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LX200 turns off immediately

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#1 sickozell

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Posted 07 February 2021 - 04:28 AM

Hello,

I have a problem with my father's LX200. It has been fully functional for 20 years, but when I turned it on one month ago I found this issue:

When I turn on the power switch the test test leds and the one on the power supply light up for one second and then they all light down, and if I retry to switch on the telescope seems dead (power supply too), so i have to disconnect power supply from the telescope and power grid and wait about 30 seconds to be able to retry, but with same results.

I've been looking for similar issues on forums but I didn't find the same problem. I checked capacitors on main board but they all look good. Perhaps C3 of the control panel has a little stain.

I attach pictures of all, and I would like to know if the power supply is the original one or an after market model, because I read that there are problems with original ones.

Thank you.

Fabio

Attached Thumbnails

  • scheda madre.jpg
  • control panel c3.jpg
  • alimentatore2.jpg


#2 sg6

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Posted 07 February 2021 - 04:50 AM

I would find another power supply - well at a guess.

The one you have states 18v and I thought the Meades all used 12v.

 

I have had one complain of "Over voltage" and maybe the bigger model have a sort of autoshut down feature. But 18V seems way too high. On a Synta derived model you wouldn't have a scope. They tend to go Pop.

 

My guess, and that is all it is, that the scope shuts down due to too high a voltage.



#3 jrcrilly

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Posted 07 February 2021 - 05:06 AM

It is not too high a voltage; that model was designed for 18VDC. That is not the early, problematic supply model but could still have problems of its own. If the light on the PS goes out as you describe, it is either defective or is shutting down due to excesive current draw from the telescope. This is unlikely, as the power supply is intended to put out 2 Amps and if the telescope were trying to draw enough to kick in the current limiting the internal fuse should have already blown. I'd think that the PS is likely to be where the failure is - and especially the AC cable feeding it. You could prove this by temporarily connecting the telescope to a 12VDC source. It will do no harm to operate briefly on 12VDC. It will either work normally (if the supply has failed) or the fuse will pop (if the telescope has failed). That will tell you which direction to look. Make sure that it has the correct value fuse installed behind the control panel first!



#4 Supernova74

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Posted 07 February 2021 - 05:28 AM

Personally like a previous member stated I would try a new Meade power supply thay are not exspensive around $30 I believe.try and troubleshoot first however you already opened up a can of worms there by opening the unit up especially if it’s a relatively simple fix.



#5 jrcrilly

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Posted 07 February 2021 - 05:38 AM

It has probably been twenty years since Meade offered an 18VDC power supply. When they did, they were much more than $30.



#6 astrovoyeur

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Posted 07 February 2021 - 09:13 AM

It is not too high a voltage; that model was designed for 18VDC. That is not the early, problematic supply model but could still have problems of its own. If the light on the PS goes out as you describe, it is either defective or is shutting down due to excesive current draw from the telescope. This is unlikely, as the power supply is intended to put out 2 Amps and if the telescope were trying to draw enough to kick in the current limiting the internal fuse should have already blown. I'd think that the PS is likely to be where the failure is - and especially the AC cable feeding it. You could prove this by temporarily connecting the telescope to a 12VDC source. It will do no harm to operate briefly on 12VDC. It will either work normally (if the supply has failed) or the fuse will pop (if the telescope has failed). That will tell you which direction to look. Make sure that it has the correct value fuse installed behind the control panel first!

You're logic is spot on.   I have 12" LX200 classic and it runs on 18vdc.   
I would only add that the OP check the fuse to insure its not been replaced with a value greater than 2amps first before testing with another 12-18vdc source.   

 

Could be you've got a shorted cap and a oversized fuse so the power supply is shuting down from over current.  

Or you've got a weak bad power supply.


Edited by astrovoyeur, 07 February 2021 - 09:20 AM.


#7 michael8554

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Posted 07 February 2021 - 09:17 AM

You do have a fuse in the power supply cabling I hope ?

 

During the Classic LX200s life, for some reason Meade suggested running it at 18V, even though the circuits were only just good enough for the original 12V.

 

C3 looks unhappy, innumerable posts on how to replace the old capacitors that have blown/may be about to blow.

 

Unless you like slewing around at full speed, waking up the neighbours, run it on 12V.



#8 NearVision

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Posted 07 February 2021 - 09:43 AM

As an owner of a LX200 Classic and 40+ years working on various electrical equipment I've seen similar things.

