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Dead Celestron AVX motor board NXW445 rev H

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#1 Mike_Conley

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Posted 20 October 2019 - 11:38 PM

In the process of repairing my AVX mount I wound up with an extra motor board that shows obvious damage to part U2.  I'm curious to see if replacing it will bring it back to life (I'm assuming someone like Mouser or DigiKey would carry the part if I can figure out what it is).  As a former electronic tech I think desoldering and resoldering a new part is just barely within my skill set.  Can anyone tell me what part it is?  It has 724G on the body.  I tried to chase that down on the internet but have now given up.  I noticed that U4 is the same part and most of its pins go to the Autoguider port making me think it might be some kind of interface chip or a resistor network.   


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#2 ZL4PLM

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Posted 21 October 2019 - 12:56 AM

i tried a few - its a 3.3v reg you can get them at mouser

 

you need a hunk of heat as the ground of the reg is through the pcb pad

 

none of them i was able to make function again - they seem to fail short and then that must kill the processor

 

I havent been able to recover any of the 4 i looked at 

 

celestron needs some serious questions of their design - poor low current component and little in the way of protection which causes the regulator to die and a very expensive board 

 

 

I stopped suggesting people buy them - Ive done too many of them !

 

good luck fixing it :)

 

cheers

 

Simon


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#3 macdonjh

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Posted 21 October 2019 - 07:50 AM

I'll betray my mechanical background with this question:

 

Rather that try to desolder the ground connection, would it be better to drill it out?  That way you don't pour a ton of heat into a PC board near an IC.  Could you use a small drill bit in a Dremel tool to have better control than a 1/2" DeWalt drill?



#4 pyrasanth

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Posted 21 October 2019 - 08:09 AM

I'll betray my mechanical background with this question:

 

Rather that try to desolder the ground connection, would it be better to drill it out?  That way you don't pour a ton of heat into a PC board near an IC.  Could you use a small drill bit in a Dremel tool to have better control than a 1/2" DeWalt drill?

I would not drill out unless you were sure that you were not working with a multi layer PCB board- you could inadvertently drill out a track in the sandwich. 


Edited by pyrasanth, 21 October 2019 - 08:10 AM.


#5 Mike_Conley

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Posted 21 October 2019 - 12:16 PM

  Wow, I just posted this last night and there are already three replies.  Thanks to all three for helping me with my problem.  First off my apologies for not adding a picture of my board to make it easier to discuss.  This is my first forum posting is my feeble excuse for not knowing better.  Assuming you can see the attached picture, U2 is next to U3 pretty close to the middle of the board.  And it looks like it might have been replaced at an earlier time.  That is about the only unusual thing I can see on this non working board.  By the way I don't know what U3 is either and am curious about that as well if anyone knows what that one is.  I do know virtually all the rest of the parts on the board.  To ZL4PLM I have to say that U5 found in the lower right portion of the pcb is the 3.3V low drop out regulator.  You can see it reflowed crookedly on the board but the solder joints look just fine.  However I have read in other places that the regulators tend to be the parts with the highest mortality rates, so I should check them out.  This brings up a question though: can I test both regulators (U1 is a 5V regulator at the top of the board) by properly connecting 12Vdc to the white two pin J3 in the upper left (lower pin is + , upper pin is ground) and jumpering J8 in the bottom right to simulate the power switch being turned on AND not connecting the motors and hand controller to J1, J2 and J3 respectively?  That would be easy to do on the bench.  Since its already a dead board I don't think I could hurt it further but would like a second opinion.  I have the datasheets for both regulators so I know what each pin is.

  Getting back to removing the 6 pin IC U2, I probably wouldn't use a drill but rather start by seeing if I could get an Xacto under the body being careful to not disturb any traces and heat up all pins on one side at once and pry up the chip high enough that when the heat is removed that side no longer contacted the pcb pads.  Then grabbing that side with pliers, heat the other side and it should just come off.  At least that's what worked well last time I had occasion to do it (admittedly several years ago).   Plan B would be to find my tiniest cutter and cut off the legs individually and remove them with the soldering iron, then use solder wick to refresh the pads making it ready for a replacement part.

 

AVX NXW445 rev H 191021 1SM.JPG



#6 macdonjh

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Posted 21 October 2019 - 12:17 PM

I would not drill out unless you were sure that you were not working with a multi layer PCB board- you could inadvertently drill out a track in the sandwich. 

See, that's what you get with advice about electronics from a mechanical guy.  If the component in question is mounted/ soldered through the PC board, though, will there be any other traces at that particular spot that aren't used by that component?  If there are, how to you ensure a good solder joint at all the layers that component is connected to?



#7 ZL4PLM

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Posted 21 October 2019 - 04:04 PM

I'll betray my mechanical background with this question:

 

Rather that try to desolder the ground connection, would it be better to drill it out?  That way you don't pour a ton of heat into a PC board near an IC.  Could you use a small drill bit in a Dremel tool to have better control than a 1/2" DeWalt drill?

no because the earth pad is via a multi layer PCB 

 

with a reflow heat gun you will be fine - I did mine with a soldering iron and managed ok 

 

but you will destroy it using a dremel



#8 ZL4PLM

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Posted 21 October 2019 - 04:09 PM

the problem with the reg is its a 500mA reg and poorly protected - its all too easy to get a cable wrong, have a tripod crash or plain just look at it wrong and it dies

 

typical symptoms - the LED is dim or gets dimmer after switch off - sticking yr finger on it will burn a hole in the tip of your finger as its shorted and no output 

 

I think you would be ok testing this way - the link to the motor seems to be encoder connection +v, gnd and the PWM DC to the motor

 

I tend to cut the legs off the little things and then desolder it too :) 



#9 ZL4PLM

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Posted 21 October 2019 - 04:13 PM

See, that's what you get with advice about electronics from a mechanical guy.  If the component in question is mounted/ soldered through the PC board, though, will there be any other traces at that particular spot that aren't used by that component?  If there are, how to you ensure a good solder joint at all the layers that component is connected to?

multilayer PCB's have unpopulated through holes that connect layers but on something fine its really hard to see what hole is connected to what 

 

The ground pad for the regs are usually a few mm square and have multiple thru plated holes below them 

 

their removal is compounded in that the back of the device is also usually solder to the ground pads as they act as heatsinks - surface mount regs have a lot of heat through a tiny spot

 

Celestron did a poor job on this board as the reg is underrated and poorly protected as its max voltage threshold is easy to exceeed in a pulse from connecting a power cable - everytime you turn it on is like a game of russian roulette !



#10 pyrasanth

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Posted 24 October 2019 - 08:46 AM

Can't you get a new board? rather than all the hassle of fixing what you have.



#11 mich_al

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Posted 24 October 2019 - 09:02 AM

Can't you get a new board? rather than all the hassle of fixing what you have.

Fixing it would be my first choice too (it's not a hassle).  The 'throw away' mentality that permeates our world frustrates me.  However,a poor design that likely takes out half the circuitry due to a momentary imbalance may demand it.




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