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Meade EQ Starfinder DC drive not working / encoder dead

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#1 TheHorizonAtDawn

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Posted 18 March 2022 - 08:34 PM

Hi everyone

I'm trying to get an old Meade EQ Starfinder mount working again after the previous owner stored it for >15 years in his garage.  The telescope is from 1999, it's a 8" f/6.

 

The DC drive doesn't work anymore.  I had found old batteries that had leaked into the drive assembly cover. Oxydation broke the solder of the encoder light detector and maybe also

the LED.  The 2 encoder disks were covered with oil (from the motor gear box? the mount wasn't stored assembled), the fixed disk is clear, but the rotating disk has many slits blocked by

dried oil and oxydation.   Not sure this can be fixed. A pb with a similar drive is discussed here https://www.cloudyni...erts-out-there/

 

The good surprise was to see the red LED of the electronic board turning on when I powered the board with a 12VDC laptop power supply.   The motor/encoder system is connected through

a 5-pin connector (JP4 on the picture below or in discussion above) to the board.  The motor doesn't move, but I'm able to measure 5V between the black and the red wire, and also 5V between the black and the white wire.  0 V measured between yellow and blue wires (apparently those two are going to the dc motor).

Of course I don't know if this 5V is nominal or not.

 

My questions:

- Is it possible to bypass the encoder and supply some given voltage between the yellow and blue wires going to the motor ? I don't care about accurate sideral tracking, I'm only going to do visual observtions but with a working drive if possible.

- how can I remove the motor gear box from the worm gear ?  I thought that after removing a small bolt the motor+gear would have been easy to pull out from the worm axis. But no. Is it

seized or is this normal and expected and I just have to pull a bit more ?

 

IMG-4874.jpg

 

 

IMG-4876.jpg

 

Thanks!  



#2 TheHorizonAtDawn

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Posted 19 March 2022 - 09:06 PM

I've found another post about the same drive: https://www.cloudyni...meade-dc-drive/

 

Has anyone an idea of the LED and light sensor model that could be used today to replace those > 20years old components ? I think I could try to rebuild this, acetone cleaned up the old oil on the encoder disks

 

I looked for a 'service manual' for this motor/electronics but found nothing.



#3 deSitter

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Posted 19 March 2022 - 09:20 PM

I would start slow by getting the encoder disk free and soaking it in mineral spirits for a while to clean it up. Assuming it is metal. Be very careful not to bend it. Go slow. Do you have any experience repairing electrical/electronic problems?

 

-drl



#4 DAVIDG

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Posted 20 March 2022 - 10:50 AM

 Having worked on electronics which includes telescope electronics for many years my advice is to clean up the wiring and solder it back together. The LED is an IR one so you can see if it is working by looking at in real time mode using the camera in a cell phone since those cameras can see the IR.  The LED and sensor are pretty tough so unless a lead broke off at the part the odds are good that they are working. 

   The circuit is using a PIC microcontroller to make a DC servo system with feed back. The motor is driven by the 2993 IC which is dual H -bridge. The problem is that it can only handle up to 500ma so if the motor got stalled, it could pull more then that and damage the IC. 

   To really figure out what is going on with the circuit you need to look at it with an oscilloscope to see were the signals are and are not. The PIC should be sending pulses to the 2993 to drive  the motor and  the IR receiver should be seeing  pulses coming from the encoder wheel to tell the PIC the speed of the motor and to slow it down or speed it up.

    With the motor unplugged from the circuit you can test if  the motor runs by applying  12 volts to the leads of the motor. If the motor runs then my guess is that the 2993 is bad since the PIC would most likely run the motor wide open if it didn't see the pulses from the encoder which others  have reported as a failure mode.

   I just looked  and there are some UDN2993 on Ebay since it is a discounted part but again you should check to see that the PIC is alive and sending pulses to it and the UDN2993 is the problem.

 

                  - Dave 


Edited by DAVIDG, 20 March 2022 - 12:08 PM.

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#5 DAVIDG

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Posted 20 March 2022 - 01:00 PM

  A little more reading and I see that the  black wire should be ground, it looks like white is +5 and the red is the signal coming back from the IR sensor.  Please confirm  since the white wire should go to both the LED and sensor but  the red wiring is only going to the sensor. The black wire should  go also to both the sensor and the LED

    My guess is the sensor is an PNP type so with no light it is conducting that is why your measuring +5 on the red wire. With the LED in place and you should see no voltage on the red wire and +5 volts when the senor is blocked. If you see the voltage changes from block to unblock that indicated that part is working. 

