TECHNICAL TIPS Last updated: 23 January 2001

ETX-90EC Wiring Protection

Subject:	 ETX refrito...
Sent:	Sunday, January 21, 2001 13:54:52
From:	rseymour@wolfenet.com (richard seymour)
Well, in a way, it had to happen sometime...

My ETX90/ec fried itself and my Autostar last week.

Yours might, too.  In five years or so...

The cause was a metal part of the Azimuth/RA clamp gradually
abrading the wires leading to the Alt/Dec motors.  If finally
wore through two of the wires, then casued the damage by short-
circuiting them together.

I was informed of the problem by a Motor Unit Fault message during
a Polar [goto], the Autostar faded to black, and it was accompanied
 by the Az axis continuing to spin until i killed power.  
After that the Autostar (nor the hand controller) could not make the 
Alt axis move.  Due to the short circuit, that testing with hand
controller also fried the hand controller's Alt driver.

My -particular- ETX90 was perhaps more prone to this due to the
way the wires fed out from the center of the RA clamp screw..
they were canted away from the connector they were going to,
hence had to make a turn and head down to get there.  Across
the moving metal piece.

When the base of the ETX90 is in place, the battery compartment
presses down on the wires, contributing to the problem.  When the
cover is removed. they don't show as touching the metal.

Here's a photo (from Jordan Blessing's ETX Tune Up page) of the area:



The round thick-washer-like object at the top of the shaft (just under
the four fragile wires...) is what did the deed.  It has a rough
sharpish edge which chewed through the insulation.
It was the blue and black wires which were cut/shorted.

Testing after the fact showed that it probably blew the receivers
in the Autostar which listen to the encoders, as well as the
Autostar's  I2C bus clock driver for the Alt axis.  
If i moved the Alt wires to the Az connector, i could command the
Alt motors to move.
But neither axis reported its position back to the Autostar.
Without another 497 Autostar to try (and fry?), i can't tell if
-all- the damage is only in the Autostar, or it it's shared with
the ETX90.
The Autostar could no longer cause a 4504's Alt motor to move.

Will it happen to you?
I use my ETX90 a LOT...  I frequently perform full-travel Az swings
(hard-stop to hard-stop) during my indoor testing of the Autostar.
I run it in Polar and Alt/Az.  I let it track overnight to gauge
accuracy of tracking.  I run it for hours during the day.
My average slew-to-view ratio is far higher than any sane owner's.
I wouldn't doubt that i have literally put 3 to 5 years of "moderate"
 use (mileage?) on it in the 1.3 years i've had it.

How to prevent it?
The -best- answer would be a thick teflon sleeve over that area of
the wires... Teflon to minimize friction, which would minimize wear.
Thick to add years to it.  Filing the edge of the RA clampnut (for
that round part is a nut, internally) so that it's rounded instead of
sharp would help.  Having the wires come out pointed towards the
connector panel would help.  Opening it up frequently (this is only
my second time inside the base!) and checking for wear would have
avoided it (by application of tape).

You can esily open your ETX base... 
(a) remove the batteries
(b) carefully peel off the three rubber feet to expose the screws
  (store the feet on a plastic baggie or window envelope's window to
    be nice to the sticky).
(c) Unscrew the three phillips head screws now exposed.
(d) Veeerrryyy carefully lift the base... the wires to the battery
    compartment are fairly short, and easily broken if you drop the
    base plate (the other reason for removing the batteries)
(e) there's the central shaft... -feel- each wire carefully where
    it would touch the RA metal, if it were so inclined.  The damage
    is on the -underside- of the wires, so it's impossible to see.
(f) carefully wrap those wires with a tape which will survive wear,
    heat, cold and rubbing.  A plastic coiled cable wrapper might work.
    However, place the base cover back on... feel the "springiness"?
    That's it pushing on that central wire... so any wrapping you do
    should NOT be thick enough to make the problem -worse- by 
    becoming pressed hard against the moving parts.
    (i'm almost considering cutting a hole in my battery compartment
     to give it more room... but that's "almost")
(g) check that the connectors on the back of the HBX/AUX/switch panel
    are still nice and tightly seated.
(h) reassemble carefully.  Reapply feet.  *carefully* reinstall 
  batteries, being sure to get + ends to match the molded guide.

Priority Parcel (air) mailing to Meade, in original packaging with an
additional sturdy external carton, (with $600 insurance and 
notification of receipt) was only $21.  It should get there Tuesday...
--dick

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