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Last updated: 31 October 2012

This page is for user comments and information specific to the Meade ETX-90RA (originally known as the "ETX Astro"). Comments on accessories and feedback items appropriate to other ETX, DSX, and DS models are posted on other pages. If you have any comments, suggestions, questions or answers to questions posed here, e-mail them to me for posting. Please use an appropriate Subject Line on your message per the Site Email Etiquette. Thanks. Remember, tips described on this site may invalidate the warranty on your telescope or accessories. Neither the submitter nor myself are responsible for any damage caused by using any contributed tips.

Subject:	ETX-90RA RA Slew problem
Sent:	Tuesday, October 30, 2012 12:47:17
From:	John Deriso (
When resting on the scope base, the RA motor turns the scope smoothly. 
When tipped for polar align, such as with the table-top tripod legs set
to 39 degrees, there's a click and a slight jump in RA every 9-12
seconds.  The RA clutch engages firmly and is not at end of travel.  I
opened the base and disassembled to study the clutch mechanism and
gears.  No apparent wear of plastic gears, the motor shaft and gear are
sound.  Motor is firmly mounted.  Tried adjusting the tension of the
single bolt that holds the plastic base together on the RA axis.  If too
tight, RA won't move.  If too loose, the base halves wobble sloppily.  I
adjusted tension until RA rotation barely binds.  Re-assembled, still
clicks and jumps.  Same problem whether in N or S hemisphere switch
position (motor drive reversed).  Significant stiction when tipped for
polar align and rotated manually, but much less stiction when resting on
scope base.  There's a clue here somewhere, I think the click and jump
are caused by the stiction.  Suggestions? Lubrication?
John Deriso
Mike here: My first thought would be the small Teflon pads around the interior circumference. One or more could be missing or worn.


That occurred to me too, Mike, right after I sent my post.  I find 3
positions for pads, 120 degrees apart.  Two are missing a pad, and
there's a black "pad" in the third position.  It can be "wiggled"
slightly, so it's probably adhesive-bonded.  I see online there is such
a thing as black Teflon.  I'll take it from here, thanks.

Subject:	re: Can ETX90-RA voltage regulator survive 5 volts input rather than 4.5
Sent:	Saturday, October 27, 2012 18:54:45
From:	richard seymour (
If you have a 5v supply, you could put a diode forward-biased in series with it.
The diode will have a near-constant forward voltage drop.
Pick one that can handle the expected current (2 amps should do) and
that has about a half volt drop.
That will bring your 5v supply down to the scope's wishes.

(where are you getting a 5v supply?  Many "wall warts" are unregulated,
and will drift up to as much as 1.4 times their "rated" voltage.

Doing a little catalog digging, i see that Radio Shack sells a two-pack
of 1N5402 3 amp diodes... unfortunately they have a 1 volt forward drop.
Visiting Digikey, i see that a   1N5821  would do... it'll handle 3
amps, and drops a half volt at that current.   They're only 46 cents
each, but Digikey has (if memory serves) a $25 minimum order.
Continuing the search, i'm getting the impression that you'll need a
Schottky diode to achieve the half volt drop (normal silicon diodes have
a full volt drop).
Newark Electronics or Jameco may have more reasonable ordering policies.
You could also check locally to see if there are electronic supply
houses with pick-up desks or retail store fronts.

good luck

Subject:	Can ETX90-RA voltage regulator survive 5 volts input rather than 4.5
Sent:	Thursday, October 25, 2012 06:49:27
From:	Mike Corley (
I have just purchased an older ETX90-RA from a chap that never looked at
the bottom and so did not even know there were batteries in there.  As
you would suppose, there were a group of very corroded batteries in the
battery case.  I have been able to clean up the mess.  I have the motor
running.  I have looked at the small controller box with the toggle and
two momentary switches and intend to replicate it.  My real concern is
if I can replace the use of batteries (the bane of electronics) and use
a 5 volt power supply?  I lean toward the belief that an extra half volt
could not possible harm it, BUT in this case it does represent almost a
10% overvoltage.  What are your thoughts?
Steven Corley
Waller, Texas
Mike here: I would be cautious about overvoltage. The electronics in the ETX are rather fragile. But you have an interesting project; keep me posted. It would make a great Tech Tip article.

Subject:	Complete disassembly of an ETX Astro 90
Sent:	Thursday, October 4, 2012 17:24:52
From:	Zygmo (
I have just acquired an Astro 90 that is absolutely filthy. Inside and
out!  It had been stored for years with no caps on either end, and the
batteries still installed, of course. It looks like I will have to do
something about the circuit board, as well as clean everything up.  By
the way; surprisingly, the optics and other parts appear to be
scratch-free...cleaned up, I think it will look good as new. Of course,
I had to remove the baffle from the mirror on the corrector
had shifted about a third of it's diameter. The glue and tape removed
fairly easily, without any damage to the glass. I have read the
directions on your site on how to reinstall it. 

My question is....where can I find instructions on how to completely
dismantle this scope for cleaning? I see on your site several individual
sub assembly instructions, but not necessarily specifically to the Astro
90.  For instance, are instructions on removing the tube assembly
universal for all ETX 90s?  Also, since this only drives in R.A. without
GOTO, does that mean the circuit board is only a power supply, and the
telescope motor could just be controlled by a universal power supply
having the same voltage and amperage as the three batteries, or are
there speed control pots on the board?

