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Meade LX200 Classic capacitors

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#1 Macadoo

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Posted 21 May 2024 - 10:55 AM

Hey folks, I'm still tuning up school's telescope and I've ordered 50 volt tantalum capacitors for the handset and control panel.  I understand that if they pop in the handset, they can (will?) burn through the ribbon cable.  I'm wondering what happens in they burn inside the control panel.  Will it ruin the PCB?  I'll likely replace them all, I'm just curious.

 

Also, I tested the 18 volt power supply and it's putting out 21.5 volts.  It's no wonder so many people have had trouble. shocked.gif

This scope is missing a lot of the extras and the power supply is all I have to work with.  But all will be good with the 50 volt caps, yes?



#2 N3p

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Posted 21 May 2024 - 11:08 AM

Hopefully you will turn the remote into something durable smile.gif  Making it as it should be in the first place. Have fun replacing capacitors. 

 

Yes! 



#3 Jim in PA

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Posted 21 May 2024 - 11:57 AM

Yes, 50V caps will be fine.  That power supply seems kind of sketchy.  Just to be sure, you're saying it's labelled for 18VDC as the output, but output measured is 21.5VDC? 



#4 Macadoo

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Posted 21 May 2024 - 12:15 PM

Hopefully you will turn the remote into something durable smile.gif  Making it as it should be in the first place. Have fun replacing capacitors. 

 

Yes! 

It's what I live for crazy.gif SO much more fun than, you know, observing lol.gif

 

Yes, 50V caps will be fine.  That power supply seems kind of sketchy.  Just to be sure, you're saying it's labelled for 18VDC as the output, but output measured is 21.5VDC? 

Yes sir, it's labeled 18v, 2A, but puts out 21.5 v.  I'm surprised the caps haven't blown.  But then again, I don't know how much use this scope has gotten over the years.

I have a 12v supply sitting around but it's only 1.5 amps and has the wrong plug.  How do folks hook this scope up to a car battery?  I have no other adaptors, just the power supply.


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#5 Jim in PA

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Posted 21 May 2024 - 01:02 PM

It's what I live for crazy.gif SO much more fun than, you know, observing lol.gif

 

Yes sir, it's labeled 18v, 2A, but puts out 21.5 v.  I'm surprised the caps haven't blown.  But then again, I don't know how much use this scope has gotten over the years.

I have a 12v supply sitting around but it's only 1.5 amps and has the wrong plug.  How do folks hook this scope up to a car battery?  I have no other adaptors, just the power supply.

OK...have you measured the output under load? Say with a 10 Ohm power resistor?  That should put you close to but under the labelled output of 2A at 18V.  It's possible that under load, you'll see some voltage drop from 21.5VDC that you measured when not under load.

 

I don't use the fork mount myself as I deforked mine.  I take it you've found the MAPUG archives?  I've not done any capacitor swaps on mine and not sure if the previous owner did.

 

My gut feeling on this would be to use the best caps you can find, from Japanese manufacturers. 


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#6 Macadoo

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Posted 21 May 2024 - 02:22 PM

I have not  measured it under load, but that's a good idea.

And yeah, I found MAPUG the other night.  Tons of info.



#7 michael8554

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Posted 22 May 2024 - 03:12 AM

Whether or not the Meade 18V PSU gives 18V under load or not, just bin it.

 

Get a PSU that is Regulated, so that it always gives the rated voltage.

 

And I'd go for a 12V or 13.8V PSU.

 

There are many online sources for the cap replacement, procedure, read them all before you start.

 

Including the Meade section here on CN...........


Edited by michael8554, 22 May 2024 - 03:12 AM.


#8 Macadoo

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Posted 22 May 2024 - 08:11 AM

Whether or not the Meade 18V PSU gives 18V under load or not, just bin it.

 

Get a PSU that is Regulated, so that it always gives the rated voltage.

 

And I'd go for a 12V or 13.8V PSU.

 

There are many online sources for the cap replacement, procedure, read them all before you start.

 

Including the Meade section here on CN...........

