24 replies to this topic

[q192]

I have a 10" LX200 classic manufactured in 1994. I don't use it very often and when I last went to use it, it wouldn't power up.  The hand controller was non-responsive and the display was blank.  The only response was all of the LEDs in the ammeter (left side of the power panel) would flash rapidly, but very dim, and keep doing it until I switched the power back off.

 

I have access to a second identical unit that is 100% functional (but I think that one was manufactured a few years later).  I swapped everything external to the power panel.  This included the PSUs, cables and hand controllers and the problem remained unchanged.

 

The scope I am trying to troubleshoot has never had a capacitor failure and I had replaced the two highest risk tantalums proactively a few years ago.  I did't verify the reference designators when I had it open, but one was in the hand controller, and one was on the main power panel.  I believe it was C1 and C3.  And it has worked since I did that cap replacement.  When I did that proactive cap replacement, I got rid of the Meade 18VDC PSU and started using an IBM laptop PSU that is 16VDC.  This newer PSU was working fine with the scope prior to this incident.

 

I did a visual inspection of all the boards in this scope including the top side of the motherboard in the base and the hand controller and nothing is obviously blown.  There are no craters or burn marks!

 

But before I go any deeper into this, I thought I would ask this forum if anyone else has experience with a dead LX200 that flashes the ammeter LEDs dim and fast.

 

Has anyone had this experience?  Did you find what part was the cause of the problem?

 

Thanks!

[donniesoprano]

Curious to the troubleshooting steps here as well.....

 

ds

[John Rose]

I had a 12" LX200 fail on first power up after long storage.  I had already replaced the cap in the hand controller. I use SMD Tantalums mounted on the bottom side of the board. I do not remember what the LED display was doing. The cap on the main board looked OK. I had one of those blow up on an 8" LX200. I just crushed the remains to get access to the leads and soldered a new one to them. Saves the hassle of removing the main board. On the 12" I traced it to a capacitor on the azimuth/right ascension motor board. Looked OK but it had shorted.  The power supply I was using had overcurrent protection that prevented the fuse on the power panel from blowing. So it is possible that the LED display was acting like you describe. There are five capacitors to be concerned about in the classic LX200. Hand controller, power panel, main board and the two drive motor boards.  The one in the hand controller is the most important. I have seen one that went up in flames and destroyed the ribbon cable.   

[michael8554]

Knowing there is a capacitor issue with the Classic, why didn't you replace them all a few years ago  ?

 

As John found, a visual inspection is not enough.

 

Now you'll have to replace the rest, reassemble, test, hoping it was only the C's that were faulty.

[WaveGuide]

Hello q192,

There are some very useful videos for troubleshooting the LX200 Classic electronics on YouTube.

For example: https://www.youtube....troubleshooting

Ensure you check out the series posted by Clearline Technology Corp.

All you need is a digital multimeter and a little skill if/when soldering is involved.

If you detach the ribbon cable behind the power panel, it's vital you refit it correctly. The poor design dual multipin connector allows for several, ofter disastrous re-connection errors (been there, done that).

Detailed notes on every vulnerable capacitor are available on this and the other forums. You simply need to know where to look. Find the 'files' repository on the ioGroups forum if you can. However, YouTube covers almost everything that goes wrong with the LX200.

Cheers,

- Jack T

Edited by WaveGuide, 01 January 2024 - 06:43 AM.

[NearVision]

You may want to check the power plug and switch. With age they will often develop a layer of dirt or corrosion on the contacts that will cause problems like intermittent or low voltage that can look like what you are experiencing. A simple fix that sometimes works after sitting for a long time is to plug/unplug the power connector and rotate it several times. This wipes the contacts and will often clean them enough to work for a while. Same thing for the power switch. Turn it on and off several times to wipe the contacts. It's best to do this with the power supply/battery not connected so that you don't turn the power on and off rapidly. This is likely to cause spikes in the circuitry in the scope which is a bad thing.

 

I've also seen a few times where the fuse will go bad and do similar things. In this case a replacement fuse is the best solution.

 

If these don't work you may need to start checking where the power is going with a volt meter.

[DAVIDG]

 The flashing LEDs that are dim indicates to me a capacitor is shorting and it is forming a RC circuit. The cap charges to a certain level, then shorts so the charge goes to zero and the cycle starts again. 

   Does it happen with the hand controller unplugged if so then the problem is in the scope. There are a couple of hidden caps  that you need to take the bottom of the scope off to see them on the main board. 

