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Drive jerk on LX85 mount

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#26 atsidor

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Posted 30 June 2021 - 10:40 AM

No, there is nothing more to be done unless you are experienced in electronics repair and troubleshooting.

Yep - pretty much blew it on this one (pun intended).



#27 atsidor

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Posted 30 June 2021 - 10:54 AM

Gday Atsidor

It sounds like one of the HBridge Mosfets may have died then,

Someone with  CRO could trace the signals from the CPU to the motor bridge

but you need the proper probes and skill set etc to do that.

That said, if all the signals look good out to the driver chip,

then replacing it should be simple, but paying time to a tech

might end up being more than a new board.

 

Andrew Johansen Melbourne Australia

Andrew - thanks for the info. I'm hoping to feed some info to a colleague who is experienced with electronics, though not necessarily familiar with astronomy and telescope mounts. I'm assuming the underlying application is not too different from other motor control applications.

 

Just to clarify, are you thinking that it is potentially the MOSFETs internal to the HBridge IC, yes? Not the standalone transistors (Q1, Q2 in the pic of PCB).

 

Edit: I'm not holding you to any of this, just trying to collect as much info as possible and appreciate your expertise.


Edited by atsidor, 30 June 2021 - 10:56 AM.


#28 OzAndrewJ

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Posted 30 June 2021 - 03:37 PM

Gday Atsidor

 

I'm assuming the underlying application is not too different from other motor control applications.

Correct. Its ( so far ) been a simple H Bridge.

I cant read the chip nos on the board, but it looks like U5 might be a single Mosfet HBridge driver.

( Earlier Meades use 2 dedicated mosfets, one PCh and one NCh  )

The relevant PCh gate gets solidly switched on/off via the transistors

and then PWM is supplied via a logic chip fo feed the NCh gate.

 

Just to clarify, are you thinking that it is potentially the MOSFETs internal to the HBridge IC, yes? Not the standalone transistors (Q1, Q2 in the pic of PCB).

Correct.

Just a first guess, but If you simply shorted the motor lines, it would overload the HBridge ( or 12V traces ),

so the gate drive transistors should be safe, unless the Mosfet shorted into the gate as well.

 

Andrew Johansen Melbourne Australia



#29 atsidor

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Posted 30 June 2021 - 04:03 PM

Thank you! Here is a better pic of the PCB that I took. I'm sorry I realized I was referencing this when I was writing my reply but neglected to put it in this thread. 

 

There appear to be 2 HBridge driver ICs (U5 and U8). As far as I can tell, it is this dual bridge Toshiba IC: https://toshiba.semi...dName=TB6552FNG

 

It seems with 2, dual-bridge ICs it could control four motors in total. I'm guessing one IC is for the RA/DEC motors and one IC is for... the focuser motor? That's three so not sure if I'm missing something.

 

Attached Thumbnails

  • IMG_1325.jpg


#30 OzAndrewJ

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Posted 30 June 2021 - 04:36 PM

Gday Atsidor

 

and one IC is for... the focuser motor?

More than likely

Meade used to have an external device ( APM909) to handle ST4, reticle and focus.

This was built into the LX85, so if you look at the annotated piccy i posted earlier,

you can see the reticle and focus port is near U5

I suspect they just use U5 to drive them ( ie allow Fwd/Rev for the focus, but code for Fwd only for reticle????

A simple continuity check from the header pins for RA, DEC, Focus and reticle

back to the relevant pins on the driver chips will tell you which one is which

 

Andrew Johansen Melbourne Australia



#31 OzAndrewJ

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Posted 30 June 2021 - 04:43 PM

Gday Atsidor

And after zooming in ridiculously, i see what looks like a shadow

sooooooooooo, is there a pimple in the surface of U8 ?????

Andrew Johansen Melbourne Australia

Is this a pimple.jpg



#32 atsidor

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Posted 30 June 2021 - 07:24 PM

Yep you nailed it! Wow, didn’t even look closely at that IC since I just figured it was far away and unrelated.

#33 atsidor

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Posted 01 July 2021 - 06:39 AM

Ok, I had certainly given up hope on repairing this board but now I’m cautiously optimistic, thanks to your help. I’m ordering a replacement IC, and I think I have a colleague that will do the repair work.

I guess there’s a chance that other components may be damaged though too? I suppose I’ll start by getting that IC replaced and then go from there.


Edited by atsidor, 01 July 2021 - 06:41 AM.


#34 OzAndrewJ

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Posted 01 July 2021 - 03:30 PM

Gday Atsidor

I guess there’s a chance that other components may be damaged though too?

I doubt it ( based on no other burnt bits ).

ie It was a result of a simple short of the outputs, not a reverse polarity on the inputs,

so if there are no other visibly blown traces on the back of the board,

i would assume the mosfet has simply gone open circuit and save the rest.

 

Andrew Johansen Melbourne Australia



#35 atsidor

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Posted 01 July 2021 - 04:59 PM

Gday Atsidor

I doubt it ( based on no other burnt bits ).

ie It was a result of a simple short of the outputs, not a reverse polarity on the inputs,

so if there are no other visibly blown traces on the back of the board,

i would assume the mosfet has simply gone open circuit and save the rest.

 

Andrew Johansen Melbourne Australia

That's great! I don't see anything else visibly damaged on front or back, so that is some potentially promising news. I ordered a replacement IC and am now trying to line up someone to do the repair :) Many thanks again!



