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Friday, 31 October 2014

Meade EXT105 Telescope repair.

A while ago a friend contacted me with regards to a Meade telescope that belonged to a colleague of his. Time elapsed, and eventually it made it's way into the workshop.

This is some bit of kit. Now I'm no star-gazer (although I've always fancied the idea) but this thing looks fantastic. Once calibrated, you can search it's database and set it to automatically point in the right direction of the celestial body you wish to view. Brilliant..... except for this one, which threw a fit when asked to point to something, and smoke bilged out from the motor that makes it move up and down (I would say it's an EL or elevation motor, but apparently when dealing with telescopes, it's a DEC or declination motor, lord knows why).



Beast of a thing. First challenge was to get the optics out and placed to one side, out of harm's way... Two cap screws this side, and two the other hold the complete optics onto the base. It takes some gentle persuasion to extract them. 
There's some sort of electronics attached to the top of the optics ( not really sure of their purpose! Let me know! ) . This board and it's cover are held in place by a single cap screw. There's a 4 pin connector to undo to release it.



Once the optics are out we can disassemble the stand. Removing the cover off the motor for the DEC drive, reveals some damage!









Ugh... 
Unidentifiable burned-out IC's ...

We must never forget that electronics is driven by high-pressure smoke. You can see here where the failure has been caused by the smoke escaping from the IC's.

Emailing Meade and a few of their dealers gets me nowhere. Not even a reply. Thanks guys.


It does, however dawn on me, that the circuit that drives the Azimuth motor may be the same...



..and on closer inspection, it is certainly similar. The components are in a different position, but the circuit appears the same. 

By a process of elimination, the bits we need are a SI4947ADY & SI4936CDY. eBay is our friend. 

Each of these IC's house two MOSFETs. The SI4947ADY is P-channel and the SI4936CDY N-channel.


Having identified the parts, we give ourselves another problem! Which is which....

Having a careful look at the remains under a lupe, one IC gives us a clue, the SI4936CDY is on the left, the SI4947ADY in the middle (It's a 7805 voltage regulator on the right)



The old components are removed, and the board cleaned up. There's some damage to the print under the SI4947ADY in the middle. I have to add a tiny piece of wire here to make up the pad, and attempt to fit the IC on the top....





New IC's are fitted. I check for shorts & open circuit connections. It all checks out OK. The tiny wire under pin 1 is good to go!














 I decide to add a 0.01uF capacitor across the motor terminals to try and protect the electronics a little more from the spikes coming from the motor. I wouldn't mind betting this is what damaged the electronics in the first place.




... and then the daunting task of re-assembly. Nothing really to note here, other than be careful to route the wiring through the DEC motor board which connects to "the mystery electronics" on the top of the optics!

And then , bullet firmly between teeth.... Power-up!

video













  

1 comment:

  1. As an owner of a 'mere' Meade ETX80 I was fascinated by yet another splendid article by Mr Doswell regarding the Meade ETX105.
    A great read on a Sunday afternoon with a decent cup of coffee. Thank you Andy, it was educational as well as entertaining.

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