Observatory harmonics?

The other night I had a dew problem. Noticed the hair drier emitted a nasty buzz when used on low, but worked fine on high. I've been fighting a vibration problem in my SCT's dec drive, so I thought maybe the hair drier was giving me a clue, and it had something to do with the 100m of wire coming out from the house. Re-booted the scope on a battery, but it didn't help.

So I'm back out there today during the day stripping the dec drive down (again). I brought a fan-forced heater out with me, and noticed that when I set it on the floor, it also produced a nasty buzzing. Perhaps a harmonic frequency of my 10' diameter timber studded, timber floored, 10' fiberglass dome observatory? Something around 50hz, perhaps?

Here's a wild idea. Is there any chance that the vibration in my dec drive (while tracking, particularly bad around the north celestial pole) is a result of the motor producing a bit of 50hz vibration, which is then amplified by the harmonics? Could they really amplify it that much (Polaris get stretched into an 8" dumbell when the motor buzzes)? The buzzing the motor produces is, with vague recollection, similar to the pitch of the hair-drier (and heater).

Or am I clutching at straws here?

Thanks,
-- Jeff.

Jeff,
Before dealing with the question, is your scope, pier and pier footing completly isolated from the observatory slab, floor and wall system ?

I ran some more tests today. My pier doesn't ring like a bell, but my floor does vibrate like a drum. The hair dryer produces some electrical feedback into my 12V power supply, which is sitting on the floor and makes it vibrate at a resonant frequency. (At least I think it must be resonant -- it fills the whole dome with a humming sound.) Isolating the power supply from the floor removes most of the hum (some remains coming directly from the power supply itself). Given that I'm in a 10' diameter cylinder, that suggests the frequency is more like 90hz, which doesn't make much sense for the hair drier itself, but I suppose a switching power supply could vibrate at almost any frequency.

Sadly, I'm not sure this means anything for my dec drive. Putting my ear to the fork arm, the intermittent hum/vibration is unmistakable (one might even say loud). Putting my ear to the drive base it is quite a bit softer (but still louder than the RA drive, which is right behind my ear). Putting my ear to the pier, it can still be heard, but the sound arriving at my other ear (ie: through the air) is louder.

This, I suppose, is consistent with a vibration being amplified by the dome and/or floor. But if I hug the right fork arm (still with my ear to it), the sound doesn't change any.

Theory 1: Given the vertical dampening (ie: softer as you go down), the pier doesn't sound like the problem.

Theory 2: The dec drive is on the end of the fork, which might make it more susceptable to the 90hz resonance of my floor.

Theory 3: I can't imagine this resonance not being damped by my hugging the arm. (But I'm no sound engineer.)

Theory 4: Even if the vibration were being reinforced, it seems like it's stong enough to produce problems even without the resonant reinforcement.

Any feedback on these theories would be *greatly* appreciated.

-- Jeff.