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Unhappy Meade 10" LX200-GPS owner

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#1 mannimmond

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Posted 16 September 2021 - 10:50 PM

Hi Folks,

 

after cleaning and re-greasing the RA drive of my Meade 10" LX200GPS, I put everything together carefully, but apparently not careful enough. When switching on, the red power LED wouldn't come on. Suspecting the power supply I replaced it with the motorcycle battery that I use for outdoors and produced a small smoke signal from the front panel electronics. In the aftermath troubleshooting I found a trace on the board that had been fried, but everything else  seemed to be ok when comparing continuity tests with the circuit diagrams I have, so I soldered in a small wire, replacing the burned up trace. Switching on, the scope looked like it worked again.

 

This was when the bottom cover was still missing. Then I put the cables back into their places and fastened the electronics boards in steps, testing in between whether it still worked. Eventually I closed the bottom plate and suddenly the mount started to behave erratically when switching on, after it had worked at every step. It looked like runaway motors in RA and DEC as described for the classic LX200, although this is a GPS. Removing the bottom plate and going back the steps didn't bring back the normal behavior. I could never get it to work again with this main panel.

 

However, swapping the panel out for a good working one, a friend was so kind to let me try, brought the scope back to life. So I know the main panel has a problem but I can't figure out what might be broken. We even updated the firmware on the bad board which worked fine. We just disconnected the board from the motors and it operates ok. All menus are there but switching it on with the motors connected, it first starts to move in DEC and then after a few seconds in RA as well. It doesn't recognize the end points and I have to open the clutches to prevent the motors to stall and smoke up. When doing this a little longer, the motors eventually stop and go into the opposite direction, so it seems the software is trying to do something but isn't quite getting the feedback it expects.

 

In any case. I have two obvious questions to this community: 1) Does anybody have an idea what could be broken that still can be fixed without sending the unit to Meade and 2) Does anybody have an unemployed LX200GPS power panel lying around that I could obtain for a reasonable price?

 

All the best,

Bernhard


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#2 Delta608

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Posted 17 September 2021 - 10:59 AM

Well, as far as I know, Meade isn't Meade anymore...It has been absorbed by Orion after loosing a lawsuit  to Orion...They still have a website, which is confusing (to me anyway, as Orion has the Meade stuff on their website.) Call Orion and ask for a power panel...Good Luck ...or even Meade if their phones still work from their website...(Maybe they will just give ya one..)  800-626-3233


Edited by Delta608, 17 September 2021 - 11:00 AM.

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#3 NearVision

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Posted 17 September 2021 - 06:00 PM

If a trace was fried on the circuit board there's a good chance that something else got fried also. There are several discussions here on CN about these scopes have the tantalum capacitor go bad. From your description it sounds like you might have a bad cap and the power isn't being filtered enough causing the logic circuits to get confused by the random noise on the power being interpreted as signals telling the scope to move. Take a close look at the board for any components that might look bad. This includes looking at the other boards in the scope. When you worked on the RA & Dec assemblies you may have accidentally reconnected the plug wrong; backwards or offset by 1 pin.

 

I don't know your comfort and experience level with soldering but there is a good amount of how-to's and schematics on these scopes on the web.

 

Can you post some pictures on both sides of the board? Someone here may spot something.


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#4 mannimmond

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Posted 17 September 2021 - 07:10 PM

If a trace was fried on the circuit board there's a good chance that something else got fried also. There are several discussions here on CN about these scopes have the tantalum capacitor go bad. From your description it sounds like you might have a bad cap and the power isn't being filtered enough causing the logic circuits to get confused by the random noise on the power being interpreted as signals telling the scope to move. Take a close look at the board for any components that might look bad. This includes looking at the other boards in the scope. When you worked on the RA & Dec assemblies you may have accidentally reconnected the plug wrong; backwards or offset by 1 pin.

 

I don't know your comfort and experience level with soldering but there is a good amount of how-to's and schematics on these scopes on the web.

 

Can you post some pictures on both sides of the board? Someone here may spot something.

