Franc Zabkar wrote:
What is the function of the primary-secondary coupling capacitor (eg
http://tinyurl.com/ha435) in a SMPS? I ask this because I suspect it
may be the cause of a video "hum" issue.


I think it's there to provide a bit of RF grounding for the secondary
circuits.

Unfortunately it also helps couple any RF noise from the line into the
ground loops in your entertainment setup.

Things I would try:

(1) If you have a scope, CAREFULLY probe the hot and neutral leads for
any spikes. You may want to track down and kill the source of the
spikes. Typical devices that cause spikes-- anything with a SMPS or a
light dimmer or an electronic power switch.

(2) If you don't want to bother doing that, put a spike filter between
the wall and your devices-- a good "surge protector" that has a few
toroid common-mode filters. Or carefully make your own with a few
millihenries of 15 Amp capable toroid and a good line-rated 0.1uF
capacitor.

(3) If you don't want to bother doing that, try bonding your A/V
equipment chassis' together with a thick, say 12 guage stranded wire.
That often surpresses ground-loop voltages enough to make the TV's
watchable. Keep this wire as short as possible. Use Litz wire if
available.