That's a good suggestion and I will run it by the Dean, but keep in mind that this isn't my telescope, it's the school's.  I'm just doing all of this as a favor (and I don't like seeing a good piece of equipment mistreated).  They'll pay $20 for a few capacitors but I doubt they'll shell out for a new power supply.  I have a few computer ATX power supplies sitting around.  One should have a +12v connector, if memory serves.  It'd be a bit bulky, though.

Now, to measure the barrel connector that fits into the control panel.  It seems to be of an unusual length, if not diameter.



#9 jdupton

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Posted 22 May 2024 - 09:05 AM

Mac,

 

That's a good suggestion and I will run it by the Dean, but keep in mind that this isn't my telescope, it's the school's.  I'm just doing all of this as a favor (and I don't like seeing a good piece of equipment mistreated).  They'll pay $20 for a few capacitors but I doubt they'll shell out for a new power supply.  I have a few computer ATX power supplies sitting around.  One should have a +12v connector, if memory serves.  It'd be a bit bulky, though.

Now, to measure the barrel connector that fits into the control panel.  It seems to be of an unusual length, if not diameter.

   I don't think running with 12 volts will hurt anything after replacing the capacitors but 12 volt power can create new issues. That's why Meade ended up moving from 12v to 18v for the LX200s in the first place. That move to a higher voltage is also what started the tantalum capacitor mess in the first place.

 

   As I understand the issue, the early Meade LX200's were supplied with (or at least specified as needing) a 12v power source. Some number of users reported motor stalls, blown fuses, and loss of alignments among other issues. The boards and hand control were designed for 12v operation and thus used 25 volt capacitors. That was an ample safety margin for the capacitors. Unfortunately, Meade decided to start shipping with the 18v (21v+) power supply to ensure proper operation which fixed the motor stall issues, blown fuses, and power brown-out conditions. Meade did not change the design specification for the capacitors used in the electronics.

 

   That is where the issues with the capacitors started. The 25v capacitors used in the electronics were now being subjected to up to 21v on a regular basis. As tantalum capacitors age, they can start to break down at lower than rated voltages. The 21 volts from the updated power supply is too close to the rated break-down voltage of new capacitors -- aged capacitors are more at risk. When the tantalum capacitors fail, they go into thermal runaway as they short circuit inside and draw a lot of current. Sometimes, the fuse in the telescope blows before the capacitor explodes or goes into "blowtorch mode." Sometimes, the fuse is not fast enough to prevent the total breakdown.

 

   I will also note that any tantalum capacitor that has been subjected to (even temporary) reversed voltage is very likely to fail soon after application of correct voltages. 

 

   The long term solution, as everyone with one of these scopes now knows, is to replace all of the tantalum capacitors in the electronics even if they have not failed yet. As the capacitors age even more and are subjected to 21 volts again and again, there is not enough design margin to prevent a future failure. Using 35v or 50v replacements is the best long term fix to keep these old telescopes running reliably. It is also a good idea to add reverse polarity protection to the power source. Even a momentary reverse voltage application will set up the capacitors for quick failure (if they don't don't self destruct before the fuse blows with the reversed polarity).

 

 

John


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#10 michael8554

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Posted 23 May 2024 - 01:51 AM

"Now, to measure the barrel connector that fits into the control panel.  It seems to be of an unusual length, if not diameter."

 

A standard 5.5mm x 2.5mm.

 

Not 5.5mm x 2.1mm.


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#11 rutherfordt

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Posted 23 May 2024 - 07:34 AM

 They'll pay $20 for a few capacitors but I doubt they'll shell out for a new power supply.

If you replace the capacitors with 35- or 50-volt ones, then you won't need to replace the power supply so you should be good.

 

Tom

 


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#12 Macadoo

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Posted 23 May 2024 - 04:23 PM

Mac,

 

   I don't think running with 12 volts will hurt anything after replacing the capacitors but 12 volt power can create new issues. That's why Meade ended up moving from 12v to 18v for the LX200s in the first place. That move to a higher voltage is also what started the tantalum capacitor mess in the first place.