Thanks, I hadn't thought about such noise yet. I am adding some pictures of the board and my repair. Unfortunately the fried trace is beneath the plug for the RA (base) motor board. I am still a bit suspicious of the transistor Q50 that switches the 12V for the left fork sensor board. I am happy soldering electronics, however semiconductors on here are pretty tiny and will require some special concentration.
 

Attached Thumbnails

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#5 mannimmond

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Posted 17 September 2021 - 07:12 PM

Here are pictures of my fix. After that I couldn't find anything wrong when testing with an Ohmmeter.

Attached Thumbnails

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#6 MikeBY

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Posted 17 September 2021 - 07:21 PM

The tantalum cap issue is for the Classic scopes.

This is a LX200 GPS. 



#7 OzAndrewJ

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Posted 17 September 2021 - 07:29 PM

Gday Bernhard

That pin is the +12V line going to the RA board

On the basis the current RA card works with the new board,

you can probably discount a short in the motor card/motor,

but the fact the damaged board works without motor cards attached is odd.

It sort of indicates possible damage to the PIC

or possibly something else on the +12V bus has been damaged ???

so maybe use the DMM to probe all points on the +12V line to see if they have a solid connection.

Otherwise, you need to put a logic analyser on the motor card headers to see what is getting sent.

You can get cheap knockoffs of the salae 8ch analyser for about $10 these days

and i have some software that can convert the data into I2C comms.

 

Andrew Johansen Melbourne Australia



#8 MikeBY

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Posted 17 September 2021 - 09:51 PM

G'day Andrew.

I'm going to jump in here real quick because I tested the scope with Bernhard.
It works perfectly with my LX200GPS main board running old firmware 1.7k
His board comes up as LX200 GPS SMT version 4.51 and it's acting like it's not getting encoder data from either motor as they will run (and sometimes reverse) but eventually hit

hard stops or I'll get a motor stall error pop up.
There's some strange verbiage about this can happen if a handbox is swapped between mounts. 

So I'm wondering if somehow the problem is firmware on the main board or some other strange error with motor calibration that's gotten out of wack. 

 

Thanks in advance,

 

Michael.


Edited by MikeBY, 17 September 2021 - 09:55 PM.


#9 OzAndrewJ

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Posted 17 September 2021 - 10:37 PM

Gday Mike

His board comes up as 4.51 SMT

No idea what that is?????

What does the menu show ( setup > Statistics )

 

it's acting like it's not getting encoder data

That is all handled in a closed loop in the motor card.

The main CPU tells the PIC on the main board to tell the relevant motor card to move at a set speed.

After that, the motor card does everything by itself.

Every now and then the main board asks for position data, and if it doesnt get any over a set period,

it declares a "Motor Unit Fault"

ie if the motor card constantly works "properly" with your Mainboard,

then it indicates the encoder feedback loop is working.

 

The handbox has nothing to do with any of this, its a dumb button box.

 

So I'm wondering if somehow the problem is firmware on the main board

Thats what i am wondering as well, but the only way to tell is to see what comms are going to the motor card.

ie for a normal slew in say DEC, you would see commands like

a)  send x01 send xHHLLhhll    ie start moving at speed xHHLLhhll

b) send x08 rcv HHLLhhllPPcc ie read encoder offset = xHHLLhhll, PWM = xPP, Sensor events = xcc

    If xCC <> #x00 then the mainboard PIC will send a series of requests for relevant data

    This is normally the saved position data when a PEC or Home sensor changed state

c) send x05                               ie stop

 

That is all that gets sent to the motor card, and it does the rest

You need to log the comms over a period of time to see if/when it loses it and then

see if it shows in the plots??

 

Andrew Johansen Melbourne Australia


Edited by OzAndrewJ, 17 September 2021 - 10:46 PM.


#10 MikeBY

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Posted 18 September 2021 - 12:09 AM

So, my LX200GPS has a very old firmware on it.

