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LXD75 RA card burnt

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#1 Sylvain Weiller

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Posted 27 June 2021 - 04:02 AM

Hi all,

Thanks for helping !

As you can see on joined picture, part of RA card is burnt with also exploded ICs.

 

I am ready to buy a new card or to have it repaired.

 

I do have some spare ICs but it does not look nice around ...

 

Could you tell me the best solution and from where it is possible ?

 

BR

Sylvain 


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#2 michael8554

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Posted 27 June 2021 - 06:59 AM

No image.

 

Before you go any further, have you determined the cause of the burnout ?



#3 Sylvain Weiller

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Posted 27 June 2021 - 10:48 AM

Hi Viking 1

Thanks!

Sorry forgot to click "Attach This File"!

Now Ok!

 

The failure happened progressively. I could no more correctly move in RA... It was not smooth at all, more and more jerky !

Best

Sylvain

Attached Thumbnails

  • IC LXD75 RA.jpg

Edited by Sylvain Weiller, 27 June 2021 - 10:49 AM.


#4 OzAndrewJ

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Posted 27 June 2021 - 05:56 PM

Gday Sylvain

That is a classic fault with Meade motor cards, and it happens all the time.

Depending on how bad the blow is, most people simply replace the burnt mosfet

and it comes back to life.

That said, i cant see under the 2 pin header plug to see if thats a reflection or other damage.

There are dozens of threads on this problem on many sites ( ie here, IO, Weasner )

Just search this forum for "mosfet" or "vishay" and you wil get lots of info.

Also read

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

for a possible cause

 

Andrew Johansen Melbourne Australia



#5 Sylvain Weiller

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Posted 28 June 2021 - 11:06 AM

Hi Andrew,

Thanks for help !

Joined is a picture without the 2 pin header.

I don't see there damages and you ?

 

Thank for interesting link.
The added 3nF capacitor , which type ? How to solder it in parallel on the R2 resistor ?

 

Also what is the best tech to remove and put new mosfet as I am not at all to desolder/solder surface components on cards ? (I am near 74 years old!)

Best

Sylvain



#6 Sylvain Weiller

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Posted 28 June 2021 - 11:11 AM

Hi Andrew,

Thanks for help !

Joined is a picture without the 2 pin header.

I don't see there damages and you ?

Thank for interesting link.
The added 3nF capacitor , which type ? How to solder it in parallel on the R2 resistor ?

Also what is the best tech to remove and put new mosfet as I am not at all to desolder/solder surface components on cards ? (I am near 74 years old!)

Best

Sylvain



#7 Sylvain Weiller

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Posted 28 June 2021 - 11:12 AM

Picture

Also I guess a number of copper lines have been burnt.
Could some one post a good resolution picture of this part of the circuit ?

I guess the repair should be done with thin copper wires ?

BR 

Attached Thumbnails

  • P6280001LXD75.jpg

Edited by Sylvain Weiller, 28 June 2021 - 01:14 PM.


#8 OzAndrewJ

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Posted 28 June 2021 - 05:39 PM

Gday Sylvain

That looks more like the mosfet blew up and spewed debris around, ( vs damage traces. )

Carefully clean off the soot and you should find the main PCB and traces  is still actually there.

I have hand drawn circuit diags for the LX75 boards so you could do a quick continuity test as well.

My boards are Rev C and these are slightly different to yours ( ie i dont have C20 and C21 and

R2/R6 are in different locations ) but it llloks like the new caps wont affect the circuit trace.

 

As to replacing SMD components, i am like you ( ie fat fingers ) but there are lots of people with

the skillset who might help there.

 

Andrew Johansen Melbourne Australia



#9 Sylvain Weiller

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Posted 29 June 2021 - 08:48 AM

Hi Andrew,

You were (at least it seems) right !

I cleaned the PCB with 95% alcohol and it looks much better.

With what is visible now, would you say the PCB is fully Ok ?

Thanks !

BR

Sylvain
 

Attached Thumbnails

  • P6290002PCBCleaned.jpg


#10 OzAndrewJ

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Posted 29 June 2021 - 03:41 PM

Gday Sylvain

 

That looks like the classic PCh mosfet "volcanic" fail ( vs just a pimple in the case )

that many a Meade owner has seen.

