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Is this Losmandy 492 fixable?

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#1 MockTender

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Posted 24 January 2021 - 07:08 PM

Went outside to look at stars a couple of nights ago and everything worked fine, but the 492.  At first it would not sart up after plugging in to a AC to DC adaptor.  The indicator on the adaptor looked like the 492 was getting dc, but no lights came on the unit.  So I went to my truck to use the 12vdc outlet.  As soon as I turned on the 492, it started to smoke. I opened up the case and everything, but the small capacitor above the on/off switch looked fine.  That small capacitor was fried at each end.

 

So I have three questions:

 

1) I have read about problems with 492 switches- is this one of those problems? 

 

2) Could I relpace the capacitor or does this look and sound like a new motherboard?

 

3) What are the ways to safe guard against this happening again? 

 

 

Mark

 

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#2 D_talley

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Posted 24 January 2021 - 07:15 PM

I see you posted this twice.  I had that happen to my 492 and gave Scott a call.  He said that there was a batch of bad diodes that would let out the magic smoke.  I replace the diode and the system booted up fine.  Please check the board for a bad diode. I see one above SW7.  Clear evidence that it is the source of the smoke. 



#3 RichA

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Posted 24 January 2021 - 07:23 PM

I see you posted this twice.  I had that happen to my 492 and gave Scott a call.  He said that there was a batch of bad diodes that would let out the magic smoke.  I replace the diode and the system booted up fine.  Please check the board for a bad diode. I see one above SW7.  Clear evidence that it is the source of the smoke. 

Electronic manufacturers tend to use the bare minimum standard components they can get. When replacing diodes, caps, it's best to "uprate" them if possible, higher voltage for the caps.



#4 ngc7319_20

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Posted 24 January 2021 - 07:43 PM

Mine did the same thing.  The part is a diode.  On mine the +12V bus trace on the circuit board (underneath) touched one of the mounting studs / case, so +12V got shorted to ground through the diode.  Should be easy to fix.  You might pull the circuit board and check for un-insulated traces near the mounting studs.  Since it is a direct short of +12V supply line to ground, only the diode should be affected.


Edited by ngc7319_20, 24 January 2021 - 11:43 PM.


#5 ngc7319_20

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Posted 24 January 2021 - 11:26 PM

Found my notes...  Here are some pictures of the problem.  There is a sharp metal edge on the case that scribes into the +12V bus line on the board.  You  could check for same problem, then put some plastic tape where it is rubbing in the case.

 

1 opened case.JPG

 

2 PC board.JPG

 

3 closeup.JPG


Edited by ngc7319_20, 24 January 2021 - 11:51 PM.

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#6 ngc7319_20

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Posted 24 January 2021 - 11:29 PM

Here's another board showing similar problem...

 

542019 xx.jpg


Edited by ngc7319_20, 24 January 2021 - 11:37 PM.

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#7 MockTender

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Posted 25 January 2021 - 04:34 PM

Thanks guys- great info!

 

Add mine to the list as well.  Contacted Losmandy.  The woman that answered the phone said that they no longer have any motherboards or parts for the 492 (they just weren't selling any).  She suggested I contact Brian in Tech Support and that he may know someone in the country that can supply the diodes and other parts.  Sent him an email- waiting.

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Edited by MockTender, 25 January 2021 - 04:36 PM.


#8 ngc7319_20

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Posted 25 January 2021 - 05:01 PM

This should work to replace the diode...  $0.35

 

https://www.jameco.c...Amp_879641.html

 

If you can pull any numbers off the old diode, we could try to get a more exact replacement.  But with 100 Volt, 4 Amp rating, this should work.  The diode needs to be installed the correct way around -- the silver band needs to be pointing to the right (like the old one).

 

The function of the diode is simply to block current if the input power polarity is accidentally reversed.  


Edited by ngc7319_20, 25 January 2021 - 06:16 PM.


#9 jp071848

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Posted 25 January 2021 - 05:06 PM

Thanks guys- great info!

 

Add mine to the list as well.  Contacted Losmandy.  The woman that answered the phone said that they no longer have any motherboards or parts for the 492 (they just weren't selling any).  She suggested I contact Brian in Tech Support and that he may know someone in the country that can supply the diodes and other parts.  Sent him an email- waiting.

Over on the Losmandy and Gemini Groups.io forums, there are three gentlemen (in California, the UK and in Oz) who do board level repairs on certain Losmandy control units. Sorry, not sure if the 492 is one of them. You may want to check over there and ask. Brian Valente, who Tanya suggested you contact, is active both here on CN as well as over on the Groups.io forums.
 



#10 MockTender

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Posted 18 March 2021 - 10:16 PM

I contacted Losmandy and they gave me the name of Michael Herman.

 

We texted back and forth.  I asked if he minded if I shared with the CloudyNights people.  His answer was yes and here is some of his email to me.