When it boots the 2nd screen on the hbx (after the copyright) comes up as " LX200 GPS"  and "Version 1.7k"

Statistics shows

Version 1.7k 

171.8KB Free

and Serial# 006182P0322016 

 

When I boot his

The 2nd screen comes up as "LX200 GPS SMT" and "Version 4.21"

Statistics shows

LX200 GPS SMT

Statistics shows

Version 4.21

454.8KB Free

Serial # 092103R1320001

 

Physically the boards appear identical,

The repair on the burned trace is straightforward and checks out ok.

His has SMT menu entries in setup, mine does not. 

We know that motor cards and everything else is working just fine with my main board.

With his.there's obviously a comms issue from what I'm reading of your last reply. 

I checked pins 2 and 3 of both connectors to U11 with a DMM. They look the same on both boards.



#11 OzAndrewJ

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Posted 18 September 2021 - 12:26 AM

Gday Michael

and "Version 4.21"

OK, 4.2l is the last firmware issued by Meade to try and fix the GPS rollover problems

but several users have mentioned having problems with it during aligns etc

( I have not heard of runaway problems. )

As long as the repaired main board starts and you can connect,

i suggest using StarPatch to load patched 4.2g firmware as a first step, and see what happens.

 

His has SMT menu entries in setup, mine does not.

No probs. That is just a new chunk of firmware that came in later in the version history. Perfectly normal

( but dont use it as it is buggy )

 

I checked pins 2 and 3 of both connectors to U11 with a DMM. They look the same on both boards.

You cant use a multimeter, you need to capture what happens on the clock and data lines during operation

to see if they contain glitches.

 

Andrew Johansen Melbourne Australia



#12 MikeBY

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Posted 18 September 2021 - 12:53 AM

Ok,

Well, I'll roll it back and give it a try 

It was on g and we went forward to I using ASU to see if it would clear things up.

 

The pin check was just a sanity check on that connector because of it's proximity to the burned trace. 

 

Thanks for all your help.

 

M



#13 OzAndrewJ

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Posted 18 September 2021 - 01:13 AM

Gday Mike

 

The pin check was just a sanity check

OK, but essentially, U11 has direct connections to the RA and DEC headers for clock and data.

The pin order on each header is +12V, data, clock, gnd,gnd,gnd

The data lines should be tied to each other and the clocks should be isolated.

After that check is done ( ie neither the clocks or data have partial shorts to +12V or ground or each other )

all you can do is log what happens during operation.

 

Andrew Johansen Melbourne Australia



#14 MikeBY

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Posted 18 September 2021 - 05:53 AM

Gday Mike

 

OK, but essentially, U11 has direct connections to the RA and DEC headers for clock and data.

The pin order on each header is +12V, data, clock, gnd,gnd,gnd

The data lines should be tied to each other and the clocks should be isolated.

After that check is done ( ie neither the clocks or data have partial shorts to +12V or ground or each other )

all you can do is log what happens during operation.

 

Andrew Johansen Melbourne Australia

Yes, that is what I found.

I rolled back the board to 4.2G to no effect.

You mentioned software that might allow us to monitor or log the comms?

I'm going to recheck the voltages on the board, perhaps there's noise somewhere now that's interfering with the comms.

 

thanks again

 

M



#15 MikeBY

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Posted 18 September 2021 - 06:08 AM

On the DEC header, on pin 3 I see a slow cycle of 5v then back to 0

on the RA header pin 3 I don't see this..

this is with nothing connected except the hbx.

 

Don't have a scope or data analyzer available so will stop here for the moment. 

Pin 2 reads 4,88 vdc,, I'm thinking this is some pwm data or clock.

 

5 volt power is good. 

 

regards,

 

m



#16 OzAndrewJ

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Posted 18 September 2021 - 04:07 PM

Gday Michael

On the DEC header, on pin 3 I see a slow cycle of 5v then back to 0

OK pin 3 on each header is a clock line so "pseudo PWM" when its clocking

Pin 2 is data, and should be identical on both headers.

 

You mentioned software that might allow us to monitor or log the comms?

