This implies to me that the issue is RF getting
into the computer via the AC power line. Or in the case of the laptop,
it could be that the RF is getting into the system because the cable
between the power supply and the laptop is acting as an antenna.

That's the aspect that I would work on. It sounds to me like RF is
getting into the computers. When the laptop is running on battery, it's
sufficiently shielded so that it's able to operate. If the desktop has
a metal case, it might be that bypassing the AC feed would help.
Shielding the cable between the laptop's AC supply and the laptop might
help. Even moving the physical location of the various components may help.

One of the things that tends to poke a hole in this whole theory is the
fact that Alan is running QRP. If there were a lot of RF floating
around the shack, my idea would make more sense. On the other hand,
computer equipment is remarkably sensitive to RF at specific
frequencies, and Murphy suggests that that's where you're operating . . .