 

   As I understand the issue, the early Meade LX200's were supplied with (or at least specified as needing) a 12v power source. Some number of users reported motor stalls, blown fuses, and loss of alignments among other issues. The boards and hand control were designed for 12v operation and thus used 25 volt capacitors. That was an ample safety margin for the capacitors. Unfortunately, Meade decided to start shipping with the 18v (21v+) power supply to ensure proper operation which fixed the motor stall issues, blown fuses, and power brown-out conditions. Meade did not change the design specification for the capacitors used in the electronics.

 

   That is where the issues with the capacitors started. The 25v capacitors used in the electronics were now being subjected to up to 21v on a regular basis. As tantalum capacitors age, they can start to break down at lower than rated voltages. The 21 volts from the updated power supply is too close to the rated break-down voltage of new capacitors -- aged capacitors are more at risk. When the tantalum capacitors fail, they go into thermal runaway as they short circuit inside and draw a lot of current. Sometimes, the fuse in the telescope blows before the capacitor explodes or goes into "blowtorch mode." Sometimes, the fuse is not fast enough to prevent the total breakdown.

 

   I will also note that any tantalum capacitor that has been subjected to (even temporary) reversed voltage is very likely to fail soon after application of correct voltages. 

 

   The long term solution, as everyone with one of these scopes now knows, is to replace all of the tantalum capacitors in the electronics even if they have not failed yet. As the capacitors age even more and are subjected to 21 volts again and again, there is not enough design margin to prevent a future failure. Using 35v or 50v replacements is the best long term fix to keep these old telescopes running reliably. It is also a good idea to add reverse polarity protection to the power source. Even a momentary reverse voltage application will set up the capacitors for quick failure (if they don't don't self destruct before the fuse blows with the reversed polarity).

 

 

John

Ahh, so a Band-Aid fix that caused even worse problems.  Makes sense.....not.  Your explanation, however, does.  Thank you.

 

Now to throw in another wrinkle, it looks like someone soldered in a resistor with the caps, possibly to head off the over-voltage problem.  But why do that when you could just put in caps of a higher rating?  And maybe they did, I don't know yet.  The ribbon cable is kind of a booger to remove.  Any tips on that process?

 

20240523_152936.jpg



#13 jdupton

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Posted 23 May 2024 - 04:43 PM

Mac,

 

attachicon.gif 20240523_152936.jpgattachicon.gif 20240523_152936.jpg

Ahh, so a Band-Aid fix that caused even worse problems.  Makes sense.....not.  Your explanation, however, does.  Thank you.

 

Now to throw in another wrinkle, it looks like someone soldered in a resistor with the caps, possibly to head off the over-voltage problem.  But why do that when you could just put in caps of a higher rating?  And maybe they did, I don't know yet.  The ribbon cable is kind of a booger to remove.  Any tips on that process?

 

attachicon.gif 20240523_152936.jpg

   The image of the capacitors shows a diode across the cap and not a resistor. Under normal operation, it is back-biased and so does absolutely nothing. However, if there is a reversed voltage applied to the mount the diode conducts so that hopefully the fuse will blow before the tantalum caps are damaged. In theory, that would work but the implementation appears to be flawed. They added a low current signal diode which will itself blow long before the fuse in the mount does. It will not save the tantalum caps from being damaged by reverse polarity power.

 

   The idea is sound but that implementation won't work. If you replace the caps with 50 volt versions, you can also replace this cap with something like a 5 amp+ 60 volt rated diode. That will force the fuse to blow fairly quickly if reversed polarity power is given to the mount.

 

 

John



#14 Macadoo

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Posted 23 May 2024 - 05:16 PM

Mac,

 

   The image of the capacitors shows a diode across the cap and not a resistor. Under normal operation, it is back-biased and so does absolutely nothing. However, if there is a reversed voltage applied to the mount the diode conducts so that hopefully the fuse will blow before the tantalum caps are damaged. In theory, that would work but the implementation appears to be flawed. They added a low current signal diode which will itself blow long before the fuse in the mount does. It will not save the tantalum caps from being damaged by reverse polarity power.

 

   The idea is sound but that implementation won't work. If you replace the caps with 50 volt versions, you can also replace this cap with something like a 5 amp+ 60 volt rated diode. That will force the fuse to blow fairly quickly if reversed polarity power is given to the mount.