Hardware + software

You need a salae 24MHz 8ch "clone" eg

https://www.openimpu...logic-analyzer/

https://www.sparkfun.../products/15033

There are dozens of suppliers for these now and prices range from $6 to $40

Ebay etc is littered with them.

To drive it, you use the freeware "PulseView" software from "SigRok"

https://sigrok.org/

This will show you what is actually on the logic lines for the clock and data.

You can log an hour or so continuosly if reqd to trap errors.

 

Then, if you want to convert the data, i have a private app i wrote that can import the sigrok files

and decode them tosuit the type of Meade scope being tested.

Andrew Johansen Melbourne Australia

What a "normal" trace should look like

LX200 Clock.jpg

 

 

 

 

 

 

 

 



#17 MikeBY

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Posted 21 September 2021 - 10:42 AM

So in re-thinking the initial symptoms and with all the troubleshooting have to recall that we started with an intermittent problem. 

Bernhard originally was having intermittent tracking problems in EQ mode, essentially intermittent RA tracking issues. 
So, I reviewed looked at the board again and lifted the plastic connector guides up to find cold solder joints underneath the RA motor board

connector. This could have been the source of these problems to begin with and only got worse with the handling of the connectors. 
So I cleaned that up and gave things another go. 
1st with both DEC and RA connected I got motor stall, but no motion.  That was different!

Disconnected the RA drive and using 'Mode' key to back out of alignment steps that required RA movement got through the process verifying proper DEC movement and left arm sensors including GPS and the 12V switching on the left arm controlled by Q50 which was near the damaged 12V trace.
So all that's left to test is the RA motor function. 

I reconnected the RA motor controller, and tried again, but this time I had a worsening condition - the Power LED did not light. Power on the handset but nothing on the display.  I checked the connector and it was NOT misplaced. 12V and 5V were OK.  I did note that the Power LED is controlled by RA0  on pin 2 of the PIC at U11 and the clock for the RA motor control is RA1 -pin 3.. In discussing with Bernhard, he said he'd run into that also once before. With no display, it seems like there may be a PIC failure going on here. I'll go over the board again because of the possibility of something unseen causing these issues like the cold solder joints just found.and give it a good cleaning as well. I did re-verify that my panel still works normally in this mount so the problem is definitely with this board.

 

I'd like to emphasize that @Mannimmond (Bernhard) posted this thread because he is looking to find a replacement LX200GPS main board.

So for anyone reading this thread that has a used LX200GPS mainboard they would be willing to sell please refer to the original post at the top of this thread. 
 

I've been working on this as we are good friends and this part is pricey and hard to get.  I have an LX200GPS that has seen very little use and I've had time (retired) and experience with Meade scopes to help him troubleshoot this issue. I appreciate all the support and guidance  Andrew,  If you've got any ideas I'm always open. 
B. has experience with data analyzers so if we get back to that it'll be up to him.  I don't know about for this PIC if it the programming can be copied in circuit off a good card and then a new device programmed. Looking at the spec sheet it's a maybe. It's beyond my skillset, but maybe not beyond B's It would be an interesting experiment I suppose, but probably last ditch if we can't find a replacement.  

 



#18 OzAndrewJ

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Posted 21 September 2021 - 04:41 PM

Gday Michael

I have no more ideas until the data gets probed.

Ihave one board here that has a runaway problem and i managed to get a logic plot of it once only.

That showed that the I2C comms had gone awry, but i dont know why.

The PIC is a protected burnt device so AFAIK, no one has ever found a way to burn a new one.

 

Andrew Johansen Melbourne Australia



#19 mannimmond

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Posted 22 September 2021 - 04:10 PM

Hi Michael and Andrew,

 

let me thank you guys for all the brain grease you put into this. I have been following the discussion with a huge amount of (self) interest but hope that this gives others who fight with technical issues like these may also benefit, at least for their understanding how this scope electronics works.

