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HEQ5 Pro - Reverse Polarity and Magic Smoke, my turn this time!


jimboozle

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Yep, you've guessed it! Fumbling around in the dark and I got the polarity wrong.  You never think you are going to be that person, until you are... I've fixed the cause of the issue by upgrading my power supply this morning - stable door after the horse and all that.  Rather annoying since I haven't used the kit for a couple of years, and am rekindling my interest following a fascinating day at PAS2023.  Anyway, I digress.

It's pretty hard to faultfind the board properly, as i sold my handset years ago, relying on a Lynx Astro cable for communications.  Which was fine until it wasn't.  ASCOM now rather predicatably has a comms error with the mount.

However, now it's broken, obviously i'm not getting comms with the mount, and the power LED doesn't light up, which from what I've read is a clue.  I've stripped down the mount and have the input board and control board on my bench, and I'm tracing the power through the system - I found an old thread where @malc-c went through this same saga - your posts were fascinating and awfully helpful, thank you.  In particular, i've inspected the two 470uF capacitors - they look ok, but I'll dig through my spares box and see if I can find some since I've probably got some lying about.  I've checked conductivity across all the resistors and inductors, and everything seems fine, so far.  Except obviously it isn't!

I've got another board on order, as it's been twenty years since my electronics degree, so i'm not too hopeful, but I'd love to fix this one if I can.

My plan:

Replace the two big capacitors (C41 and C3, by Jumper 1)

See if it works (unlikely!).

Any other likely culprits to hit next? A schematic would be useful if anyone knows where to find one, so I can trace the power through the board until it stops...

Thanks, Jim

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well, it wasn't those two capacitors.  Both replaced with comparable parts but no dice.

I have noticed that when I connect the power supply, it cuts out, indicating a short circuit.  It's evidently downstream of C41 and C3. Any ideas?

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Thanks for the tip Keith - on your advice I've just tried that.

All is ok if I just power up the power board (well, nothing happens, and the power supply is happy to stay running).  Connecting it to the control board (only) causes the power supply to cut out, hence my thinking there is a short early on in the control board power circuitry.  I guess I'm going to have to follow the power circuit through the board and find out where it stops, using my trusty old multimeter only.  It's going to be a fun evening!

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The next component I'd worry about is U1 on the underside. I think its a 12V to 5V regulator, so would have been exposed to the full -12V.

As it's a short you have, checking out the circuit with a multimeter won't help I'm afraid. The 12V is likely to go to about 20 places, and anyone could be the short. The meter won't say which.

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17 minutes ago, AstroKeith said:

The next component I'd worry about is U1 on the underside. I think its a 12V to 5V regulator, so would have been exposed to the full -12V.

As it's a short you have, checking out the circuit with a multimeter won't help I'm afraid. The 12V is likely to go to about 20 places, and anyone could be the short. The meter won't say which.

Well you can sometimes find the short circuit by measuring the milli-volts across two power pins on each chip with your meter with a current limiting power supply connected to the PCB. If a chip is the possible culprit then you'll get almost zero mV across it's power pins, where as on the other chips you may well have a few mV's on them, but that can depend on knowing the routing of the power track(s). It works due to PCB tracks having resistance.

You can also find a short on a PCB with the same current limited power supply connected by using a thermal imaging camera (if you're lucky enough to have one), it can be quite magical when you see the PCB track temperatures, the shorted chip might also show up as being a bit hot.

But anyway, whilst you have it apart, solder a nice fat reverse biased diode across the power input connector - it will save the boards if you do it again.

Edited by EarthLife
typos
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Thanks for the tips - I'll have a look at U1, it sounds like a very likely culprit.  Thankfully my replacement will be here in a day or two, but I'd like to get this going as a spare, and I will DEFINITELY get a diode attached! Great idea, and no idea why Skywatcher don't spend the extra 20p doing so...

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No expert, I was just lucky that replacing the two electrolytic caps worked for  me.  Other issues have been where the boards power up but there are no communications between handset and mainboard, which are cured by reprogramming two new PICs and swapping them for the originals.

