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Need help repairing LXD75 Electronics

Meade mount DIY
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#1 Raumhund


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Posted 03 April 2020 - 10:56 AM

I think I broke my LXD75 yesterday and could need some help fixing it.

I have an older LXD75 mount with an Autostar Handbox (not sure which one, it States 42G).


The state:

Only the RA Axis Motor runs. The DEC Axis Motor is dead.

Trough a lot of try and error I managed to pin down the Error to the small PCB mounted directly at the motor.

I managed to desolder the cables and changed the RA and DEC modules.

I also changed the motors - the work both.

Thereby I was able to pin the error down to one of the PCBs - they are labeled "LXD75 Mainboard".


From reading I got the idea the motor drivers might be dead, but the do not get any signal from the PIC controller.


I used an Oscilloscope to see the traffic and it looks quite different between the functional and the defective PCB.

I was not able to figure out the encoding yet, but it looks like one of the line is RX and the other one is TX.

There is traffic on both lines but when I try to engage the motor the traffic from the functional PCB looks different then the one from the broken pcb - but I am certain the error is not solely within the Handbox.


Things I did:

Reset the Handbox

Checked Powersupply

Checked Cables


Changed RA and DEC Modules -> quite tedious work, but I know  are certain that one PCB is broken.


Questions I have:

How should I proceed? Does anyone have deeper knowledge of the protocol between the Handbox and the "Mainboards"?

Is there communication between the "Mainboards" and the Handbox - do they need to register to funcion?


I am not able to change to rotation speed - the motor always rotates with the same speed (it might be load-dependent).

Are the encoder discs read, and if so how? I was not able to read any signal at any pin of the sensor (might be a current signal).

But I was also not able to measure some square wave at any of the pins of the controller?!




What I did prior to the failure:

I try to recap what I did and explain what broke, but I'm not 100% sure yet.


Yesterday I build managed to connect a Raspberry Pi running Astroberry, I managed to control the Scope using the software - I was really happy and wanted to try my new scope control at night!

The connection is made (understand bodged) using an Raspberry Pi 3, a FTDI board and a UART-RS232 Module - but it works!

The Setup runs on a 12V Powersupply (I also tested another one).

I am not sure if the Autostar expects the RS232 logic levels, but the communication works, so I assume its ok?

May I have killed my whole setup with the + / - 12V voltage levels? 



After i set up the scope during day I transfered it outside in the evening and tried to calibrate it.

Only one axis was turning and the scope hit the tripod - I then shutdown the power supply to avoid damage.


One thing was strange - I powered on the mound and started the Handbox, the Handbox is the supposed to point the scope towards a star, but the scope did not move at all. I pressed some buttons and some strange movements happened.

I thought my PC connection might have compromised the Autostar setup, I therefore tried to reset and initialize it - without success.


Sometimes I got erratic movement of the mount, and later on a motor test, which seems to go endless. Sometimes it ends with an error, but not always.


Sorry for the wall of text!

Any help is really appreciated!

I can upload some pictures later...



#2 OzAndrewJ



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Posted 03 April 2020 - 03:41 PM

Gday Raumhund

The protocol between the handbox and boards is well known and is detailed in several papers on the roboscope IO group.

Basically, the protocol used is a Meade proprietary I2C and all clocking is controlled by the handbox

They use one data line and 2 clocks, one for each motor card.

The comms use TTL level voltages so not sure if you have killed the cards or not.

The simple way to test is to watch the comms using a logic analyser.

The motor cards simply respond to commands

The data line has a weak pullup on it.

When the handbox wants to talk to a card, it sets the data line to input and pulls down the clock.

The relevant card responds by pulling down the data line.

If that happens, the handbox raises the clock and sets the data to output.

It then clocks out the required command, and if needed, sets the data line back to input and clocks back a reply.

The motor card does the rest.

That should get you going :-)

Andrew Johansen Melbourne Australia

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