 

No problem at all...please do mention it in Cloudy Nights...I just don't visit there frequently.  I am one of 3 fellows worldwide who repair Gemini units.  The others are Brendan Smith in Australia and David Partridge in the UK. 

It will be $50 to replace the diode.  I will thoroughly check the rest of the operation of the unit.  Shipping is about $16 using a USPS small carton that is a good tight fit to the Gemini-1, with a plastic bag for moisture protection.  I don't need the cables or handset...I have all that. 

 

But for $100 total I will do much more valuable work for your G-1.  I will put in a transistor circuit I created to bypass the D1, eliminating the voltage drop and saving the CPU and SRAM.  I will add zener diodes to the serial port nodes protecting them from over voltage.  I will update your eprom to the latest level 4 v1.05 if it needs it.  I will replace your battery if it needs it. 

 

A full repair of a damaged unit generally runs $200.  To "bulletproof" a working unit is $100.  I can take PayPal or a check.   

 

This is obviously not my main living.  I only do these repairs and upgrades as a service to our fellow Losmandy gemini owners.  I myself have 4 or 5 of these units and they work fine for all my needs. I can send you upgrade PDFs too about souping up your mount to about 2 arcsec RMS PE or better.  Let me know if interested...(sorry if I already sent some your way!)

 

Very best,
Michael*

 

*I decided to withold his number and address- If you are interested in contacting him, pm me*


Edited by MockTender, 18 March 2021 - 10:22 PM.

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#11 Geo.

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Posted 21 March 2021 - 03:16 PM

You can probably get away with with a 1N4001 (last digit is the amperage rating). I buy these by the 100. A 1N4004 is really common. Any old electronics should have a few on it. Wall warts usually have 4. These are simple rectifier diodes. 

 

These, OTOH, are surface mount Schottky diodes (SS310) that serve to protect step motor drivers from back EMF surges. 

 

P1016886.jpg

 

These actually rather large by today's standards, almost two 10ths of an inch long. 

 

If they can be repaired for $50 it's a pretty good deal. I generally charge $125-150 for a working 492. A modern step motor controller is a lot less complex as all the analog switching is done in software and a touchscreen. Due to IoT and 3D printing technology the prices of the controller components can be $15 a unit. When you are used to controlling 5 printer motors, you are not very taxed by something as simple as telescope control.

 



#12 aa6ww

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Posted 22 March 2021 - 11:03 AM

I'd get an X-acto blade and cut off the corner of the trace on the board that is touching the ground on the case so there's clearance. Then just replace the diode, or get anyone you know with a soldering iron to do it for you. Make sure you note the polarity of the diode, it only goes on one way.

 

...Ralph



#13 t-ara-fan

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Posted 23 March 2021 - 01:50 PM

You can probably get away with with a 1N4001 (last digit is the amperage rating).

The last digit in the 1N400x series indicates the Peak Repetitive Reverse Voltage.  Not the amperage. 50V for the 1N4001 up to 1000V for the IN4007.   The whole series has a 1A Average Current Rating. 

 

For DC blocking (if that is indeed the purpose) any of that series will do the trick.
 



#14 mherman346

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Posted 31 March 2021 - 02:39 PM

A very interesting discussion of the 492 boards and what can go wrong...thanks for all the points above.

 

I repair 492 units.

 

I can supply replacement EPROMs in case your unit starts to "behave funny" where the LEDs may not work as expected.  The EPROMs were originally designed to last for 100 years, but of course... that's marketing!  They do go bad sometimes.  The EPROM for the GM8 drive is different than that of the G11 drive, so be sure to specify what version you need.  (The crystal oscillator for the GM8 version is 6MHz, and the crystal for the G11 version is 12MHz.)

 

I just repaired the unit shown at the top of this thread.  The diode D1 labelled on the board was blown to smithereens.  Replacing the diode alone was not sufficient.  The owner had tried several different 12V sources, including ones in his camper, and so I suspect that one of these DC supplies was reversed-bias supply (supposed to be center positive only) and this blew the diode up.  But perhaps that original Schottky diode had worn out and blew up, as someone above reported.  Schottky barrier diodes use a surface metal coating on a heavily doped silicon crystal, and over time, the metal can diffuse into the silicon and degrade that form of diode..  Once it is blown up...the 5V regulator chip does not get its 12V input supply.  The diode is the only "polarity protection" in the 492 unit, and it is not a fuse.  There is no polarity protection circuit inside the case, but I can provide one if interested.

 

All the best,

Michael Herman

mherman346@gmail.com


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#15 Joe Bergeron

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Posted 31 March 2021 - 04:10 PM

I’ll keep you in mind if my current 492 board goes bad. The first one failed after 20 years or so due to a blown, smoky diode or capacitor, I forget which. 




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