I can't see any badly damaged traces so i am pretty sure just replacing the mosfet

will bring it back to life.

As noted above, it is the PChannel mosfet that has blown ( as per usual )

and there are lots of available replacements.

 

Andrew Johansen Melbourne Australia



#11 Sylvain Weiller

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Posted 29 June 2021 - 05:44 PM

Hi Andrew,

Thanks a lot for your nice help !
The IC on the left looks ... not so good ?

Should I change first the right one and test or change both before applying tension ?

BTW could you confirm the name of the 2 ICs ?

Best

Sylvain



#12 OzAndrewJ

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Posted 29 June 2021 - 07:14 PM

Gday Sylvain

 

The IC on the left looks ... not so good ?

It looks old, but they rarely blow up.

It wont hurt to change it, but its not normally required

 

BTW could you confirm the name of the 2 ICs ?

The undamaged one on the left is a 4936 ( NChannel)

The blown one is a 4947 (PChannel)

These are normally made by Vishay-Siliconix

but there are now lots of variants used

ref https://www.cloudyni.../#entry10662644

for a list of ones known to have been used by Meade

 

Andrew Johansen Melbourne Australia



#13 Sylvain Weiller

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Posted 30 June 2021 - 05:24 AM

Hi Andrew,

Thanks a lot for your precious information !

Time to work now :)

BR

Sylvain



#14 Geo.

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Posted 11 July 2021 - 10:21 PM

There may be damaged traces under the bad MOSFET. If you have an astro club ask around for someone who does reflow work.

 

I usually find the safest way to remove the damaged part is to carefully cut the legs away from the epoxy package and remove each leg separately. It's less likely to damage any traces this way. I use a surgeon's scalpel, by sliding it under the pin and cutting upward. Cleanly removing the damaged part is the hardest part of the operation.

 

Surface mounted devices (SMD) are the way of the world now, so more hobbyists are learning to work with it. This should be a walk in the park for any one with some hours under their belt. The MOSFET is a fairly large device and easier to place.

 

I'm pulling the parts together to reflow this motor control PCB designed by a friend in the Chech Rep.

 

Small.jpg

 

Generally, repairs are done with a $40 reflow heater like the one on my bench, next to the soldering station. But the motor board will be done in a $300 reflow oven.

 

CaveM.jpg

 

Good luck!

 


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#15 Sylvain Weiller

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Posted 20 July 2021 - 10:15 AM

Hi all,

Just back !

 

Well reflow look fine but here guess not available at all ...

I remove the bad MOSFET with small blades.

 

Here is the actual state of the board after the operation.

 

The picture shows after minimal work ...

 

What is the most simple way to restore cut lines (I have a thin USB soldering iron, some thin wires of different kinds ...) ?

 

The 2 other pictures at bottom show the circuits and on the right my thought for possible wiring work. BUT maybe it is wrong and maybe is is not the best way to do !

Thanks for your help in this matter ! 

 

 

 

BR

 

Sylvain

Attached Thumbnails

  • StatePostRemoval2.jpg
  • idea of wiring null.jpg
  • idea of wiring.jpg

Edited by Sylvain Weiller, 20 July 2021 - 11:19 AM.


#16 CltFlyboy

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Posted 20 July 2021 - 11:00 AM

With those traces torn up like that, especially around the pads, you are looking at dead-bug wiring the affected pins. Basically what you do is clean up the mangles traces, record where the pins end up going, solder the part in place using the good pads, then use hookup wire to make the connection from the remaining pins to the nearest termination point on their original circuit. It's a pain but it can be done, and at least you left the majority of the pins intact.

 

For future reference, I take a pair of very fine side snips and cut the legs at the silicon to remove the chip, then I use a hot air workstation to clean the leftover legs/solder.

 

Good luck!



#17 Sylvain Weiller

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Posted 20 July 2021 - 11:18 AM

Hi Apollo,

Thanks !

