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A tale of two 10" Classics (DEC runaway on one/inoperable condition on the other...)

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#1 SteveRosenow

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Posted 29 June 2021 - 09:42 PM

Hi, all.

I spent a great bit of time on the phone this afternoon with Garry Ingle of meademods on the subject of my woes, and decided I'd try here as well to gain a double-forked approach to solving this dilemma I am having. The more minds, experience, and wisdom the merrier, right?

Here's the backstory...

A couple months back, I had been hired to fix an ailing Meade LX200 Classic whose tantalum capacitor on the power panel PCB went and turned into a thermine bomb. Fortunately, it didnt do any damage to any nearby components, so, I'd removed the 6uF 25V capacitor which was the culprit and replaced it with a 10uF 50V electrolytic. I did this to the main PCB and hand controller as well.

To ensure functionality, I'd swapped all of the hardware out of my own 10-inch LX200 with the hardware I had been hired to repair (a local guy hired me through a Facebook group, I don't offer those services here nor do I advertise them anywhere), and after a night of testing all worked out well...

 

...until it came time for him to reassemble his own scope. He had found that his was not working correctly after I had fixed it (I'd done this many times with great success, and this was the first failure for me), so after a week of discussing the issue I agreed to look at his entire scope instead of just the electronics bits... This was to iron out any issues, and to test it against my own identical scope just to see if there were issues not present at the first try.

On his end, his scope will not post. Upon power-up, the MEADE text appears in the hand controller display then a random string of characters appears, followed by inaction from both the RA and DEC drives. In short, the hand controller freezes at the MEADE display and that's it. Nothing. Nada... Well, except for a lot of heat from the heat sink in the base, but that's about it.

Compounding issues is that with his telescope, in addition to the original Meade controller, also came supplied with the Clearline Tech aftermarket hand controller. When that hand controller is used on his scope, a "COMM ERR" readout happens followed by the same inaction of the drive motors and inability to slew the scope. On a side note, Clearline is sending a replacement hand controller because it literally melted in the heat of an intense heatwave we experienced here in the Pacific Northwest - great customer service on their end!

On my end...

My 10" LX200, which I've had the last seven and a half years, is failing in its own right. It had been a couple months since my scope had last been powered on. The last time it had been powered, it worked perfectly and all functions were present. The only two times I have ever had an electronics failure of this scope was when I had reversed polarity of the power cord at a star party in Goldendale back in 2015, and when I'd popped the power panel fuse in it when I was attempting to fix the blown capacitor from that previous issue. 

At issue, is that on my scope, I have immediate DEC motor runaway whenever the power switch is turned on. All connections look to be okay at both RA and DEC motor drive boards, and both drive boards seem to be somewhat pristine. The main PCB in the base looks to be perfectly okay as well. The capacitors on my scope (both hand controller and base) were replaced with 10uF 50V electrolytics years ago). 

I cannot recall ever hot-plugging anything and my hand controller is minty inside. 

What could be causing these issues?

Mind you, I don't have access to an oscilloscope nor do I have acce$$ (<-- see what I did there?) to fancy calibration equipment. 


Edited by SteveRosenow, 29 June 2021 - 09:44 PM.


#2 John Rogers

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Posted 29 June 2021 - 11:14 PM

I had a similar problem with the Dec runaway.  This calibration tool fixed the problem quite nicely and was easy to use:  https://clearline-te...ation-tool.html

 

As far as your friends scope, clearline-tech has replacement front panels in addition to the hand controller.  One of the versions is designed to boot without the hand controller in place, for folks who want to control the telescope with a computer or blue-tooth device.



#3 SteveRosenow

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Posted 30 June 2021 - 02:45 PM

I had a similar problem with the Dec runaway.  This calibration tool fixed the problem quite nicely and was easy to use:  https://clearline-te...ation-tool.html

 

As far as your friends scope, clearline-tech has replacement front panels in addition to the hand controller.  One of the versions is designed to boot without the hand controller in place, for folks who want to control the telescope with a computer or blue-tooth device.

Thank you! I have been reading into the Clearline products for quite some time. 

If the CTC boards don't solve the problem, then what would be the means of troubleshooting these issues and what could be the possible causes?



