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LX90 Encoder Change

Meade
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21 replies to this topic

#1 tom4star

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Posted 19 September 2021 - 12:51 PM

Dear all,

thanks for becoming the change to be part of this community.

I am a newbee in Astro and I have bought a Meade LX90. But the electronic seems not to work.

So I disassembled the drive and checked some signals. I came to the conclusion that the encoder signal of one axis is defect. So I could disassemble the gear box, but I am not sure how to disconnect the motor from the encoder electronic (see attached photo). Anyone who can help here?

I would appreciate any help.

Thanks

Tom

Attached Thumbnails

  • Lx90MotorEncoder.JPG


#2 OzAndrewJ

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Posted 19 September 2021 - 04:28 PM

Gday Tom

The encoders look like the US Digital E4 series as used in the LX200s

In them the disk was normally held on with a small grub screw

but i am not 100% sure how the LX90 version goes together with the gear.

( The LX90 uses a 108 vane disk )

In my test bed motors,it "looks" like the nylon gear is acting as a bush inside the disk

and is then a light press fit onto the axle, but i cant confirm that.

One problem is the old encoders were a reflective type, but they are now discontinued.

The new ones are transmissive, and as such require a properly fitting cap to hold the reflector

so may not fit.???



#3 Piet Le Roux

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Posted 20 September 2021 - 06:34 AM

Here is an excerpt from Jan's LX90 manual: 

 

PS carefully note the last paragraph......

Attached Files


Edited by Piet Le Roux, 20 September 2021 - 08:49 AM.


#4 Piet Le Roux

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Posted 20 September 2021 - 06:58 AM

More info on the LX90:

http://web.archive.o...rvatory.org:80/



#5 tom4star

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Posted 20 September 2021 - 11:54 AM

Dear OzAndrew, dear Piet Le Roux,

thank you for you fast reply.

I do not see any screw to loose the gear from the shaft.

So no way to replace the LED transmitter/receiver without destroying the motor/encoder/gear combination?

Do I need to open the motor to get the shaft out of it?

It would be a pitty to destroy all that for a 50 cent device...

 

Thanks

Tom

 

PS: I have attached some photos here (I do not know how to attach some more pictures here and not exceeding the limit)

https://hidrive.iono...hare/c5h17jhygk


Edited by tom4star, 20 September 2021 - 12:01 PM.


#6 OzAndrewJ

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Posted 20 September 2021 - 04:56 PM

Gday Tom4star

I dont see a screw on my unit either, hence the assumption it is a light press fit.

I doubt you would destroy anything by trying to gently lever off the disk+gear assy

but it looks fiddly and i have never actually disassembled any LX90s that far.

Maybe others can chime in.

 

Andrew Johansen Melbourne Australia

 

PS the photos link results in a white screen with no warnings for me :-)



#7 tom4star

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Posted 20 September 2021 - 11:31 PM

Dear Andrew,
ok. I will try next weekend to go further.
The link should give access to some more photos. I currently do not know why it does not work for you.
Greetings to Australia from Germany.
Tom

#8 OzAndrewJ

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Posted 21 September 2021 - 12:36 AM

Gday Tom

 

The link should give access to some more photos.

Nope, still just a white screen.

 

Andrew Johansen Melbourne Australia



#9 Piet Le Roux

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Posted 21 September 2021 - 07:04 AM

I resized 3 of the photos and attached them....I think Andrew will find the waveforms interesting.

Attached Thumbnails

  • SignalVertical2.jpg
  • SignalHorizontal2.jpg
  • Lx90Disassembled.JPG


#10 tom4star

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Posted 21 September 2021 - 11:28 AM

Thanks for the Update.

#11 tom4star

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Posted 21 September 2021 - 11:45 AM

The Waveforms are of the vertical and horizontal axis between blue and yellow cable.

#12 tom4star

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Posted 21 September 2021 - 11:45 AM

The Waveforms are of the vertical and horizontal axis between blue and yellow cable.

#13 OzAndrewJ

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Posted 21 September 2021 - 04:31 PM

Gday tom4star

 

I have no idea how to read plots done that way for this type of encoder.

