Here's the problem that prompted me to feel so deficient in shielding
knowledge. Unfortunately, the maker of the DXD doesn't have any ideas, so
I'm turning to a broader knowledge base...
My station uses a TopTen DXD to switch microphone and headphone audio
between two radios. The DXD is a pretty carefully-engineered device, which
has active ICs and transistors solely to switch power to relays, which
handle the actual audio. There are NO active devices in the audio channels.
I've been troubled by a sharp rasping noise -- a buzz, really, not classic
AC hum) in my transmit audio. When the microphone goes through the DXD,
the noise is present. When I plug directly into the transceiver, the noise
is gone. Turning down the mike gain on the transceiver also turns down the
noise.
My DXD is currently right under my computer monitor. I turned off the
computer monitor (CRT) while listening to the audio in my transceiver
monitor, and the noise went away. When I turned the monitor back on, the
noise came back only a very little bit, until the monitor synched up with
the incoming video signal and began to display an image, at which point it
came back full force.
It seems to make no difference whether the DXD case is grounded or not. If
I disconnect the radio cable from the DXD to the transceiver, but leave it
lying right next to the DXD and under the monitor, the noise goes away, so
it's apparently not being picked up on the cable itself. I've disconnected
each of the other external cables from the DXD, without result. The
external 12v power comes in through a short length of RG-174 coax from a
modified wall-wart. When I disconnect it, the noise goes away, but so does
the microphone audio, because the DXD defaults to no audio to either radio
when the power is off. If I leave the DXD powered up but unplug the
microphone at the DXD's front panel jack, the noise gets MUCH worse. The
power jack is not bypassed, and does have a reverse polarity protection
diode, but the manufacturer does not think this is an issue.
It does seem as if some low-frequency RF from the monitor is getting into
the DXD, but it's hard to see how. The DXD's case is all steel and with no
obvious gaps in the shielding. Aside from throwing away the monitor (a
noisy brute in the HF bands too) or moving the DXD (which would disrupt a
good station layout), I'm stumped.
Thanks in advance for any suggestions.
73, Pete N4ZR
Sometimes a tower is just a tower
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