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LX90 - ALT only slews in one direction?

Meade
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#1 enskye

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Posted 08 August 2022 - 11:02 AM

I'm looking for assistance in correcting a problem with my LX90 10".  Purchased in 2010, it has been working admirably until recently when it refused to slew down in ALT.

 

I first thought the problem was with the controller, but after reversing UP/DOWN, it still will not slew down.

 

Since it is slewing in one direction, I'm assuming the problem is not with the declination drive motor (though I'm not certain this is a valid assumption).

 

This would leave the motor drive circuit board as the source of the problem... which after contacting Meade support, no longer seems to be replaceable.

 

So I'm at an impasse and am reaching out to the community for possible solutions.

 

 



#2 OzAndrewJ

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Posted 08 August 2022 - 05:29 PM

Gday enskye

The symptoms are classic for the PCh mosfet on the board burning out.

( This drives the top side of the motor )

You need to open the fork arm and look at the Mosfet to see if it shows burn marks

or a pimple in its surface.

It is an extremely common fault and many have fixed it by just replacing the mosfet.

Any competent electronics tech can do it as the chips are relatively large

and easy to get to.

Lots of threads on this problem here and other sites

Just search this forum for "Mosfet" to get a lot of background

 

Andrew Johansen Melbourne Australia



#3 enskye

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Posted 08 August 2022 - 06:03 PM

Thank you Andrew!  

 

I see two mosfets on the board... and one of them looks a bit wonky, so that's probably the culprit.

 

I'll follow your advice and let you know how it turns out.

 

--enskye



#4 OzAndrewJ

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Posted 08 August 2022 - 06:27 PM

Gday enskye

The wonky one should be the PCh unit ( Q2 )

This was originally a Vishay Siliconix SI 4947

but there are several modern alternatives.

If you are interested, there is a discussion on the failure mode here

https://www.eevblog....ircuit-design)/

 

Andrew Johansen Melbourne Australia


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#5 Piet Le Roux

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Posted 09 August 2022 - 02:29 AM

I was thinking that it maybe a good precaution to attach these heatsinks to the mosfets :

https://www.amazon.c...0?ie=UTF8&psc=1



#6 enskye

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Posted 10 August 2022 - 08:21 AM

I verified that the problem is with the Q2 mosfet (4947A) and would like to attempt to replace it on my own.  I've searched for a source to purchase a replacement, but have not had any success.

 

Does anyone know of a reliable place to source the replacement mosfet?



#7 OzAndrewJ

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Posted 10 August 2022 - 04:03 PM

Gday enskye

Cant help with the SI4947, but it is a simple dual channel mosfet

and Meade have used several alternatives over time.

It only switches ON or OFF, it doesnt do PWM so that makes it easier to select.

My notes have the following
 

Over the years, Meade has used different pairs of FETs.
The schematic shows what appears to be
F9933   and F9936   by International Rectifier or Vishay.
F9953   and F9956   by International Rectifier's
SI4936  and SI4947  by Vishay Siliconix        <<< most used
FDS6982 and FDS6975 by Fairchild

but thats just what i have pinched from posts over time.

The EEVBlog link i noted above also has some suggestions for a better modern replacement.

 

Andrew Johansen Melbourne Australia



#8 DAVIDG

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Posted 10 August 2022 - 05:54 PM

 You can still get them on Ebay. I bought these a few months  ago when I repaired my LX200GPS

 

         https://www.ebay.com...1cAAOSwL~hbRXFK

 

 It was easy to remove the bad one with low melting solder and flux sold as a  kit to remove surface mount IC's

 

                   - Dave 


Edited by DAVIDG, 10 August 2022 - 05:55 PM.


#9 enskye

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Posted 14 August 2022 - 09:32 PM

Thank you both for the info.  I've tried ordering from ebay and will post the results of my replacement attempt once I receive the mosfet.

 

--Skye



#10 todick

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Posted 20 August 2022 - 02:49 PM

hi

I have a lx90 with the same symptoms will not move down all other direction ok.

I have taken the board out but cannot see anything wrong.. under a magnifying glass i can see a pimple on the top mosfet marked 02 on the top of the pcb.

 

would somebody here please repair it for me (i will pay all costs)

I am in the uk 

thanks

dick

Attached Thumbnails

  • lx90 wiring.jpg

Edited by todick, 20 August 2022 - 03:32 PM.


#11 enskye

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Posted 03 September 2022 - 10:37 AM

UPDATE:  My only previous soldering experience was with simple electronic learning kits... so the LX90 circuit board looked daunting by comparison.  The blown mosfet was much smaller than anything I'd worked on... and it is a surface-mount device (SMD) (so no soldering on the back of the circuit board).  The replacement mosfets I'd ordered arrived (using Dave's link above), and after watching some YouTube videos on how to remove and replace SMD's (flux on the pins is key) the process went surprisingly smoothly and the problem appears to be corrected.  I'll take her out tonight if the skies are clear.

 

Here's a link to one video which was particularly helpful: https://www.youtube....qGuH6Tfw80&t=2s and a link to their webpage (https://blog.sra-sol...-soldering-iron). An Amazon search on "Fast Chip Kit for Quik SMD Removal with a Low Temperature Alloy" will pull up what I used.

 

Thanks again Andrew and Dave for your assistance and pointing me in the right directions.

 

Dick... The repair is easier than I thought, so give it a try if you have any soldering experience.  If not, I live in the US (Connecticut), and would be willing to attempt the repair if you're able to send me the faulty circuit board.

 

--Skye




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