 

                  -Dave 

[rferrante]

I would echo what John Rose said about the power supply. Nice power supplies have overcurrent protection that causes them to shut down until the current drawn goes down, then they switch on again which triggers the overcurrent protection again, etc. in a loop, which would look like blinking ammeter bars on the LX200. The LX200 uses a slow-blow fuse which takes a couple of seconds to blow, and by that time the power supply has already shutdown (temporarily) so the fuse might never blow.

 

The reason for the slow-blow fuse is that the problem the fuse was to solve was perceived to be a stalled motor (not a shorted capacitor), for instance caused by the telescope bumping into something. The motor uses most current at zero RPM, which of course it always sees for a second when it first starts up. So it needs high current for a short while, but if it saw high current for a longer while then something was stalling the motor and then the fuse should blow.

 

I'd suspect a short somewhere inside the scope.

[q192]

Thanks guys.  I'll check the power switch and connector and then start looking for shorted caps.  I already switched hand controllers and the problem remained so I'm pretty sure it is in the drive base somewhere.  I'll see if I can find it.

 

Thanks again and if anyone else has any other ideas, please let me know!

[q192]

Ok here is an update and I need some additional advice.

 

I had to put this project aside for awhile, but I circled back to it and got the capacitors replaced.  I pulled out the main board and replaced all 9 tantalum 6.8uF caps that were on it.  This included C1, C2, C3, C4, C5, C6, C7, C8, and C27.  Testing them outside the circuit, C8 was definitely shorted but had not burned up.  But I think this was definitely the problem preventing startup.  In retrospect, I wish I had read the recommendations again prior to doing this and just went after C8 and the other 4 caps that are on the incoming 18V rail.  I was trying to be proactive and since I had the thing out, I figured it was easier then than ever.  While I had things apart, I went ahead and replaced C1 and C3 on the power panel (I thought I had previously replaced it but I hadn't - C1 in the hand controller was replaced years ago).

 

So I put it back together and the good news is that the short is fixed.  It stays powered up.  It acts normally for a second or two then the RA motor starts humming loudly but steadily for a few seconds, changes, hums again, and keeps doing that until I shut it off.  It never actually moves.  The hand controller display shows "Meade" but never moves beyond that.

 

The replacement caps I used are aluminum electrolytic 6.8uF rated at 35C.  All my solder joints look good and I'm pretty sure I did not make a mistake with polarity on any of them.  But I'll check again on that.

 

I did save the old capacitors and marked where each one of them came from so in theory I could try to place them back on the board and in their original locations (except for C8 of course!).  However they are old, I did bend some of the leads taking them out, and replacing them was a real pain.  So I'd rather not just to see if it makes a difference.

 

Does anyone have any recommendations on what to do here?  Is this a circuit tuning issue with the comparators, or is it likely I damaged another component with heat or ESD when I was working on it?  Please let me know if you have run into this or if you have any recommendations.

 

Thanks!

[DAVIDG]

 I would double check that you have the polarity correct on the new caps since they are electrolytic  from your pictures and the stripe on them indicates the negative lead vs the original  tantalum cap were the stripe indicates a positive lead.  These are just filtering caps so the exact value shouldn't matter but you replaced them with the same values so I don't see that as a problem. 

  The humming motor is  a bit weird. When the scope boots it looks to see movement in the RA motor by looking at the signals from the encoders and turns the RA motor one in one direction then the other. Usually when it doesn't see movement it keeps ramping the speed until the scope is running away and not stay in one position. So it sounds like the main board is OK but it not seeing the signals from the encoder on the RA board  and the program stops that point.  So I would confirm that the motor is humming and not moving vs turning in one direction then the other  If it just humming you can unplug the encoder board on the motor assembly  and  the motor should just run when you boot it.  If not then it sounds like the power IC that controls the motor might have died which is not uncommon.  I would double check also that you have the main cable and the RA motor cable plugged in correctly and have not missed a pin. when plugging it in. I would also unplug the DEC motor since it should get passed the MEADE text on  the hand controller even with it not plugged in. 

 

                  - Dave 

[q192]

I visually verified that the polarity of all the caps is correct.  I reseated the two connectors that go into the mainboard and the connector that goes into the RA control board and there is no change.  I don't see a connector that plugs encoders into the RA board, but it is pretty tight in that spot.  I think everything to the motors and encoders is soldered down to that board but I could be wrong.

 

I see no missed pins or misaligned connectors in any of this.  

 

I'll try to troubleshoot further tomorrow.  The noise the motor is making is substantial.  When it does it, the ammeter on the power panel illuminates 9 of the 10 red LEDs in the bargraph.  Whatever it is doing is using some current to make that noise.  Is it possible it is trying to move the motor in both directions at the same time and it is chattering back and forth rapidly?  That might explain what it is doing.