 

If it's been sitting for a while it may just be gunk/corrosion on the contacts in the switch or connectors. With it unplugged from AC power try this: Switch the power switch on and off several times quickly and plug and unplug the power plug from the supply to the scope. Also rotate the plug in the socket. This wipes the contacts against each other and helps to remove the gunk/corrosion for a better connection. While you have the base open check that the fuse is reasonably tight in it's clips and not flopping loosely. (It happens!)

 

If that doesn't help try another power supply. You can use either 12 volts or 18 volts. The scope works with either it's just slightly quieter and slower at 12 volts. Wear and heat on the parts from the higher voltage at 18 is minor. The danger is the Tantalum capacitors are rated for 25 volts and the original Meade power supplies were notorious for being poorly regulated and would often spike over that causing the caps to blowup. And they did! This generation of Tantalum caps would explode like a miniature Thermite grenade and take out anything close by like the cable in the handset. I would STRONGLY recommend replacing all of them with Electrolytic caps rated for 35 volts as soon as possible. (You can use higher than 35 volt caps they are just as safe but larger and may not fit physically as well.) They can blow just because they feel like it even with a well regulated supply at any voltage. I've seen them do and it ain't pretty!

 

If none of that helps it's time to get a meter and start checking where it's disappearing.

Good Luck!

Tom



#9 carolinaskies

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Posted 07 February 2021 - 10:26 AM

The LX200 18v models easily run on 12v and the only difference in running is top slew speed.  Because of the issue with the old capacitors, running on 12v is typically the best practice.  

As to the particular issue of powering off, I can see a probable cause being the power supply being below recommended amperage and given age of the mount.  While 2A may 'run' the telescope, even 22 years ago when I bought my first LX200 Classic we would use a 3-5A 12v.  Since the telescope is powering up and then shutting down and will only do so repeatedly after a 'cooling' off period, that indicates the mount is trying to work with less amps and is overheating the PS as it tries to work.  

A good test is to power the mount with a 12v battery source like a car battery that has more than enought amps to supply the mount.  Or if you have a jump pack for starting car batteries, this is acceptable because they typically have a 17aH battery.  

If the mount will power on and not shut down immediately using a battery source versus the AC adapter then definitely replace the PS with a 12v 4-5amp unit.   If it continues to shut down, then you'll need to upgrade the capacitors.  I would still suggest only running 12v, again the 18v isn't necessary.  



#10 astrovoyeur

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Posted 07 February 2021 - 11:09 AM

Speaking from my own personal experience with a XL200 classic 12" that I've always run on 18vdc, I've never seen the amp meter (LED bar graph) upper left corner of power panel, read more that 4 bars, or 1.2 amps.  Each segment represents 300ma.   Usually when you see a steady 4 bars or more while slewing indicates the OTA isn't balanced.   My scope will normally slew at 3 bars without its dew shield attached. 

 

My guess is the power supply has a regulator with bad caps.   I've seen this quite frequently with wall-warts.  If you can crack it open you'll probably find some swollen caps that can be replaced for a few cents. 


Edited by astrovoyeur, 07 February 2021 - 11:17 AM.


#11 astrovoyeur

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Posted 07 February 2021 - 11:39 AM

Sickozell,  I'd stay clear of high current supplies as long as you've got tantalum caps in your scope.   Because of their low ESR they tend to create a lot of internal stress and heat when they are first energized by power supplies with an abundance of amps.  Which is another reason they fail. 



#12 sickozell

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Posted 07 February 2021 - 01:56 PM

Thanks to everyone who answered me.

I found the self-made 12v car battery box that my father used to power the scope. I checked its charge and checked the 5A fuse of this box and it was blown.

I replaced it with a new 3.15A (at the moment I had only of this value), so I connected everything and turned the scope on, the amp meter has lit for a fraction of second and went out.

I rechecked the fuse: blown again.

(sorry for my bad english, hope you understand)

 

btw: the fuse on control board is 2A, fine and tight in socket. I tried also to switch on and off several times as suggested by NearVision


Edited by sickozell, 07 February 2021 - 02:07 PM.


#13 NearVision

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Posted 07 February 2021 - 08:05 PM

Sounds like you need a new fuse in the battery box and then start tracing the power inside the scope. There are schematics available online. Google is really trying to be a friend. :)

 

On the power panel you've got a capacitor and a 0.1 ohm resistor before the power switch and then the 2A fuse. With a volt meter you should be able to check at the pins on the backside of the power jack, the switch pins, and then on each side of the fuse holder. If you get 18 volts (or 12 volts if you use the battery) at each point you should be able to tell where it's loosing the connection.