#4 John Rogers

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Posted 30 June 2021 - 03:17 PM

Thank you! I have been reading into the Clearline products for quite some time. 

If the CTC boards don't solve the problem, then what would be the means of troubleshooting these issues and what could be the possible causes?

Unless you can find someone who likes to tinker with these electronics as a hobby, then it might be more cost-effective to pick a likely component and replace it.  There is always the option of upgrading the classic electronics to the newer Audiostar configuration from George Dudash, if he is still offering the kits.  I did this some years ago and have not looked back.  



#5 MikeBY

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Posted 01 July 2021 - 03:05 AM

You best check (and change) the caps on both the DEC and RA  motor/sensor boards. The runaway condition is likely a failure on the DEC sensor board. There's a 7805 5 volt regulator on these tiny boards (looks like a transistor) that powers the IR LEDS and optical sensors and IC.  The other common problem are broken connections at the LEDs or sensors.  The wires at the LEDs and sensors is very fragile.  

Those two caps, like the ones on the front panel, main board and front interface board are little bombs waiting to go off. These caps are really tough to change out because the double sided traces are very thin. If you break a trace you may end up with no 5 volts on the board and DEC (or RA) runaway is the result. The boards are the same, but the wiring to them is going to look different. The one on the RA drive has extra wires for the PEC hall effect sensor. Also the wiring harness will have different empty positions. Also check the wires on the connections. They are crimped, not soldered, and a non-gentle hand will cause loose connections galore.  Consider soldering.  One suggestion was to break the tantelum caps with pliers and then solder the new cap to the old legs.  That's actually a good idea, but no matter how you do it, these are tricky to do.

 

There are common problem when re-assembling. Connectors off by a pin or wires loose on the connector inserts as mentioned above. 

 

 

I remember there is a resistor that is mounted vertically on the DEC/RA motor/sensor boards, (R3) and the end that is UP is connected to the +5 volt rail on the board.  Meade uses a differential drive circuit so the voltage readings are not quite what you might expect.feeding the motors   No fancy O-scope (hey those tiny Chinese digital guys are dirt cheap these days) needed.  A multimeter that reads true RMS voltage is sufficient for these classic scopes. (Clever troubleshooting techniques for adjusting the sensors and offsets by Bruce Johnston and all the detail schematics are on the LX200 classic site http://www.lx200classic.com/

 

I just went through a LX200 classic 12" myself. (cheap promotion - it's in the classifieds).
Also suspect is those RJ11 connectors and the cables.  Common failures that cause noise and problems between the handset and the scope.

 

Hope this helps you figure this one out.   PM if you need more.

 

Michael

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Edited by MikeBY, 01 July 2021 - 03:33 AM.

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#6 MikeBY

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Posted 01 July 2021 - 03:21 AM

Oh.. and while I was in the one I just did, I fixed the heatsink on the main board that attaches to the bottom plate of the scope. The way it was done (afterthought clearly) by Meade, it doesn't

sit flat because of the package lead that attaches to the heat sink.  I removed the heatsink, and filed down the edge where the packages attach so that the result is completely flat. 
This about tripled the contact with the bottom plate of the scope. The one pic was as it came from the factory. A bit of heat compound barely around the screws. (the panel is unpainted so it's a bit difficult to see that there is only compound around a small part of it). The second pic is after the mod.  

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Edited by MikeBY, 01 July 2021 - 03:28 AM.

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#7 SteveRosenow

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Posted 02 July 2021 - 05:35 PM

UPDATE:

After replacing both CR2032 batteries (one each in mine and in his) I managed to get the runaway to stop in mine. It now no longer runs away but now just sits at the Meade logo and doesn't go past that.

I feel like I am getting closer.



#8 John Rogers

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Posted 02 July 2021 - 06:01 PM

Interesting.  I am fairly certain that the battery is only used to maintain the internal clock.  I suspect you have a loose or intermittent connection somewhere.



#9 SteveRosenow

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Posted 02 July 2021 - 06:12 PM

You best check (and change) the caps on both the DEC and RA  motor/sensor boards. The runaway condition is likely a failure on the DEC sensor board. There's a 7805 5 volt regulator on these tiny boards (looks like a transistor) that powers the IR LEDS and optical sensors and IC.  The other common problem are broken connections at the LEDs or sensors.  The wires at the LEDs and sensors is very fragile.  

