On  Wed, 19 Aug 1998 Kevin Staggers <[log in to unmask]> wrote:
>
>Len Warner wrote:
>> Try the following tests:<snip>
>
>I tried all you suggested, except ...  It does seem to be in the
> building wiring. ... Anyways, the hum comes through with
>the computer turned off.  If I unplug the computer, the hum is still
>there, being transmitted over the parallel port cable from the printer.
>... The computer is plugged into an UPS, everything uses grounded plugs.
>
>Also, is this dangerous to my equipment?  It seems to be
>throughout the building, as I tried running a grounded extension cord
>from outlets in other rooms ... and the hum never went away.

Unless you share your mains supply with a particularly nasty piece
of heavy equipment, this is probably neither a mains fault
nor dangerous to your equipment.

Hum continues with:
* all equipment powered from the same distribution strip,
  so mains ground quality is probably not a factor
  and this is NOT an external "ground loop"
* computer off or unplugged,
  so source is not in computer

Now I know about this UPS, I'm very suspicious of it:
many have an awful staircase pseudo-sine-wave output
with lots of harmonics of 50Hz/60Hz. If so, they may be
leaking through any unit's PSU or be picked up by cabling.

Another possibility is a failure of the ground connection at your
stereo's input. This sounds likely because of the printer effect.

To check this: (using a phono plug or known-good cable to connect)
* touch the phono centre conductor to make sure you get a hum
(this is your test signal(!), but you could use an audio source)
* short the centre conductor to the outer conductor and
  touch the centre conductor again - should be NO hum

If you still get a hum then the stereo has a faulty input:
perhaps its ground wire has dropped off inside or the
connector/PCB joint is dry or cracked.

If your stereo seems good, continue testing with:
* ALL equipment powered from the same distribution strip
* no other inputs connected to stereo system, if possible

Try:
* system powered without connecting UPS (to anything!),
  then reconnect UPS
* system operated normally but without printer connected (to anything!)

This should tell you whether the printer or UPS is the source
and if the printer is conducting the hum.

If that doesn't work, I'm out of ideas: you need a technician or
Hi-Fi buff on-site to wave a magic wand over it.


Len Warner <[log in to unmask]> http://easyweb.easynet.co.uk/~len/ ICQ:10120933