The encoder puts out a simple quadrature output on the A and B channels

Sooo what you need to do is plot both A and B "relative to ground"

on the same plot, then look for gaps.

I have attached a plot i got using a logic analyser for an LX90 testbed

tracking at sidereal in Polar, then slewed a bit.

The "ringing" you see on many transitions is tied to the way the

logic limit oscillates as the pulse decays ( which your first plot shows well )

 

Andrew Johansen Melbourne Australia

 

sample encoder plot for an LX90

LX90 PWM.jpg

 



#14 tom4star

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Posted 22 September 2021 - 01:48 AM

Dear all,
the horizontal axis encoder signal is correct, the vertical wrong on my lx90.
So I have disassembled the driver unit. Ijust need to get the gear detached from the motor. I try it next weekend and inform you if I was sucessful.

Thanks
Thomas

#15 OzAndrewJ

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Posted 22 September 2021 - 02:03 AM

Gday Thomas

I still dont know if i understand your terminology of vertical or horizontal

( I was thinking of the CRO in X/Y mode lol.gif  )

or do you mean the two plots above are one for the Alt motor and one for the Az motor???

In which case was your CRO plot for the voltage between Blue and Yellow???

To properly see whats going on, you really need to plot both channels relative to ground

at the same time, ( as it removes any cross contamination )

but if your plots are really the delta voltage between A and B channels, then yes,

the upper plot shows one channel is practically dead. ( Tho it is trying to work at times )

 

Andrew Johansen Melbourne Australia



#16 tom4star

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Posted 22 September 2021 - 04:06 AM

Hello,
the bad plot is for the Alt Motor.
If you like I can give you measurements earliest @ weekend (no time currently).
Best Regards
Tom

#17 OzAndrewJ

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Posted 22 September 2021 - 05:01 AM

Gday Tom

OK, i think i understand the plots now.

It would be interesting to see the chA and chB plots by themselves

but not necessary, as it is clear one of the channels isnt playing.

Just as a hail mary, maybe wipe the lense of the sender /receiver chip

with an ear bud and some isopropyl alcohol

to see if something is blocking the light path????

 

Andrew Johansen Melbourne Australia



#18 tom4star

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Posted 23 September 2021 - 09:19 AM

I could remove the motor from the encoder board. I desoldered it.
The encoder is a HP8k (from ?Agilent) or a replacement maybe HEDR8000 (Avago) or
It is still available at aliexpress. 5pcs arount 17euro + tax + delivery costs.

I have added 2 more pictures @ https://hidrive.iono...hare/c5h17jhygk

Edited by tom4star, 23 September 2021 - 11:19 AM.


#19 OzAndrewJ

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Posted 23 September 2021 - 04:48 PM

Gday tom

I still cant see anything on that link site,

but it sounds like you are making progress.

I assume the disk was a light press fit ????

Maybe post a low res piccy of its underside here so other can see it in future.

 

Andrew Johansen Melbourne Australia



#20 tom4star

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Posted 24 September 2021 - 12:29 AM

The figures of the motor/encoder unit.
Meade LX90 Gear/Motor/Encoder Unit
Encoder PCB Chip HP8X One Side Up
All figures of the disassembled parts are stored here: https://www.cloudyni...0-disassembled/

Edited by tom4star, 24 September 2021 - 01:14 AM.


#21 OzAndrewJ

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Posted 24 September 2021 - 01:12 AM

Gday Tom

OK, that looks pretty close to the PCB used in the earlier LX200s, tho the one i have piccies of didnt have the capacitor alongside the led/receiver unit.

Also looks like the plastic gear does have a shoulder, and the disk is pressed onto it

so that explains that one.

 

Andrew Johansen Melbourne Australia

E4P PCB.jpg



#22 tom4star

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Posted 24 September 2021 - 01:21 AM

Dear Andrew,

great. So I will order the encoder and will inform you if it worked. (Probably that takes more than a month, since delivery from China is a bit slow.)

Thanks Andrew and Piet La Roux for your help :-).




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