----- Original Message -----
Sent: Sunday, September 28, 2003 9:45 PM
Subject: [M]: RE: Re: fuse blew


> Roger,
>
> Thanks for the help. I changed the fuse this afternoon and the scope came
> up OK. Then I set the slew rate down to 2 as always and moved the
telescope
> around a bit. I slewed it a bit N/S and that went OK. After a couple of
> moves E/W the fuse popped again. :-( It looked like a problem in the
> motors/gearing. I use the clutches to completely free the drives and then
> put another fuse in to see if it was the motors under too much mechanical
> load or the motors having electrical problems. With both clutches
> disengaged, the drive motors freewheeled without a problem. I did this
> several times and numerous rounds of the worm with the thought that if
there
> was corrosion that was contributing to some excess friction in the drive
> train, maybe this would free it up. Then I rebalanced the heck out of it.
> It is much better balanced now. I engaged the clutches and it slews
without
> popping the fuses. It's possible it was just a balance issue but it was
> pointing at the zenith when it popped both times so that seemed unlikely.
> Maybe there was some corrosion/stiction going on too. I don't know but I
> will hold my breath for now.
>
> Clearest skies,
> Walt Cooney
It sounds as though it may have been a combination of a stiffness, and/or
balance. Is there anything that would make the load assymetric when near the
Zenith?. Cables (it is amazing just how much 'moment', a cable from the back
of a camera at the rear of the scope, can produce), guide scope without 3D
balancing etc.. If you have the clutches unlocked, and position the scope
tube close to the 'problem' area, does it feel as easy to move in both
directions?. Any resistance should be investigated.

Best Wishes

> -----Original Message-----
> Roger Hamlett
> Sent: Sunday, September 28, 2003 8:59 AM
> Subject: [M]: Re: fuse blew
>
>
> > Half way through a scripted observing run last night, the fuse in the
line
> > between the 110VAC to 18VDC converter blew. I'm off to Radio Shack this
> > afternoon in hopes they have fuse part number 270-1022 per the Mapug
> > archive. What worries me is not knowing what actually caused the fuse
to
> > pop. I'll find out more this afternoon hopefully when I find out if a
> > replacement fuse stays in tact. If the fuse does stay in tact, any idea
> why
> > this might have blown? The telescope does not seem to have been in a
bind
> > and it is pretty well balanced. It is a 4 year old 12" classic that has
> > been in an outdoor (non-A/C) observatory for the past three years. The
> fuse
> > doesn't even look like it popped but apparently it has. It is an
original
> > Bussman 1.5A slowblow with a spiral winding.
> >
> > Thanks for any suggestions.
> >
> > Clearest skies,
> > Walt Cooney
> > http://members.cox.net/waltc/BLACKBERRY_OBSERVATORY.htm
> You are asking exactly the right question. A fuse that is operating at a
> reasonable percentage of it's 'rating', will blow eventually with
'ageing'.
> This is why on some commercial equipment, it is part of the maintenance
> cycle to replace fuses after a couple of years. Hence trying one new fuse,
> is the correct thing to do. However if this blows, then it shows a
problem,
> and should not be replaced again till this is found. Unfortunately, there
> are a lot of possible 'failure' causes, if it does do. One is a mechanical
> problem (your comment about the balancing is a good starting point). Is is
> possible that a setting has got changed in the hand controller, resulting
in
> the slew speed going back 'up' to it's default setting?. On the 12", if
> there is a reasonable amount of attached kit, then slewing at high speed,
> will blow the fuse. Other obvious things are a motor or gearbox having
> developed some corrosion, and stiffened up. The next possible area, is any
> one of a number of electronic failures (one of the motor driver IC's, is
> perhaps the most likely).
> Start with the new fuse, verify that the maximum slew rate is set below
the
> maximum, and if it works, smile. When you apply power, make a point of
> looking at the hand controller display. If this starts to come up, and the
> fuse fails when the scope tries to move, then it 'narrows' the fault down
to
> the motor system(s), and will help reduce the number of things that have
to
> be looked at....
>
> Best Wishes
>
>
>

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