I can't afford to spend much on this, so I need to be able to do
everything myself.

Thanks for any help on this. I have other scopes and your site has been
invaluable for information.

Mike here: The ETX-90RA mount and circuitry is different (but similar in some respects) from the ETX-90EC/AT models. But removing the OTA is the same remove the two screws on both sides of the ETX where the tube adapters are located; then carefully expand the fork arms to release the small plastic tabs on the tube adapters. For cleaning the optics, see the article "Cleaning Optics" on the Helpful Information: Buyer/New User Tips page. If you need to replace the circuit board, try Telescope Warehouse (link on the Astronomy Links page).


Ok. Thanks. I did not get back to my mail till just now, but I went
ahead and removed the OTA last night. It came off fine. I dismantled the
OTA, and was surprised to find the main mirror clean enough.  The flip
mirror, and corrector were filthy though. I hate cleaning the secondary
mirrors on ALL telescopes because as far as I have seen, none are
overcoated with Silicon Monoxide. Having no choice, though...I washed
all the back, including the flip mirror. Flip mirror cleaned as good as
can be expected....the rest nicely. The OTA is back together, with
everything but the Secondary baffle on. Am picking up some two part
Epoxy today. I did look through the scope this morning, though....nice
and clear, with a slight bloom on one side due to the lack of the
baffle, I assume. However, I think I forgot to mention yesterday that
the eyepiece tube was inserted crookedly. REAL crookedly!  I had to tap
one side several times from inside (scary!), and it may not be exactly
right, so that could explain the bloom too. I will work on that some
more, after I install the baffle.

The battery holders and some of the circuit wires were green from the
battery leakage. The motor did not operate after I cleaned that and
installed the batteries...however, I did discover something there too.
All I had were some standard non-alkaline batteries.  While working on
this scope last night, I decided to take some pics of the parts with my
digital camera, which reported bad batteries when I turned it on. I
removed the new batteries from the scope, installed them in my camera,
and the camera reported "No Batteries".  Checked them with my meter,
which reported full charge. Put them back in the camera, which again
said "No Batteries"!  I had two expensive batteries for my other camera,
installed them in the camera, and it worked perfectly.  So, I will also
buy some good Coppertop batteries today, and try the scope motor again.
Maybe I will luck out!  Will let you know.

I have one more question, please. I would like to remove the fork arms
to be able to take them apart for cleaning too. When you remove that
silly self tapping screw under the circuit board, does the top just lift
off then?  Nothing going to go flying off in the distance never to be
seen again...right? And while it is apart, can you direct me a post that
will come closest to telling me what to do while I am in there?

Thanks for all your help. I have far better scopes than this one can
ever be....but I like the challenge of restoring it, and the portability
of this scope could be handy. 
Mike here: Thanks for the update. Glad things are going well. There are many disassembly articles on the Helpful Information: Telescope Tech Tips page. And while we're discussing disassembling, be certain to read the Disassembly Procedure article there. And while you're documenting everything you do, if you want to write it up (with photos), I'll be happy to post it so that others can benefit from your experience. Thanks.

Subject:	Basic ETX-90
Sent:	Saturday, September 22, 2012 12:35:28
From: (
Hi Mike -   today at the flea market in Glendale I purchased a used
ETX-90 - an EXT-90RA based on info gleaned from your website.  No
tripod, no Autostar.  I had to find the manual online to discover where
the battery compartment was hidden.  3 AAs still installed with one
battery clip fully corroded.  Removed the PC board, de-soldered the
leads from the motor, de-soldered the offending battery clip and cleaned
it up.  Put it back together.  The OTA is currently away from the fork
unit and I may keep it that way and use it on an EQ mount when I want a
quick look at something.,
Had I not found your mighty websites while I was waiting for my LX90-10"
last spring I'd not known anything about the scope.  At least I had
knowledge when I saw it sitting on the table this morning, and knowledge
on where to start looking to I.D. this scope.
Another success story!

Subject:	your help please
Sent:	Friday, September 21, 2012 09:52:28
From:	Carl Weil (
i just got a old broken side arms ra90 meade with missing top eyepiece
assembly and have found new arms but dont know how to open botom
it was made in 1997 - can i purchaser a instruction manual or help from
Carl Weil, Director 
Mike here: Apologies for the delayed response. Please see the Announcements: Site page on the ETX Site for more on email. Also, PLEASE read the Email Etiquette page on the ETX Site. Your message was originally deleted UNREAD as SPAM due the Spam-like subject line. Thanks for understanding.
Not certain what you mean by the bottom canister but there are many disassembly articles on the Helpful Information: Telescope Tech Tips page. As to a manual for the ETX-90RA, you can download that from Meade's web site.


I was able to get bottom plate off by gently hitting with hamer when unit was inverted
was unable to pry off earlier
Mike here: Glad you got it off. Odd that it took a hammer.


please what epoxy would you recommend to hold top lens holder in when i
get one? there is a hair line crack in the plastic housing at the lens
holder hole
Mike here: I don't have a recommendation. Try searching the ETX Site for "epoxy" or "glue". You don't want to use a glue that can outgas and coat the optics.

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