 

Looking at the caps more closely, it looks like I may have damaged the lower edge of one of them with my soldering iron.  That is the cap that is next to U10 (CD74HCT14E) on the board.  I already have the cover back on it and I can't see the reference designator for it in my pictures, but I think that is C1.  If that is intermittently shorting it might be causing some problems so I may pull the main board back out tomorrow and replace it with a new one.

 

Any thoughts?

[DAVIDG]

 If replacing the possible damaged capacitor doesn't fix the problem I would unplug the RA motor assembly from the main board. If the system is still drawing high current  I would  suspect that the L2724 has gone bad since that controls the motor if it is an electronic issue.  If the current draw drop down with the RA assembly unplugged then the problem is in it.  It could be possible that the RA assembly is jammed and it is now mechanical problem vs an electronic issue.

 

              - Dave 

[NearVision]

Another possibility to check is the voltage regulators that drop the 18V down to 5V for the ICs in the logic circuits. There is one on the power board near the power jack and another in the handset. 7805 is the part number for both if I remember correctly. If a capacitor that shorted was on the output of either of these it could take it out also. These are 3 pin devices that look like a power transistor. One leg is the input (18V), one leg is ground along with the tab on top of it, and the third leg is output (5V). If you've got the voltage from your power supply at the input pin and significantly higher or lower than 5V on the output it's bad.

[q192]

Thanks for the suggestions!

 

I'm still battling the same issue above, but I have done and checked a few things.  I have temporary use of another almost identical LX200 that I can use for reference

 

I replaced the damaged C1 on the main board again.  I had burned the side of it with a soldering iron when I installed it, so I switched it out for another new capacitor.  No change, the motor still makes lots of noise and doesn't really move.  

 

DAVID G:

I unplugged the RA motor assembly and powered it up.  It still doesn't get past "Meade" on the HC, but the current draw is much less.  Two bars on current meter (200mA I believe).   This may indicate that the problem is in the RA motor assembly or the high current draw and motor noise is due to a failure on the main board causing the motor to fight itself (if it is plugged in and can try to move and therefore draw current).  I'm not 100% sure how to narrow it down to a main board issue or RA board issue.
 

Is there a way to easily test the L2724?  

 

As for a mechanical jam of the RA drive, that may be a real possibility here.  However, I don't see a way to manually turn the motor or worm gear to give it a boost, or release a bind.  Opening the RA clutch the fork spins completely free of the main RA gear.  When the motor starts up and makes the noise, the motor and the worm aren't really turning.  Is there a way to manually rotate the worm or the motor itself?  I don't see an easy way to do it.  The worm and main gear are well greased and clean.  My reference LX200 moves the worm gear both directions while booting up, but the subject LX200 does not visibly move the worm at all.

 

NearVision:

I checked two of these 7805 devices and they are both good.  I checked the U17 on the main board first because the shorted capacitor that caused the original failure was C8, the 18V side of the VR on the main board.  However, it's output is 5V so I think it is good.  I also checked VR1 on the power panel and it also showed 5V at the output.  I did not check the one in the hand controller.  I thought that one was low risk because I did not recently change any components in the hand controller and it seems to work ok.  The display works, the lights work.  The hand controller buttons don't appear to do anything right now, but the scope is not booting fully so I don't think that is a VR in the hand controller.

 

Does anyone have any other suggestions?  Is there a way to check the L2724?  Is there a way to manually turn the worm gear a little bit without disassembling motors and the drives?

 

Thanks!

[michael8554]

When first powered up, the RA worm should rotate until the Index mark on the RA worm shaft is detected.

 

If RA isn't rotating then the Initialising may stall, waiting for the detection.

 

You can easily remove the RA motor and worm assembly from the base, just two Allen headed bolts.

 

Run a black Sharpie around the heads, to show where to relocate the assembly.

[DAVIDG]

 Here are my thoughts based on the symptoms provided.  The fact that  the electronics are pulling a lot of current with the motor attached but not with the motor attached indicates an electronic  problem  and most likely with L2724 driver or the motor is jam so it can't rotate. If it could rotate and  the magnetic sensor could not  find the index mark it wouldn't draw current and make the noises it is. The fact the hand controller  stops booting and is stuck on MEADE is because the system isn't seeing any movement of the motor via the encoder signals. 

  You can find the schematics at http://www.lx200classic.com/

   Here is part of the schematic for the motor. Note it  says when the motor is disconnected  one should be able to measure around 19K Ohms between the pin 1 and pin 3 on the L2724.  If  it doesn't shows that value that would indicate the L2724 is bad. The other possibility is C9 is shorted since that would short the leads from the motor together but I don't think that is the issue since it would  pull current without the motor attached if C9 is on one the main board since it would always be in the circuit. If I'm wrong and C9 is on the motor control board then it could be the issue. 