You could try removing D1 and then see if the short goes away.  The other possibility is the main regulator 2171WU (marked U4).  This is responsible for providing the 30+v DC to the stepper drivers.  The 7805 mentioned above does indeed provide the 5v TTL voltage.  The rest of the 5v line is made up of a couple of filters, and the aforementioned 470uf caps.  The Schottky diode D1 is between these two filter circuits.  If you get 5vdc on the output of the 7805 (pin 3) then the Diode is good.

Detecting shorts often needs a thermal camera, or flux resin, or finger test to see what's getting hot.  Sometimes it can be as simple as a small surface mount cap that is shorting to ground.

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Thanks for the info! I’ll try some of those suggestions when I’ve finished work. Looking at the 2171 it does look a little discoloured, which could well be due to it cooking.

as previously mentioned, I’m awaiting a new board so this isn’t top of the list, but I’d like to get it running mainly out of interest, and to keep as a spare I guess.

thank you for taking the time to reply, and I’ll update you all on progress once the slow boat from china brings my 7805.

 

in the meantime, a hefty diode is now across the power input so it doesn’t happen again, and an Anderson connector based 12v distribution board is in construction…

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I might have gone overboard labelling, but I’m not having this happen again!

I’ve velcroed the board to the mount so I can transfer it to my big setup when I eventually get the RASA collimated again…

Cable management to follow, I promise!

 

CBDAC0EE-D900-4171-864B-16D32581959E.thumb.jpeg.1f60be66e8df575fc05f5ffc2399d4b0.jpeg

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Sorry, actually I am not sure what went wrong.  The Anderson connectors are keyed to only fit in the correct way aren't they? Or did you connect the input the wrong way round and everything saw reversed polarity. I assume the mount's barrel connector is wired conventionally. I just want to make sure I am not running the same risk.

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Hi Lumos,

The problem was that I hadn't pulled my finger out and fitted the anderson connectors.  Instead I was powering directly from a bench mounted 12V supply, and I got the polarity wrong.  Now the powerpoles are in place, it won't be happening again!

The mount has a standard 5.5 x 2.1 mm DC jack input, with the tip positive.  It's pretty hard to get it wrong, but these things happen!

Edited by jimboozle
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  • 7 months later...

Hi. 
I’m sorry to admit that misery enjoys company. I cannot believe that I made this mistake too. 
Would you please let me know where you ordered your boards from. I don’t have the wherewithal to fault find at the moment but I will keep the old boards and fix it sometime. Hopefully to give it to some poor soul who makes the same mistake. 
Thanks

Shaun

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19 hours ago, ZS1RA said:

Hi. 
I’m sorry to admit that misery enjoys company. I cannot believe that I made this mistake too. 
Would you please let me know where you ordered your boards from. I don’t have the wherewithal to fault find at the moment but I will keep the old boards and fix it sometime. Hopefully to give it to some poor soul who makes the same mistake. 
Thanks

Shaun

You can get the board from RVO or Grovers Optics. You could also try FLO, they have some Skywatcher boards listed on their website but not for the HEQ5, but worth asking.

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22 hours ago, ZS1RA said:

Hi. 
I’m sorry to admit that misery enjoys company. I cannot believe that I made this mistake too. 
Would you please let me know where you ordered your boards from. I don’t have the wherewithal to fault find at the moment but I will keep the old boards and fix it sometime. Hopefully to give it to some poor soul who makes the same mistake. 
Thanks

Shaun

Shuan,

How old is the mount?  - I've repaired several HEQ5, EQ6 and boards from dobsonian mounts, but I'm limited to the older generation boards when it comes to replacing the main processors.  However most blown boards that occur through power supply issues such as wrong polarity tend to be a reasonable easy fix as they often (not always) are a protection diode that blows as a sacrificial "fuse"

I'm willing to take a look at it for you, or if you do go and get a replacement form Firtslight Optics or Rother Valley Optics then please drop me a PM and donate the board to the exchange program.  I set this up so that (where possible) boards can be recycled and if I can effect a fix then they can be offered to any other SGL member who needs a replacement board for just the cost of the parts and a drink to cover my time and postage.  But to make it work I need people to donate their faulty boards to get the ball rolling.

Let me know what you decide to do 

Regards

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