 

I posted a picture above with my thought on wiring ... could somebody with a good understanding of this particular circuit tell me if it is accurate or if I made some mistakes ?

 

Best

Sylvain



#18 CltFlyboy

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Posted 20 July 2021 - 11:43 AM

That looks correct to me, but I'll defer to someone who has worked with that specific circuit.

 

One other thing that might work: conductive repair "ink". It's not really an ink, more of a silver suspension in a glue. Since you have traces that have lifted, you might be able to use that to coat where they were (to the FR4) and then lay the traces on top and solder to the ink. I personally rather do the dead bug type wiring but that's because I've been doing it the same way for decades. Might be worth a shot!

 

https://www.amazon.c...g/dp/B00B88B9KI



#19 Sylvain Weiller

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Posted 20 July 2021 - 12:09 PM

Hello Apollo,

 

Thanks !

Found that silver glue easier for me on Ali...
For the time it comes I guess my repair plan will be "certified" :)

 

BR

Sylvain



#20 OzAndrewJ

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Posted 20 July 2021 - 05:36 PM

Gday Sylvain

 

Looks OK to me

Have attached my hand drawn sketch for an earlier board. ( It doesnt have C20/C21 on mine )

If you look at that, all the traces you have linked match,

but the blue line you show going to C21 could really go to any other point that ties to the 12V bus

ie the outboard side of R2 or even short to the other 12V in on the chip

Andrew Johansen Melbourne Australia

LXD75 Card Trace.JPG



#21 Sylvain Weiller

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Posted 21 July 2021 - 12:55 AM

Hi Andrew,

 

Thanks a lot.

I have worked out a little bit your image ... I hope you like it.

I will study what you wrote ...

 

BR

Sylvain

Attached Thumbnails

  • post-162435-0-77400800-1626820555_thumb.jpg


#22 Sylvain Weiller

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Posted 21 July 2021 - 03:28 AM

Hi all,

Not easy to solder perfectly in place the MOSFET with big fingers ... would have liked to have the "doigté" of young chinese girls smile.gif

I used a pair of thin bended and static free tweezers.
Well here is the result of this first step ...

 

Wiring next ...

Please, where to solder the lower end of the blue-green line as it seems in the schematic not directly linked to one of the power supply pin !?

Also are the bases of R2 and C20 linked (see picture with red arrow) ? (I had a solder bridge in between but removed it with a scalpel. Could have hurt the link doing so. But if they should be linked I will solder them together for sure.)

 

Thanks !

Attached Thumbnails

  • CI-Soldered.JPG
  • Wiring1.jpg

Edited by Sylvain Weiller, 21 July 2021 - 05:25 AM.


#23 Sylvain Weiller

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Posted 22 July 2021 - 05:32 AM

Hello ?
Nobody can answer ?

 

Sylvain



#24 OzAndrewJ

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Posted 22 July 2021 - 05:20 PM

Gday Sylvain

I dont have a board with C20 on it

but my guess is that C20 goes from the output of Q3 to ground

ie Q3 drives the gate of the Mosfet

R2 is a pullup resistor ( to +12V ) on this line

and my guess is C20 is a filter cap going to ground.

Have attached a crop from your pre surgery pic in post #9

It certainly looks like C20 joins with R2

 

You could also test the C21, R6 pair, as they will be wired the same way

but drive the second channel in the mosfet

 

Andrew Johansen Melbourne Australia

LXD75.jpg


Edited by OzAndrewJ, 22 July 2021 - 05:21 PM.


#25 Sylvain Weiller

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Posted 23 July 2021 - 01:35 AM

Hi Andrew, much thanks !

 

One last question remains:
Please, where to solder the lower end of the bottom left blue-green line as it seems in the schematic not directly linked to one of the power supply pin (as I thought before) ... see pale purple arrow (maybe - it's not clear - pointing to a capacitor ?) then difficult for me to decide !

Same question in another way where to find an easy to reach equivalent point for the one pointed by the blue arrow in the schematic ?

 

Best,

Sylvain

Attached Thumbnails

  • Question.jpg

Edited by Sylvain Weiller, 23 July 2021 - 01:49 AM.



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