 

The power LED is right after the fuse so if it's not lit you don't have too many places to look.

When testing these that capacitor (c4) is a filter across the input jack and the resistor (r1) should be fairly big. It goes from the negative of the power jack to system ground and is used by the amp meter for sensing the current draw.



#14 astrovoyeur

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Posted 08 February 2021 - 08:34 AM

Its hard to imagine why the 3 and 5 amp fuses are blowing before the control panels 2 amp, and only after the power switch is turned on.  

 

I've never noticed before until now. but it appears the control panel's amp meter isn't protected by the 2 amp fuse.    You may want to disconnect the amp meter to see if the short goes away.   If you look at bottom of schematic for switch SW1 you'll see a +PWR terminal that feeds the amp meter at top right of schematic as input to U1A. and U2A

 

http://lx200classic....Power Panel.pdf

 

One other thing you can do to test the theory is to pull the 2 amp fuse in the control panel and see if the power supply fuse blows.  This will at least isolate your problem to the circuits that proceed the 2 amp fuse. 

 

My guess would be one of the 2 LM339s is bad on the amp meter.

 

Another thing you might check is if the power jack is isolated from the chassis.  The schematic implies there are 2 different grounds earth and circuit.  I'm pretty sure the jack has to be isolated to insure all power returns through R1. 


Edited by astrovoyeur, 08 February 2021 - 09:22 AM.


#15 BPoletti

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Posted 09 February 2021 - 11:48 AM

Try running the scope off of batteries instead of the PS.  



#16 sickozell

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Posted 09 February 2021 - 01:03 PM

Yes, I checked that the power jack is isolated from chassis.

Before starting disconnecting components I tried to remove  the control panel's fuse. Turning on the power switch, nothing happens on the amp meter or power led, but the power supply doesn't die.



#17 nitegeezer

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Posted 09 February 2021 - 03:47 PM

I would expect that there is a shorted capacitor or IC. If the fuse is not a fast blow, the power supply fold back current limit could easily shut the power down before the fuse gets hot enough to blow. If you have a multimeter, check the power lead to see if it is shorted both with switch on and off. Hopefully any blown parts will have a burn mark so it is easy to identify. The tantalum caps are the first suspect.
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#18 astrovoyeur

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Posted 10 February 2021 - 09:49 AM

Yes, I checked that the power jack is isolated from chassis.

Before starting disconnecting components I tried to remove  the control panel's fuse. Turning on the power switch, nothing happens on the amp meter or power led, but the power supply doesn't die.

How weird that a 5 amp fuse is blowing before a 2 amp.   Are you sure its a 2amp and not 20 amp? 

Based on the fact the fault cleared with the removal of the 2 amp fuse.  I would have to say its one of the Tantalum caps.  

 

There is a tantalum on the power panel board, main board, RA motor, Dec motor and HC.   5 total.   You might be able to probe each it see which one is shorted given that is their most common failure mode.

 

If you find the bad one. simply crush it with needle nose  and separate the remaining leads so they aren't touching anything but don't cut them.   The cap isn't a critical component and you can test without them in the circuit.   When you find a suitable replacement you can simply attach/solder it to the old cap's leads.  Or you can go through the tedious task of desoldering the old and risk damaging the circuit board.   


Edited by astrovoyeur, 10 February 2021 - 09:57 AM.


#19 rferrante

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Posted 10 February 2021 - 02:00 PM

The fuse on the LX200 power panel is a slow-blow fuse. You have a short, but the power supply's short circuit protection is kicking on before the slow-blow fuse will blow. The short is probably one of the 5 tantalum caps that are known to fail with age.

 

Since you have a limiting power supply, you can diagnose this without needing to buy a lot of replacement fuses. Just disconnect everything from the power panel (including the rear ribbon connector) and see if the problem remains. If it does, you probably have a bad C1 on the power panel pcb. If the problem goes away, start plugging connectors in one by one until you see the problem again. Then you know where to look.

 

--Rob



#20 astrovoyeur

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Posted 10 February 2021 - 02:34 PM

The fuse on the LX200 power panel is a slow-blow fuse. You have a short, but the power supply's short circuit protection is kicking on before the slow-blow fuse will blow. The short is probably one of the 5 tantalum caps that are known to fail with age.

