I'm in the middle of a multi-year project to digitize my vinyl collection and
I've got a reucrring problem that's driving me up a wall.

I have a Dual CS 528 turntable hooked to a Pioneer SX-201 which is functioning
as preamp. My homebrew PC (a dual-CPU 1 GHz Intel server board) sound card (a
late-model low-end SoundBlaster) is wired to the tape in/out jacks. A cassette
deck is connected to the CD inputs.

Most of the time, the setup works fine. It's not audiophile quality by a long
stretch, but it works well enough that I hear the music and not the technology.

The problem is that without warning, one or the other channel will drop out
partially or entirely, with a 60-cycle hum appearing noticeably (but not at full
volume). If I pull and reinsert the turntable inputs, it goes back to normal,
and pulling and reversing the turntable's power cord also clears it.

Thinking that this looks like an intermittent ground loop, I took care to plug
the amp, TT and the PC into the same (battery backed-up circuit of the) UPS.
Didn't help.

One other thing I noticed is that sometimes the signal will drop out when there
is a spike on the line (like when the dehumidifier turns off) but I can't say
I've noticed that since I put everything on the UPS. I have even replaced both
the turntable and the amp and get the same problem.

Over the course of the summer the setup was not giving me much trouble, but in
the past couple of weeks I have difficulties several times a day.

Now I'm wondering if there is some kind of diode effect somewhere that is
causing a charge buildup, but at this point I'm just veering into wild
speculation because I'm all out of ideas.

/ff