 

Andrew mentions that the PIC is protected, so this puts a damper on any ideas to read the programming out and use it to program a new chip. The only way would be that MEADE makes this code available. There may be an argument for doing this once they are not interested anymore in keeping personnel for maintenance and once the boards aren't manufactured anymore. Of course with a lot of time and logic analysis one could probably reverse engineer the programming, but that would take not only a specialist but a real enthusiast as well.

 

I am not entirely convinced yet that the chip is the problem though, as I have seen bad soldering points too often in the discussions above. I wouldn't rule out noise on the comm lines either until I can check that with a scope again.

 

Anyway, all these are just hopeful musings on my side. The chances of me just getting a new front panel are now around the 95% mark I recon ;-).

 

Cheers,

Bernhard



#20 MikeBY

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Posted 23 September 2021 - 05:35 AM

Hi Michael and Andrew,

 

let me thank you guys for all the brain grease you put into this. I have been following the discussion with a huge amount of (self) interest but hope that this gives others who fight with technical issues like these may also benefit, at least for their understanding how this scope electronics works.

 

Andrew mentions that the PIC is protected, so this puts a damper on any ideas to read the programming out and use it to program a new chip. The only way would be that MEADE makes this code available. There may be an argument for doing this once they are not interested anymore in keeping personnel for maintenance and once the boards aren't manufactured anymore. Of course with a lot of time and logic analysis one could probably reverse engineer the programming, but that would take not only a specialist but a real enthusiast as well.

 

I am not entirely convinced yet that the chip is the problem though, as I have seen bad soldering points too often in the discussions above. I wouldn't rule out noise on the comm lines either until I can check that with a scope again.

 

Anyway, all these are just hopeful musings on my side. The chances of me just getting a new front panel are now around the 95% mark I recon ;-).

 

Cheers,

Bernhard

I took a quick read of the datasheet for this device and it supports In Circuit Serial Programming.
 

10.16 In-Circuit Serial Programming™

PIC16CXXX microcontrollers can be serially pro-grammed while in the end application circuit. This is simply done with two lines for clock and data, and three more lines for power, ground and the programming volt-age. This allows customers to manufacture boards with unprogrammed devices, and then program the micro-controller just before shipping the product. This also allows the most recent firmware or a custom firmware to be programmed.For complete details of serial programming, please refer to the In-Circuit Serial Programming (ICSP™)Guide, DS30277.

 

Since we are doing a repair function and perhaps a backup function on a known good board, there shouldn't be an IP issue with this.
I find it rather strange by today's standards that I/O ports were connected board to board without so much as a resistor to protect the PIC from spikes or noise.
It does not surprise me that the PIC port would be easily damaged.  
Keep in mind this gear is over 20 years old and I don't recall any license restrictions when I purchased my scope from Meade.

This might be worth investigating further especially since Orion can't manage to ship any Meade spares for at least 6 months.

 

Here's a link to the datasheet 
https://www.mouser.c...008C-278076.pdf

 

and to the mentioned programming guide
https://ww1.microchi...eDoc/30277d.pdf


Edited by MikeBY, 23 September 2021 - 05:39 AM.


#21 OzAndrewJ

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Posted 23 September 2021 - 05:59 AM

Gday Michael

By all means investigate further.

I have spent a moderate amount of time playing with Microchip PICS but am no expert there.

Lots of people over the years have tried and not got past the fact that the chips have the protection fuses burnt.

One person i have corresponded with recons you can acid etch the cover off the PIC

and use an electron microscope to manually read out the bits in the flash memory

but thats way beyond my paygrade :-)

 

Andrew Johansen Melbourne Australia



#22 MikeBY

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Posted 23 September 2021 - 02:40 PM

OIC.... well that does sound way above and beyond.  I was hoping it would be possible  to dump the program data and configuration area.

and that would suffice.  Protection fuses?  hmmm.   I guess I'll need to read the datasheet in detail.

 

it's not my area of expertise either.

 

I hope B. can source a working board.  



#23 MikeBY

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Posted 25 September 2021 - 02:58 AM

A replacement has been located.. smile.gif




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