ETX TECHNICAL TIPS Last updated: 20 August 2004

Repairing Battery Cut-off Switch

Subject:	Reply to "Problem with battery use (doesn't work!)"
Sent:	Wednesday, August 18, 2004 15:17:48
From:	Niall Saunders (niall@njs101.com)
Further to Tom Savage's query about his ETX-105 no longer working when
using 'internal' batteries, I had EXACTLY the same problem. In my case
it WAS the little cut-out switch - BUT it is an almost impossible task
to get at the little panel to sort the problem.

Even if you work your way deep into the innards of the base of the scope
to get the panel out of the housing (which you really NEED to do in
order to fully repair the power socket), there is then the MAJOR task of
getting the panel dismantled.

I already had my scope in several thousand pieces (in order to repair
the missing RA end-stop) so further dismantling did not pose any real
worries for me. What could be done instead is to simply 'join' the two
switched zero-volt contacts on the power socket (the 'GROUND' contact is
'opened' when a power plug is inserted, thus preventing the external
power supply from charging what may well be 'un-chargeable' Alkaline AA
cells in the internal battery compartment). If you are ABSOLUTELY SURE
that you will NEVER connect an external power supply when you have AA
batteries in the internal compartment, then joining the two ground
contacts is (sort of) acceptable (but users will have to do this AT
THEIR OWN RISK).

Alternatively, if under warranty, it is best to send it back to MEADE.
Personally I would recommend an external power supply EVERY TIME, and
NOT one that runs direct from the mains - unless you have a death wish
!!! The 12V 17Ah 'car starter' units are really perfect for the job, and
many have both a cigarette socket AND a cigarette 'charge' lead that can
be adapted for driving a Meade scope - usually all that is needed is a
2.1mm power 'socket' soldered 'back-to-back' with a 2.5mm power 'plug'
(to cater for the fact that Meade has used the less commonly found 2.5mm
socket on their scopes but, never mind, this little 'gizmo' can be made
with a 'right angle bend' in the soldering, which then allows the
external power connection to sit more neatly - flush with the connector
panel on the sides of the Meade scopes).

I also have noticed that the subject of fuses for external supplies has
arisen once again. Just a reminder - it is HIGHLY UNLIKELY that ANY fuse
fitted to an external power lead will offer ANY protection to the scope
electronics WHATSOEVER. Any fuse, if fitted, should be fitted AS CLOSE
TO THE SOURCE OF POWER AS POSSIBLE (typically, and ideally, in the body
of an associated cigarette power plug). This fuse needs to be rated
large enough to accommodate the MAXIMUM current likely to be drawn by
the scope (in normal operation), with as small a 'spare margin' as
possible. In all of the measurements that I have taken, from -15C to
+20C, on scopes from the ETX-105 to the LX90 and LXD55, I have NEVER,
EVER seen currents higher than 750mA - so I would recommend a 1A fuse as
ideal (and by deduction, any 12V D.C. - fully stabilised, and WELL
regulated - power supply capable of supplying 1000mA, constantly, with
NO change in voltage would therefore be suitable, assuming that the
connection the 'mains' was not likely to kill you on a dark and damp
evening).

Basically, the fuse in such a scenario is to prevent possibly
catastrophic damage caused by a short-circuit to THE CABLE, NOT by any
internal failure inside the scope. If the scope were to fail internally,
then the desperately thin wiring used by Meade is likely to act as a
fuse in its own right!! (And it is also worth noting that there is NO
INTERNAL FUSE WHATSOEVER, despite the short-circuit current well in
excess of TEN AMPS that would be likely to flow should there be a major
wiring fault when running from the INTERNAL batteries).

Hope this helps,
Niall Saunders
Aberdeen, Scotland

(by the way, I assume that there ARE still things to see in the sky? It
is only scandalous rumour and unsubstantiated gossip hereabouts that the
universe - or at least the sky - extends above the layer of fog and
beyond the veil of torrential rain that has been our close horizon for
the last SIX WEEKS !!!)

Return to the top of this page.

Go back to the Tech Tips page.

Go back to the ETX Home Page.