 

Since you have a limiting power supply, you can diagnose this without needing to buy a lot of replacement fuses. Just disconnect everything from the power panel (including the rear ribbon connector) and see if the problem remains. If it does, you probably have a bad C1 on the power panel pcb. If the problem goes away, start plugging connectors in one by one until you see the problem again. Then you know where to look.

 

--Rob

 

 

I found the self-made 12v car battery box that my father used to power the scope. I checked its charge and checked the 5A fuse of this box and it was blown.

I replaced it with a new 3.15A (at the moment I had only of this value), so I connected everything and turned the scope on, the amp meter has lit for a fraction of second and went out.

I rechecked the fuse: blown again.

 

The op mentioned in his earlier post that he has already blown a 5amp and 3amp fuse in his power supply.. I can't image a 2 amp slow blow clearing a 5amp fault. 


Edited by astrovoyeur, 10 February 2021 - 02:37 PM.


#21 nitegeezer

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Posted 10 February 2021 - 02:50 PM

The op mentioned in his earlier post that he has already blown a 5amp and 3amp fuse in his power supply.. I can't image a 2 amp slow blow clearing a 5amp fault.


As a design engineer, this reminds me of an old joke at the office.

A transistor protected by a fast acting fuse will blow first to protect the fuse!!!

#22 rferrante

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Posted 10 February 2021 - 08:25 PM

The op mentioned in his earlier post that he has already blown a 5amp and 3amp fuse in his power supply.. I can't image a 2 amp slow blow clearing a 5amp fault. 

A BK/MDL-2-R 2A time-delay fuse will actually pass 5A for a limited time, even several seconds, according to the Bussman data sheet for that series, unless I'm reading it wrong.



#23 astrovoyeur

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Posted 11 February 2021 - 07:53 AM

A BK/MDL-2-R 2A time-delay fuse will actually pass 5A for a limited time, even several seconds, according to the Bussman data sheet for that series, unless I'm reading it wrong.

There in lies part of the problem.  The LX200 classic specifies a 1 amp slow blow. on the power board.  



#24 carolinaskies

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Posted 11 February 2021 - 04:35 PM

A standard fuse vs a slow blow will blow first, that's why they are called 'slow blow' as they can tolerate more range of value longer. 


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#25 RSX11M+

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Posted 14 February 2021 - 08:50 PM

I'd like to have had a schematic to help you, but I don't. So here is what I do have - photos of my own, working, LX200 12" Classic's display board.
 
It has been modified, with C1 removed (simply clipped out - not desoldered) and a substitute capacitor added (soldered) to the rear of the board. 
IMG_4212.JPG
IMG_4213.JPG


This added capacitor (Blue) is an electrolytic type 2200µf 25v radial lead can, soldered directly to the back of the power connector J5. In this location it is well clear of anything when installed in the drive base.
IMG_4214.JPG

 
The fuse is "original" and is what I would call an MDL 2A glass fuse. Inline fuses in the power chord usually pop before this one does. (mine did)
IMG_4215.JPG


I am an EE, and while I am fully capable of having performed this modification myself, I do not recall doing so. Furthermore, I usually initial my work and this has no markings of mine, and is slightly different than my "style" of workmanship. Hence I must conclude this was done by Meade as a factory change. You should also know that the capacitor, added in this position, does not electrically parallel C1 - but is across the power supply J5 directly.
 
I did have to make a similar change of this kind to my LX200-16" (which I do remember) but that drive base has an entirely different PCB and hardware. It does share the same type hand controller.
 
 
 
On the topic of 12v vs 18v supply - some report these scopes can be operated direct from battery, which is probably so for most. However, in some operations this may not be sufficient - and the result would possibly manifest as an unintended restart or runaway movement. At the very least these could ruin a photo session if not harm hardware. I should think a fully charged battery could be used if any could. Personally, I use the Meade 12vdc to 18vdc adapter when operating from battery.
 
Finally, If your C1 tantalum capacitor is what's defective, the display board alone plugged into your AC power adapter should cause that supply to go into protection mode - just as if it was installed in the scope base.
 
 
 
Update:
 
I see astrovoyeur linked a set of LX200 Classic schematics on http://www.lx200classic.com which include the power panel. I conclude that C1 is not required for operation if the "Blue" capacitor (C4?) is in place. I also see that my suggested test for C1 would require the front panel power switch to be ON and the fuse to be installed. Other schematics there should be of help for further troubleshooting. I believe these schematics to be fan generated, and not official Meade docs so caveat emptor.


Edited by RSX11M+, 14 February 2021 - 11:08 PM.



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