#36 atsidor

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Posted 14 July 2021 - 06:33 AM

Returning to this thread to report that my LX85 board has been repaired! Thanks in no small part to the folks here -- Andrew, Bjorn: thank you so much. It was indeed the U8 H-bridge driver IC that was the culprit -- it is now replaced and the motors are functioning again.

 

I also have to give a huge thanks to a very kind, fellow astronomer (and electronics expert) in my local community who did the repair for free. It was a ~$2 component. The repair work was certainly much more than that and, in a way, truly priceless to me.

 

I am blown away by the kindness of those in the astro community, both online and local.

 

Now... back to trying to resolve the original issue of the motor stuttering... more to come.



#37 OzAndrewJ

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Posted 14 July 2021 - 04:26 PM

Gday Atsidor

Excellent news re the repair.

re

back to trying to resolve the original issue of the motor stuttering

What test gear do you have??

Again a logic analyser is the best tool here

 

Andrew Johansen Melbourne Australia



#38 atsidor

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Posted 15 July 2021 - 01:43 PM

 

What test gear do you have??

Again a logic analyser is the best tool here

 

Andrew Johansen Melbourne Australia

I don't have much personally, but my colleague who repaired the board has offered to hook up his oscilloscope to investigate further, if necessary. Short a schematic though, I'm not sure how much can be done. Could at least look at the signals to and from the motor.

 

First, though, I'm going to plug the RA motor into the output of the DEC motor board and run it that way. I believe the RA and DEC motors are identical. Mechanically speaking, they are (aside from the RA motor have the PEC sensor hanging off one end). And they have the same 6-pin electrical connection. I still need to confirm it's the identical pinout scheme though. I can't imagine why they wouldn't, but I don't want to take any chances.

 

Anyway, if the RA motor displays the same behavior plugged into the DEC motor board, then seems like it's a motor problem (maybe encoder?). (Note here that my DEC motor operates normally without stuttering.)

 

The RA motor being bad is honestly probably the best result as I have another CN user that has offered a brand new replacement part. If it isn't, I guess we'll have to dive in with the oscilloscope...


Edited by atsidor, 15 July 2021 - 01:44 PM.


#39 OzAndrewJ

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Posted 15 July 2021 - 04:13 PM

Gday Atsidor

 

offered to hook up his oscilloscope to investigate further, if necessary. Short a schematic though,

As per before, a CRO will show if there is something there.

A Logic analyser will show the details.

With an analyser, you poll the encoder A/B lines and the motor PWM.

From that, you can see if the PWM matches the encoder

ie is the PWM smooth with rough encoder traces etc.

Re schematic, no one has a "detailed" one

but the earlier posted piccy shows the headers and their functions.

Your mate should be able to probe the RA header pretty easily.

 

Andrew Johansen Melbourne Australia

 

PS, if you do remove the motor, your mate could try a quicky Motor only test.

By running it standalone off a lab power supply ( no encoder feedback )

just run the voltage up from 0 to 12 and back at a few variable rates.

If it does stutter, it is 99.9% the motor/Gbx, and not a feedback problem.


Edited by OzAndrewJ, 15 July 2021 - 04:17 PM.


#40 atsidor

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Posted 15 July 2021 - 07:41 PM

Success! I connected the RA motor to the DEC output and found the same stuttering occurred. That narrowed it down to the motor assembly itself. I took off the plastic housing over the encoder to take a peek and... discovered that the encoder disk (not sure if that's the right term, small circular disk with black/white stripes which are read by the encoder) was loose and freely spinning on the motor shaft. Thus, it could easily slip when the motor turned.

 

I better secured the disk to the shaft (with tape temporarily) and tested again - and no stuttering! I will try to post some pictures of the details then for folks to see. So happy!



#41 OzAndrewJ

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Posted 15 July 2021 - 08:12 PM

Gday Atsidor

 

Excellent work, and glad it was relatively cheap.

 

Can you pls take clean photos of the encoder , its disk and if possible any part nos.

We know of a certain problem with the new E4T encoders used on the LX200/600/850s

so am wondering if you have a similar problem.

 

Andrew Johansen Melbourne Australia



#42 atsidor

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Posted 15 July 2021 - 09:25 PM

Andrew, I can certainly do that and will put photos together in next day or two. It could have easily gone much worse, so very thankful and fortunate it all came together in the end. Many thanks again for your kind assistance here on the forums!

#43 barnold84

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Posted 15 July 2021 - 10:45 PM

Hi Adam,

 

I'm glad to hear that you have found someone who could help you repairing the electronics and also that you have found out what caused the strange motor behavior. I should certainly take a look at the LX85 I had and see if it's the same issue.

 

Best,

Björn



#44 atsidor

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Posted 19 July 2021 - 11:24 AM

I completed the fix of the encoder over the weekend. I added a couple spots of hot glue around the encoder disk to secure it better. Is it permanent? Probably not, but I wanted something that I could re-work later if necessary. It is working well now -- I put it through a bunch of slews and saw absolutely no stuttering.

 

I've attached some pictures I took through the process. I did not see much in the way of part numbers but for what it's worth, here you go. Hopefully it will help other folks. 

Attached Files


Edited by atsidor, 19 July 2021 - 11:24 AM.


#45 OzAndrewJ

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Posted 19 July 2021 - 04:01 PM

Gday Atsidor

 

OK, good info.

Definitely nothing like any prev Meade encoders i have seen

but your fix should work OK for a long time.

Good to see it was cheap ( in the end )

 

Andrew Johansen Melbourne Australia




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