Those two caps, like the ones on the front panel, main board and front interface board are little bombs waiting to go off. These caps are really tough to change out because the double sided traces are very thin. If you break a trace you may end up with no 5 volts on the board and DEC (or RA) runaway is the result. The boards are the same, but the wiring to them is going to look different. The one on the RA drive has extra wires for the PEC hall effect sensor. Also the wiring harness will have different empty positions. Also check the wires on the connections. They are crimped, not soldered, and a non-gentle hand will cause loose connections galore.  Consider soldering.  One suggestion was to break the tantelum caps with pliers and then solder the new cap to the old legs.  That's actually a good idea, but no matter how you do it, these are tricky to do.

 

There are common problem when re-assembling. Connectors off by a pin or wires loose on the connector inserts as mentioned above. 

 

 

I remember there is a resistor that is mounted vertically on the DEC/RA motor/sensor boards, (R3) and the end that is UP is connected to the +5 volt rail on the board.  Meade uses a differential drive circuit so the voltage readings are not quite what you might expect.feeding the motors   No fancy O-scope (hey those tiny Chinese digital guys are dirt cheap these days) needed.  A multimeter that reads true RMS voltage is sufficient for these classic scopes. (Clever troubleshooting techniques for adjusting the sensors and offsets by Bruce Johnston and all the detail schematics are on the LX200 classic site http://www.lx200classic.com/

 

I just went through a LX200 classic 12" myself. (cheap promotion - it's in the classifieds).
Also suspect is those RJ11 connectors and the cables.  Common failures that cause noise and problems between the handset and the scope.

 

Hope this helps you figure this one out.   PM if you need more.

 

Michael


At your suggestion I checked the voltage of that one resistor's upward-facing lead. On the RA motor board I get between 5.1 and 5.5 volts.

On the DEC board I get 0.5

Where could the failure be?



#10 jaymcgil19

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Posted 04 July 2021 - 07:11 PM

OK Steve, first thing to do is find out why you can't get past the Meade prompt. First unplug the dec motor coiled cord, the dec motor isn't required to get to the alignment window. Now when you turn the scope on, tell me if the ra motor moves very slowly in one direction then reverse and then stops.

 

John M



#11 SteveRosenow

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Posted 05 July 2021 - 08:10 PM

OK Steve, first thing to do is find out why you can't get past the Meade prompt. First unplug the dec motor coiled cord, the dec motor isn't required to get to the alignment window. Now when you turn the scope on, tell me if the ra motor moves very slowly in one direction then reverse and then stops.

 

John M

On my scope, there is no movement past the MEADE prompt, and no movement in the RA drive while trying this.

On the other LX200 Classic, there is no movement past the MEADE prompt and no movement in the RA drive, and intermittently there are random black rectangles and random characters that go past the MEADE prompt on the display.


Edited by SteveRosenow, 05 July 2021 - 08:11 PM.


#12 MikeBY

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Posted 16 July 2021 - 01:44 AM

At your suggestion I checked the voltage of that one resistor's upward-facing lead. On the RA motor board I get between 5.1 and 5.5 volts.

On the DEC board I get 0.5

Where could the failure be?

Either there is no +18 volt at the board connector due to a loose wire or a problem with DEC cable, RJ45 connectors or cable extension coming up the fork arm. If you have 18v measured to ground at the connector on the DEC board then either the 7805 has failed or the trace on the component side of the board is damaged. Unless you've been replacing the tantalum capacitors which can damage traces the most likely issue is loose connections as the wires are simply pressed on to the connectors or the RJ45 connector are not making good contact.  I do voltage checks using ground at the board edge connector. The 5 volt supply points on each board should be tightly regulated as each board has it's own 7805 regulator. I'd be concerned with a 5.5 volt reading to the local ground with a 7805.
Connectivity from the DEC fork arm back to the base of the telescope can be checked  with power disconnected from the scope using an ohmmeter. Wiring "reverses" through the DEC connector in the fork arm. Check the schematics.

If you don't have 5 volts on the DEC board you will get runaway on the DEC motor. 




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