Last updated: 12 March 2009

First, the Problem

Subject:	ETX 125 blows 2 autostars in one night
Sent:	Sunday, March 1, 2009 01:05:22
From:	Dan Robertson (
ETX 125 blows 2 autostars in one night
I had my ETX125 operating on the 12v cable and a small car battery. The
telescope operating fine for about an hour after which time, while
slewing I noticed a burning smell coming from the autostar I immediately
turned off the telescope and disconnected the power cable. I waited 5
minutes and started the telescope up again using the internal AA
batteries. The autostar booted up and after setting the time and date I
noticed that it would not drive the AltAz motors but the software in the
handbox menus worked fine.I powered down the scope and tried a spare
autostar handbox which had been used to operate this scope before. The
telescope worked fine for 15 minutes on the internal AA cells and the
spare handbox. after that time I noticed that the handbox lights were
completely out and it was and still is dark and unresponsive

The 2nd autostar handbox was completely dead,it would make a tick sound
on the internal speaker about every 3rd time power was applied and the
odd scrambled letters on the LCD but the red LEDs are off

I tried both problem handboxes on my ETX-60 scope which I have used them
on before and I got no response from the motors The ETX-60 still works
fine with it's orginal handbox

Both autostars have the newer style 497 circuit board Inside the First
handbox which gave off a burning smell I noticed a black charred boil on
the PIC 16c57-hs chip.this autostar boots up and allows access to the
menus but no motor operation.

Inside the 2nd autostar, No visible signs of internal damage, but this
handbox is dead and It would not boot off of the serial cable for a hard
firmware reloading either.

I inspected the circuits inside the ETX125, no pinched wires, no
scorched chips or shorted out contacts. No bits of wire or bent pins on
the HBX connector.

I tried the handboxes with 2 different connector cables with no response
either. there was no camera attached to it when this happened or any
extra wieght.
My big questions are:
Will this scope keep eating autostars?
What needs to be replaced?
Is one of the drive motor boards blasting 12 volts back up the data wire?
Is this a common occurance?
Is there a way to mod the handboxes with microfuses to guard against
this type of thing?
I would like to design a safety junction box for autostar units to
protect them from things that would eat them like this. If I can find
the problem I would like to install optical isolators to protect the

I am an avionics technician, and I have a lot of soldering experience
and diagnostic equipment, I would love with the help of some experienced
ETX users to get to the bottom of this problem before buying new
Dan Robertson
Mike here: I only recall one other report of this happening and that was many years ago on an ETX-90. The problem was a fault in the ETX base, not the handset. Your problem is likely in the power circuit to the HBX port on the control panel. Question: when the problem first occurred (using external power) were the batteries still installed in the ETX? You said you disconnected the external power when you switched to the internal batteries; was it still disconnected when the next problem occurred? If either of these are true, I would initially suspect the battery cutout switch.


Sent:	Tuesday, March 3, 2009 21:34:01
From:	richard seymour (
Mike wrote:
> ... i know of one other ... was an ETX-90

So that was probably -me-, back in early 2001.

The edge of nut which held the Aziuth shaft in the base cut through
the insulation of the wires passing near it (to head up to the DEC motor).
There was no -visible- damage, but you could -feel- it with your
fingertips against the wires.

When it cut through, it shorted the signal lines to ground and +12v.
Since the signal lines are fed directly from the Autostar, it fried
the same PIC chip as yours (plus blew some traces off the board).

In my case, it took out my Autostar and my handcontroller.

I sent the whole mess back to Meade, and they fixed it
(this was the old days).

So: my opinion is YES, it will continue to consume Autostars.

good luck
Mike here: Yep, it was yours.


Oh.. one more thing:
That PIC chip is a programmed device, so you can't simply
replace it with an off-the-shelf part.
The programming is "locked", so we've never decoded it.

have fun


From:	Dan Robertson (
Thank you Guys for the great responses!
I will inspect and test as far as I can with my equipment,

I have access to a pic programmer but I supposed the Autostar code is a
trade secret,

Are there any programmed chips in the scope base itself?
I can see speed controllers and encoder wheels.
I would love to know the protocol for driving the motors and build a
testing circuit for each motor board.

How much would do you think it would run me for a Meade factory repair
of the Base?

Thanks again
Mike here: Meade typically charges something like $75-100 to repair an ETX, IF they will repair it.


Many details of the scope's operation (including descriptions
of the message protocol between Autostar and PIC-bearing motor cards)
can be found in Meade's US Patents on the devices.

The primary hardware patent is  6,304,376
others include:  6,392,799  6,445,498   6,563,636  align: 6,922,283
7339731, 7221527, 7092156, 7079317, 7053992, 7414707, 7227223

You can access them through the Patent and Trademark Office itself:
or as "PDF images" (which easily include the figures, but are not
text-searchable) from:

Specific details of hobby interest are found in the Roboscope group's
Files area on Yahoo:

have fun

Next, the Diagnosis

Subject:	Teardown Photos and culprit - ETX 125 blows 2 autostars in one night
Sent:	Saturday, March 7, 2009 04:05:06
From:	Dan Robertson (
The yellow and black wires were ever so slightly damaged, which would
indicate to me a mometary short between autostar pin 5 and +12v. Has the
protection on these wires been improved in later models?

Thanks for the lead on this problem:







Mike here: Cutting of wires has been known to happen before due to be caught in gears or pinched in other places, but this is the first time I recall wires being damaged at that location. I wonder if there are other telescopes that have the same wire damage that haven't shorted out yet.


///// This is the diagnostic I came up with for testing the wires without having to crack open the base.
///// I'm still in the process of repairing mine, I hope this helps out etx users who might have damaged wires.

Testing an ETX telescope for shorted wires:

This is a procedure I have come up with to test an ETX for shorted
internal wiring, I recommend you test your scope if you have had ANY
bizarre behavior out of it or seeming power fluctuation. I would also
recommend that anyone who loves their scope a lot should do this test,
as a one shorted wire will not show symptoms but a 2nd one will fry

I have an ETX 125 that has an underlying metal frame with plastic outer
casing fastened to it, The wires that control the declination pass up
through the bolt used to tighten down the right ascension clutch.because
of the rotation and because the passageway through the bolt on my scope
was crudely machined, sharp edges sliced through the wires shorting the
power main to my autostar data wire, destroying the computer in the
handbox. This can also destroy the internal circuit boards that drive
the motors in your scope.

You'll need:
-A multimeter that you can set to beep on continuity.
-a crocodile test clip wire

1. Disconnect all power from telescope.
2. Remove the batteries
3. Unplug the autostar Handbox
4. Plug the autostar cable into the scope base without the Autostar.
5. Unscrew the caps on the side of the fork base for the 2 Table top tripod legs
6. Try to clip your crocodile clip to the metal threads for the table tripod.
7. Make sure the lead has a good connection to the metal frame of the scope.
7. Test for a continuity beep between the other threaded hole
8. Leave one test lead clipped to the metal frame.
9. test between the metal scope frame and each pin of the autostar cable. 8 pins.
10. If you test continuity between any of the pins, that wire is shorted out to your frame and should be repaired.
11. tighten the RA clutch and test the 8 pins again
12. Loosen the RA clutch and test each pin while rotating the scope by hand.
13. Rotate the scope to each mechanical stop to ensure that no shorting is happening inside the scope.
14. Now Wiggle the RA clutch loosely while testing each pin again.

If you find continuity between your fork frame and any of the autostar
pins, you should have the wires repaired before your electronics are
damaged. Once a second wire gets shorted to the frame, lasting damage

Dan Robertson

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