Cassiopeia Observatory logo


Last updated: 4 January 2013

Sent:	Monday, December 31, 2012 23:06:06
From:	ross (
If you don't use it you'll lose it!

Having bought an ETX-90 that is about eight years old from an elderly
lady who didn't quite understand what all the electrical controls and
buttons were for, I found that the power circuits wouldn't turn on. I
assumed corrosion in the power switch, which may never have been used
since new. A quick shot of CRC in the switch did the job and restored
power; but only from the external dc adapter. When I pulled the dc plug
out, the jack wouldn't switch over to internal battery power, and no
amount of poking and prying, putting the plug in and out or spraying CRC
would fix it.

Eventually, I dismantled the base, (remove the rubber feet, undo three
screws and pull the battery cover and base plate off), and removed the
RA gearbox, (coupla leads -mark them appropriately, three screws on the
worm gear, three more on the gearbox, and it fell out -- sort of), to
get to the power board that mounted the power switch, leds, control
inputs and dc input socket. Wasn't going to happen. There didn't seem to
be anyway to get the board out short of main violence; it seemed to be
moulded in place. The upper part of the board was secured by two moulded
tangs, the lower was restricted by the base housing flange. It looked
like I would have to grind the tangs off to get it out; lots of dust and
plastic debris. Hmm!

Interestingly enough, it also seemed that the main circuit board for the
gearbox couldn't be removed if the power circuit board was still in
place. There's probably a knack to it, but I couldn't see it or research
an answer; and the forum had no answer - although kudos for the editor's
kindly and timely response at an hour when everyone east of the Pacific
should have been tucked up in bed.

In the end I decided to bypass the switch portion of the dc jack and put
in a rocker switch to switch between external dc and batteries.

This involved:

Cutting a small rocker sized hole with a Dremel in the base and
inserting an on/off toggle switch. Stuffing the inside of the base with
cloth managed to keep the dreaded grinding dust out.

Cutting the negative (black lead) from the battery pack.

Re-attaching the battery side of the cut negative lead to one side of
the rocker.

Attaching a new lead to the other side and,

Soldering the other end of the new lead to the solder pad marked '1' on
the dc jack (see the image). This is the common negative for both
external dc and battery power.

The other side of the cut lead isn't needed and can be un-soldered from
the circuit board, or just taped up.

In this way, when I'm on external power, I flick the toggle to off so
that the external power doesn't try to charge the alkaline batteries.

When I want battery power only, I unplug the external dc and flick the
toggle to on.

Bit crude, but it works. Please, anyone trying this should make certain
that the pad they're soldering to IS the common negative, or at least,
some sort of dc negative. There don't appear to be any fuses in the
ETX-90 and directly wiring negative to positive will give a gratifying
display of sparks and smells!

Ross Williams
Bendigo Australia



Go back to the Tech Tips page.

Go back to the ETX Home Page.

Copyright © 2013 Michael L. Weasner /
Submittal Copyright © 2013 by the Submitter