 

 

 

 

 

One can also attach a volt meter to the motor leads on connector from the main board  with the motor disconnected and see what voltages are going to motor when the system tries to boot. One should see around +18 and then -18  going to the motor. 

 

 

Also with the motor disconnected from the main board one could use a 9 volt battery to the motor leads on the plug  to the motor control board  for the motor and the motor should run in both directions wen you reverse the polarity to the motor That would test if there is some mechanical issue.  Since the scope has been sitting for some times the grease may have hardened and stopping it from turning.

 Hope this is some value.

 

 

                     - Dave 

[rferrante]

Also you may want to check the IR devices at the gearbox visually for a broken lead. Check right at the base of the device where the leads go into the plastic.

[q192]

Good ideas.  I'll check the resistance across pins 1 and 3 of L2724 with the motors disconnected.  I assume this is while the main board is powered off, correct?

 

I'll as check for broken leads on the IR devices.  Although I suspect if that were the case, it would likely lead to runaway and it isn't doing that.

 

I like the idea of putting voltage across the motor directly to see if I can get it to move that way.  This may be a basic question, but are the motors running on +/-18vDC continuously while slewing?  So it is 100% safe to put 9vDC across them? Others have done this for troubleshooting and it works with no issues?

 

If I'm reading the schematic correctly this would be putting 9V across pins 1&2 on the connector (red & orange), correct?

 

I just want to be sure before I try it!  Thanks!

[DAVIDG]

Good ideas.  I'll check the resistance across pins 1 and 3 of L2724 with the motors disconnected.  I assume this is while the main board is powered off, correct?

 

I'll as check for broken leads on the IR devices.  Although I suspect if that were the case, it would likely lead to runaway and it isn't doing that.

 

I like the idea of putting voltage across the motor directly to see if I can get it to move that way.  This may be a basic question, but are the motors running on +/-18vDC continuously while slewing?  So it is 100% safe to put 9vDC across them? Others have done this for troubleshooting and it works with no issues?

 

If I'm reading the schematic correctly this would be putting 9V across pins 1&2 on the connector (red & orange), correct?

 

I just want to be sure before I try it!  Thanks!

 Yes you check the resistance across pin 1 and 3 of  the L2724 with the power off

 I agree that a defective encoder would results in a  run away condition so I don't think that the IR sensor or IR LED is the problem but it never hurts to check to be sure.

 The motors  are just  DC motors with a gear box and encoder assembly attached. The speed is set  using Pulse Width Modulation with the encoder feed back to make a servo type system. The original setup was to power the motor at 12 volts but users found that it didn't slew fast enough so Meade's fix was to just up the voltage to 18 volt to get it  to slew faster but without upgrading the tantalum caps from 16 volts max to ones with higher rating. Hence the problem with the caps burning up. So running the motor directly on 9 or 12 volts won't hurt it. It will just run at the fastest speed possible at that voltage which is what happens at the max slew rate.  All you need to see is  it move the scope in both directions for a few degrees to determine that nothing is jamming up. 

   On a side note last Spring the drive on  the 24" Tinsley Cass  of my groups observatory stop tracking. When I inspected the system I found the motor running but the  safety clutch was slipping  Something in the drive has jammed. When I took all the cover plates off to examine the complete drive train I discover a small stick jammed in the gears. We had birds trying to build a nest in the dome and their nesting material kept falling on the scope and some how that stick got into the drive.

   You said the scope had sat for awhile so who knows if some how something got into the drive or the grease dried out  in the gear box on the motor and is stopping it's movement.

 

                  - Dave 

[q192]

Thanks, Dave.  I'll give it a try and see if I can figure it out.  I don't think it is jammed.  Yes, it was not used for awhile but it was stored in a hardcase inside.  No wildlife had access to it.  Even while I have been working on it these past few weeks, I have closed it back up when I wasn't working on it.  The grease on the main gear and worm looks fine and it's wet.  Inside the tiny gearbox on the motor, I have no idea.  I can't see that.  

 

Good suggestions, though!  My next tasks:

 

1. Measure the resistance across pins 1 and 3 of U17 on the main board while unpowered and motor disconnected.  Try to verify 19K ohms.

2. Put 9V across the motor and see if it will turn.

3. Measure the voltage being sent to the motor upon startup.  Try to confirm +18 and -18vDC.

4. Visually inspect the connections to the IR LEDs and photodiodes and all other wires associated with the motor control board.