 

 

John

Oops, my ignorance is showing.  How would the current get reversed?  As in, someone not using the barrel plug and power supply?

So I do still need to replace the caps.  I'll put that on my list for tomorrow.  I have 50v caps ready to go.



#15 jdupton

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Posted 23 May 2024 - 05:31 PM

Mac,

 

Oops, my ignorance is showing.  How would the current get reversed?  As in, someone not using the barrel plug and power supply?

So I do still need to replace the caps.  I'll put that on my list for tomorrow.  I have 50v caps ready to go.

   Yes, you are exactly right. The diode protecting against reversed voltage is there for someone who chooses not to use the supplied power brick. Imagine a user wanting to run the mount from battery power where AC is not available. They might use one the available power cords that have the DC barrel plug on one end and alligator style battery clips on the other end to connect to a large (car / marine) battery. In the dark, if they connect the clips to the wrong poles on the battery and -- bye bye mount...

 

 

John



#16 Macadoo

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Posted 23 May 2024 - 05:54 PM

Mac,

 

   Yes, you are exactly right. The diode protecting against reversed voltage is there for someone who chooses not to use the supplied power brick. Imagine a user wanting to run the mount from battery power where AC is not available. They might use one the available power cords that have the DC barrel plug on one end and alligator style battery clips on the other end to connect to a large (car / marine) battery. In the dark, if they connect the clips to the wrong poles on the battery and -- bye bye mount...

 

 

John

Gotcha.  All I found with this scope was the power brick.  I wondered why there wasn't a car battery option.

So, this is a little beyond me, I'll admit.  How would a diode be wired-in?  Not in parallel, like it is now, I wouldn't think.  Did these bricks come with a diode soldered to the board from the factory?



#17 jdupton

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Posted 23 May 2024 - 06:34 PM

Mac,

 

   The diode is wired in just as the one on the board now. It is in parallel with any one capacitor's leads. A diode will have a line around one end of it. (Note the black line around the end of the diode in your photo nearest the end of the capacitor marked with the '+'.) That line goes towards the positive (+) side of the circuit when used in this mode. In that orientation, the diode cannot normally pass (any significant) current. 

 

   If power is applied to the mount in the opposite direction, the diode will become a near short circuit pulling a good bit of current until the fuse in the mount power input blows. That is how it is supposed to protect the mount. It sucks lots of current to make the fuse open up and disconnect the mount from the reversed power source. The Schottky diodes I pointed to usually have a very low forward voltage drop -- usually around 0.25 volts or less. That is as high as the power supply will be able to power up the board before the fuse blows.

 

   There is really no way to use a reverse polarity diode in a power supply. The issue must be addressed in the device. Power supplies can be designed shut down quickly (overcurrent protection) if too much current is pulled but it will often try to ramp to full voltage first in most cases. It all depends on the design of the power supply. Most wall wart style power bricks are not known for having the same features as a high quality bench power supply.

 

 

John



#18 Macadoo

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Posted 23 May 2024 - 06:37 PM

After thinking about this more, shouldn't the protective diode be the first line of defense, as in on the board in the controller, where the power first comes in?



#19 Macadoo

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Posted 23 May 2024 - 06:46 PM

Mac,

 

   The image of the capacitors shows a diode across the cap and not a resistor. Under normal operation, it is back-biased and so does absolutely nothing. However, if there is a reversed voltage applied to the mount the diode conducts so that hopefully the fuse will blow before the tantalum caps are damaged. In theory, that would work but the implementation appears to be flawed. They added a low current signal diode which will itself blow long before the fuse in the mount does. It will not save the tantalum caps from being damaged by reverse polarity power.

 

   The idea is sound but that implementation won't work. If you replace the caps with 50 volt versions, you can also replace this cap with something like a 5 amp+ 60 volt rated diode. That will force the fuse to blow fairly quickly if reversed polarity power is given to the mount.

 

 

John

Oh, I missed your link somehow,  Thanks John.  And thanks for the explanation.  I think I might have some of those exact diodes sitting around (from an Arduino project).  I'm glad I posted the pic and that you caught the inappropriate diode.




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