Another LX200 Classic 8" Declination Runaway

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KG4ONJ
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Another LX200 Classic 8" Declination Runaway

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Post by KG4ONJ »

Hello all.. I'm coming here from AstronomyForum.net and just saw that it's pretty much inactive. I have owned my 8" LX200 Classic for many years and have gotten so many tips from just searching around however this time I'm not sure where to go..

I pulled my LX200 out of the closet after about a year and when I set it up found that it had a few issues. The fuse on the main control panel was blowing whenever I attached the DEC cable so I decided to go ahead and replace all the recommended capacitors in the main board, the control panel, the RA & DEC drive boards and the handset. All went well without incident however I found that the capacitor on the DEC board had blown. Other than a slight burn on the plastic flat connector there didn't appear to be any obvious damage.

I began powering things up - starting with the control panel by itself, which was fine. Plugged in the control panel to the flat cable, powered up, all fine. Added the keypad, powered up, keypad initialized, RA motor whirred a bit, got to the menu and all was fine (even added a brighter map light LED!). Finally I plugged in the DEC motor cable and powered up:

The DEC motor IMMEDIATELY started moving the scope down as soon as I flipped the power switch.

I checked all the connections - both ends of the DEC cable, the flat DEC cable running from the control board to the opposite side of the coupler, everything good. Powered up again, same thing. Tried unplugging the DEC cable while power was applied - that stopped the motion but as soon as I plugged it back in the motor started at full slew downward again.

I've just done some cursory checks at this point, looking for broken / loose wires, bad solder points, etc. Nothing yet. I disconnected the plastic flat connector on the DEC control board and powered up again with the DEC coiled cable attached - same thing, runaway downward.

The only thing different I've noticed is that sometimes it will go the other direction - "up", but 9 times out of 10 it goes down.

I've spent 2 days reading about allllllllllllllll the zillion possibilities it could be. I thought I would try here first to see if anyone had a "magic bullet" they could send my way. I think I feel more bewildered now..

Thanks in advance!
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Re: Another LX200 Classic 8" Declination Runaway

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Post by Graeme1858 »

Hello KG4ONJ

I'm sure an LX200 user will be along soon!

Welcome to the forum.

Regards

Graeme
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Re: Another LX200 Classic 8" Declination Runaway

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Post by 515bonner »

I owned several of the classic 10" SCT over the years. Have you checked the hand controller for any electronic issues?
It seems that years ago, another person I knew had a classic 8" that was having problems with runaway dec issues and he
replaced the hand controller as a last resort and that seemed to take care of the issue. (this was probably 20 yrs ago).
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Re: Another LX200 Classic 8" Declination Runaway

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Post by KG4ONJ »

515bonner wrote: Mon May 04, 2020 3:01 pm I owned several of the classic 10" SCT over the years. Have you checked the hand controller for any electronic issues?
It seems that years ago, another person I knew had a classic 8" that was having problems with runaway dec issues and he
replaced the hand controller as a last resort and that seemed to take care of the issue. (this was probably 20 yrs ago).
Thanks for the reply! I have checked the controller - all seems to be working properly with it. I also have an "extra" brand new controller that I've checked it with, same results. It's actually happening regardless of whether the controller is plugged in or not. As long as I have the DEC cable plugged in from the control panel to the drive the DEC motor turns on as soon as I turn on the switch and apply power.

I'm thinking it has to be something fairly obvious. A power issue. Maybe something stuck "on" or something shorted. Hard to say.
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Re: Another LX200 Classic 8" Declination Runaway

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Post by Lady Fraktor »

A possibility? Hopefully you can get it fixed soon.
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Re: Another LX200 Classic 8" Declination Runaway

#6

Post by Sky »

The "Mapug Astronomy Topical Archive" has plenty of information on troubleshooting DEC runaway problems for the "Classic" LX200 scopes. Scroll down to the bottom of the page:

http://www.skymtn.com/mapug-astronomy/M ... hvList.htm

Good Luck getting your scope fixed ....
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Re: Another LX200 Classic 8" Declination Runaway

#7

Post by robf »

KG,

On the LX200, the main board will send power to the motor, and then check how much the motor has turned (and how fast it is going) by listening to two signals coming back from the encoder PCB to the main board. If these two signals are interrupted, the main board will continue to drive the motor, infinitely. So connection problems are a possible culprit, and there is a connector inside of the fork arm that should be examined and reseated.

Another possibility is that during the replacement of the capacitor on the DEC board, something was damaged. The adjustment potentiometers are old and one may have died. That exact thing happened on my DEC board, so it is possible.
--Rob
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