 

I very much appreciate the suggestions and feedback in this forum!  Thanks for all the help!

[q192]

And now for an update.

 

For the items above:

1. Resistance across pins 1 and 3 of U17 was somewhere between 15-20K ohms (I'm using an old analog meter since the batteries in my digital one leaked.  I need to replace it!).  I verified it is the same as the value across the same pins of U18.  So I don't think there is a problem with either of the motor driver ICs.

2. I put 9vDC across the RA motor and only a tiny click can be heard when applying the voltage and taking it off.  No rotation.  From this I concluded there is a problem in the motor/gearbox itself.  More on that below.

3. I didn't do this one based on the results of #2 above.

4. I visually inspected the connections and found nothing remarkable.

 

So based on the results of #2 I assumed that the either the motor has serious issues, or the gearbox was too sticky to move.  The worm and main gear grease is very wet and in good shape so I didn't think that could be an issue.  I tapped the motor and the gearbox many times while giving it voltage - nothing.  I tried turning the output shaft of the gearbox with a pair of needle nose pliers (with rubber between them and the shaft to not gall it up) - nothing.  I also racked the worm up and down to engage / disengage it from the main gear all while applying power off and on etc.  Nothing seemed to change it.  So I reconnected everything and the boost from 9v back up to 18v did the trick after I had essentially knocked things loose.  The RA motor now spins.  It was pretty jumpy at first so I guess it was working out the stickiness of the grease in the gearbox.  I actually hadn't run this scope in several years so that makes some sense.  The more I ran the RA motor back and forth, the smoother it seemed to run.  It does still occasionally lose its mind and I have to restart it, but my plan is to run it more and more in the coming days.

 

So for now it is somewhat functional but still isn't 100%.  It makes some strange motor noises sometimes on startup, and it occasionally goes crazy when you are slewing it at the slower speeds.  I'm not sure why it is doing that, but my plan is now get it back under the stars and see if the GOTO accuracy is still working well and if it will track smoothly across a few hours.  

 

Any recommendations on things I need to do if the motor speed jumps around occasionally for no reason?

 

And thanks again for all the help, guys.  I wouldn't have made it this far on this thing without this forum!

[DAVIDG]

 I have disassembled the gear boxes on LX200 GPS and the ones on the LX200 Classic are similar.  The LX200 gear box is pretty simple. There are a couple of plastic gears between the motor and the worm and also the metal encoder disk . I believe the encoder has set a screw to hold the  disk to the shaft   or might be a press fit and the disk may be loose   There are also a screw that holds the one of the gears onto the shaft that drive the worm. The fact that things were jammed, I would take a look inside the gear box to see what is going on.  The front plate is held on by a couple of screws so you take that off to see the gears and encoder disk. 

 

                     - Dave 

[WaveGuide]

- - - -   The front plate is held on by a couple of screws so you take that off to see the gears and encoder disk. 

 

              

In the Classic (and presumably the GPS) gearbox, there's also the encoder mask sandwitched in place between the gearbox case and the front plate. This mask is a very thin metal 'shim' with slots to 'chop' the IR beams going to the photodetectors. If this mask falls out or displaced, it's a heck of a job repositioning it. So, before slackening the front plate, I'd strongly recommend reading through the thesis by the late Bruce Johnstone: Repairing the most common problems on the LX200 Dec/RA drive.

A copy of this is kept in the MUPAG archive here: https://www.skymtn.c...stro/decfix.htm
 

Replacement encoder disk and mask are available if needed from Rob Ferrante at ClearlineTech. Corp. Here: https://www.clearlin...lx200-misc.html

 

- Jack T

[Skywatchr]

 I have disassembled the gear boxes on LX200 GPS and the ones on the LX200 Classic are similar.  The LX200 gear box is pretty simple. There are a couple of plastic gears between the motor and the worm and also the metal encoder disk . I believe the encoder has set a screw to hold the  disk to the shaft   or might be a press fit and the disk may be loose   There are also a screw that holds the one of the gears onto the shaft that drive the worm. The fact that things were jammed, I would take a look inside the gear box to see what is going on.  The front plate is held on by a couple of screws so you take that off to see the gears and encoder disk. 

 

                     - Dave 

The encoder hub on the Classic is press-fit and nearly impossible to get off without serious damage.  The encoder disc is glued on and can be removed quite easily.  Plus there is an encoder mask to align that is only held in place by being pinched between the gearbox cover.  The original encoder mask was longer to be able to easily align it, but then the tab gets cut off leaving only a tiny stub. Of course making it a bit more difficult to align a used mask.  If that encoder mask is misaligned, strange things can happen.