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Help diagnosing NEQ6 Pro electrical problem


ForrestR

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I guess this is more or less my first post here, apart from one I posted in a sales thread some time ago.

I have been lurking here for some time, became a member not that long ago, and haven´t had anything to contribute I am sad to say.  And now it seems I need help.  I have searched this and other forums but can´t seem to find my exact problem.

I recently purchased a second hand NEQ6 mount and I tried to power it up yesterday and I think I damaged the mount.

I used a 12v 4A transformer (analogue heavy transformer, intended for halogen lighting).  I did not check polarity.  There was no telescope on the mount, but I forgot to remove the counterweight which was pointing down towards floor.

I turned on the mount with handset attached, it initialized, asked if I wanted to return to park, I said yes, there was a brief, faint, sizzling sound, and then nothing.  No movement.

The red LED is on, the handset works fine, but now when I power down and power up, the handset doesn´t ask about park anymore, but it seems fine otherwise.  It goes through setup, if I go to three star alignment at the end, it will say "slewing" but nothing happens.  

When the handset is powered up, and until I start alignment, there is a faint high frequency sound coming from the vicinity of the motors, not the board, but that sound stops as soon as the alignment attempt starts.  

I can see no visible damage to the board, no burn marks, nothing.

I suspect I have fried the board, not sure how, but cannot imagine what else it might be.  I have seen reference to people using up to 15v, saying that 12v was not enough for them to run the mount, but the burning/sizzling sound that first time suggests it was something I did.

My own thoughts (based on total ignorance):

1.  the nature of the transformer - it reads 12.2v with voltmeter, but perhaps that type of transformer will damage the mount?

2.  polarity - perhaps this was the problem, but if so, it damaged the mount, reversing polarity now has no effect, handset fine, no response from motors

3.  weight down - motors tried to slew but could not overcome weight of signle counterweight not balanced by a scope, something fried?

4  12.2v enough to power handset but not motors.... but I still come back to that first fizzle sound...  I think undervoltage is wishful thinking.

 

Any help is greatly appreciated.  I know no one will actually call me an idiot, but it is okay to think it, I certainly do.  

Thanks in advance!

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Are you reading 12.2 volt AC or DC, it must be DC (Halogen lighting units don't care, they just want low voltages). If it's AC, then you may well have fried the motherboard\drivers, the handset usually has some form of regulation on board, that's why it appears to be working...

Most peeps use either a Linear supply or a switched mode supply, both can be obtained from Maplin's i.e. http://www.maplin.co.uk/p/375w-linear-dc-variable-voltage-bench-power-supply-rp10l or http://www.maplin.co.uk/p/power-pax-120w-switched-mode-dc-fixed-voltage-12v-power-supply-n96ju

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I think that´s it.   I just looked at the transformer and indeed, it says 12v, 50w, but does not specify AC or DC.   That makes perfect sense, but damn, what a stupid mistake, and very frustrating to get off to such a bad start.

I have a 150 12v DC PS on order, but I was impatient and wanted to power up the mount and figured, I have this small transformer, read voltage at 12.2, never thought to check if it was direct current, I always assumed these transformers were, based on nothing but igrnorance.

It´s a first world problem, and it won´t kill me to buy a new board, but man I feel like a fool starting out this way.

I have seen the board on TS, about 120 Euros I think, any other suggestions for a lower price?

Thank you so much for your help.  I was pretty sure it was the board, but at least now I have a good idea what I did wrong and have learned something.

Forrest

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If it is just a transformer, then it is almost certainly 12v AC. If it has just rectification and a smoothing capacitor, I would expect to see about 17V DC off load. Some of the "Transformers" used for 12V halogen lighting, are not true transformers, but are an electronic circuit that pretends to be a transformer, but is kinder to the lamps. These often have a minimum, as well as a maximum, load for normal operation.

From your description, it would appear that the problem is with the motor board. As Julian mentioned, above, it is likely that the handset has been protected by its power regulation circuitry.

I am not familiar with the Skywatcher motor control, but if it follows the design used by sister company, Celestron, it may be like the one in this photo.

If it is similar, it is possible that the voltage regulator chip (U11 in the top left of the photo), or its adjacent electrolytic capacitors, have sacrificed themselves and protected the lower voltage circuitry. The L293DD, U5, bottom right, is the motor driver, and is good for 36V, but DC. The two PIC16F876A microcontroller chips run from between 2V and 5.5V. If one of the capacitors is bulging, or has a funny smell, then it may be possible to replace it, but I would also replace U11.

Geoff

Motor control -R.jpg

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2 hours ago, Geoff Lister said:

If it is just a transformer, then it is almost certainly 12v AC. If it has just rectification and a smoothing capacitor, I would expect to see about 17V DC off load. Some of the "Transformers" used for 12V halogen lighting, are not true transformers, but are an electronic circuit that pretends to be a transformer, but is kinder to the lamps. These often have a minimum, as well as a maximum, load for normal operation.

From your description, it would appear that the problem is with the motor board. As Julian mentioned, above, it is likely that the handset has been protected by its power regulation circuitry.

I am not familiar with the Skywatcher motor control, but if it follows the design used by sister company, Celestron, it may be like the one in this photo.

If it is similar, it is possible that the voltage regulator chip (U11 in the top left of the photo), or its adjacent electrolytic capacitors, have sacrificed themselves and protected the lower voltage circuitry. The L293DD, U5, bottom right, is the motor driver, and is good for 36V, but DC. The two PIC16F876A microcontroller chips run from between 2V and 5.5V. If one of the capacitors is bulging, or has a funny smell, then it may be possible to replace it, but I would also replace U11.

Geoff

Thanks Geoff, I removed the motherboard and took some pics.  I don´t see an obvious analogue of the chip at U11, but it does appear that one of the capacitors may be bulged, at least compared to the capacitor next to it.  It doesn´t appear to be burned over overheated, but the top is definitely bulged compared to its companion.

Pics: 

motherboard3.jpgsubir gif

motherboard2.jpgsubir gif

motherboard1.jpgsubir gif

 

 

 

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48 minutes ago, AlistairW said:

I've made a similar mistake with the HEQ5, - wrong polarity in my case. Mount would not move but controller was fine. Ordered a replacement board from FLO, and it was fairly easy to fit in the HEQ5.

Alistair

Which reminds me, how to I assure I have polarity correct when I repair or replace this board?  

I really appreciate the help, thank you all.

Forrest

 

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2 hours ago, Dr_Ju_ju said:

centre pin is positive +

Perfect, thanks!

1 hour ago, Geoff Lister said:

470uF 25V capacitors are very common. If you are OK with a soldering iron it should be easy to replace.

U1 is a low-power 5V linear regulator, and U4 is a 2.5A switching regulator. They may be OK.

Geoff

I replaced all of them in my TV once, when it took about a half hour to come on.... on recommendation of someone in a forum, worked like a charm.  I am okay with a soldering iron so I will give this a try, again, thanks for the help. :)

Forrest

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