   The fixed encoder disk is just a mask for the sensor so it is  there to  cleanly defined aperture over it. So the rest of the slots don't matter if they are blocked or not. The one over the sensor is the only important one.

   From what I saw on the other thread is if the PIC is alive the LED should flash a couple of times when the drive is turned on and then go steady. If that doesn't happen then that indicates the PIC is not working.. Since the PIC has a custom program in it can't be replaced.

    So an option is to replace the circuit with an PWM controller and adjust  it to get the rate  you need. The drive uses a 144 tooth gear so you need a 1/10 rpm to track at solar rate which is fine for visual work or make it track a tiny bit faster for Sidereal.

   Here is one of  many on Ebay that looks like it should  work https://www.ebay.com...353.m1438.l2649

 

                 - Dave 


Edited by DAVIDG, 20 March 2022 - 04:31 PM.

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#6 TheHorizonAtDawn

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Posted 21 March 2022 - 03:15 AM

Thank you -drl and Dave for your advices.

 

drl yes the disks are metal not plastic.   I used acetone to clean one of them, all slots are perfectly clean. The second disk is still attached to the motor shaft, and its rear side is more difficult to acces, but I did significant progress. I had to stop yesterday but will continue asap.

 

I built years ago some electronics to drive the frequency of a motor at +/-5Hz from standard 50Hz for an homemade EQ telescope. It's still working after 35years smile.gif   Some TTL involved (74xx if I remember correctly). I had plans how to do it but learned a lot.  I likely can have access to a oscilloscope the week-end, so yes doing more tests should be possible !

 

Dave I confirm I have measured +5V between the black wire and the white wires (actually the pins of the connector on the board, without any load connected, so this at least looks good.

In my picture above, is the transparent component removed from its aluminium holder the sensor ? (yes according to the thread I mentioned in my first post). I do confirm the white wire goes to the sensor and LED, and the red wire goes to the component still in place in front of one of the encoder disks (the LED likely). I measure 5V between the pins on the board corresponding to the black and red wires.

 

I'm not sure the black wires from the LED and sensor can be soldered again, they look thin and so deeply damaged by corrosion... but I have to look at more carefully.

 

The wires going to the motor are the blue+yellow: I did a quick test yesterday, but couldn't see/feel the motor moving. I'm also not sure the connections were good, I'll try again.

The worm and gear are not stuck so I hope the motor didn't get stalled and the board is ok.

 

The PWM controller could be a solution if I can't fix the existing electronics. I didn't know this kind of tiny controllers. thanks.

 

I'll be back in a few days !

 

Thad


Edited by TheHorizonAtDawn, 21 March 2022 - 03:21 AM.


#7 DAVIDG

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Posted 21 March 2022 - 10:18 AM

 Thad,

   The transparent device is the LED not the sensor.  The LED would have two wires going to it and the sensor three so that is how you can tell for sure. Here is link to typical LED and receivers    The LED in the unit is most likely an IR type so when you power it up you will not see it light up. To confirm it is working, look at  it with your cellphone camera since it is sensitive to IR and will show it lite up if it is working.  I would replace the corrode wire on the LED and sensor.

    The motor itself runs at fairly high RPMs and is attached to a gear box to gear it down. So you both can easily feel it running and hear it running if it is working. 

 

                  - Dave 



#8 TheHorizonAtDawn

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Posted 02 August 2022 - 11:29 PM

Hi everyone

I have found time to work again on the drive of my 1998 EQ Starfinder smile.png  

 

The bad point:  wires are so corroded that they broke too close to the LED or sensor to be soldered again. I consider these 2 parts lost.

The good point: I dismounted the EQ head which was so sticky that I couldn't observe and I removed the spur gear from the RA axis.

I then connected a 12V power supply to the electronic board, switch on the power and got the motor working ! Funny rotation speed though, way to fast most of the time and very variable speed. I guess the board is lost calculating the correct speed with the LED/sensor out of work.

 

As the board seems okay (still not sure), I would be happy if I could find the LED+sensor parts from a non-functioning drive (failing board or what ever).  If you have this sleeping in your garage, please drop me a PM !!! laugh.png

 

I can always buy a PWM controller and adjust the voltage as Dave suggested above, but it would really be funny to restore the drive electronics with a working LED and sensor pair. Removing the electronics board and putting 12Vdirectly between the blue and yellow wires in the connector made the motor working, but again way to fast.

 

Crossed fingers for a LED and sensor pair now...

smile.png


Edited by TheHorizonAtDawn, 02 August 2022 - 11:31 PM.



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