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How to fault find electronic circuit?

Posted on 2011-03-20
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How would you start to fault find, would there need to be a special equipment? For example a hdd enclosure that has gone faulty. The symptom is that the HDD doesns't spin and load. I'm just using this as an example.
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Question by:sorush
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Reece Dodds earned 500 total points
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Start with the power source.

Use a standard multimeter to test voltage levels on the low voltage (+/- 3.3v, 5v, 12v DC) rails.  You will need basic knowledge of circuits to make sure your not about to zap yourself by touching the 110/240v AC components.

If all voltage levels seem OK, you power supply is (pretty much) working.

To continue from here, let me know more about the symptoms you are experiencing and the device your haveing trouble with.
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by:dbrunton
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>>  would there need to be a special equipment?

Short answer yes.


Long answer.

Inspect and look for burnt components.  Sniff for burnt components.  Look for poor solder joints and loose wires.  After that you'll need meters and oscilliscopes to test the various electronic elements out - I'm thinking transistors and capacitors.

For a hard disk you can swap the electronic component board for another board from an identical hard disk.  Must be identical down to the BIOS version.  But see sentence at end of next paragraph.

For onboard ICs you'd need the manufacturer's equipment but at that stage it is easier to replace rather than test.
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by:Reece Dodds
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dbrunton, i think the OP was referring to the circuit board of the HDD enclosure not the HDD itself in his example.

Sorush, does the HDD work when attached to a computer mainboard?  Is it IDE or SATA?
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by:asidu
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Assuming you have the knowledge of electronics, to repair viz a harddisk would be a challenging job. You will need the circuit diagram of the harddisk which would be a difficult to obtain. It would be not would not be easy to repair.

For the motor not spinning you could only check if the power is coming on to the board. Apart from that not much could be achieved.

As suggested by dbrutnon one could only change the entire hard electronic board if  a similar type of circuit board is available.
 
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by:sorush
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So from what has been said,
1. Check power supply circuits.
2. Look for visibly damaged components on the board.

So essentially I need to have access to the circuit diagrams.
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by:sorush
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I am able to get the hdd to work via sata. But the board in the hdd external enclosure is at fault. When connected the hdd with in the enclosure doesn't spen up or seek the hdd.

So I gather an oscilloscope is also essential for fault detection.  
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by:dbrunton
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>>  But the board in the hdd external enclosure is at fault.

How is the enclosure powered?  If via USB then the USB port may not be supplying sufficient power.  For best results in such a case use a powered USB hub.
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by:sorush
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It is a powered usb hub.
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by:asidu
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Try to use two usb cable ports to power the device.
That will draw power from two ports and hopefully drive it.
You should have one USB cable with one end  having two connectors.
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by:nobus
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you best start by elimination.
-test the disk directly on a working pc, or test with another disk in the enclosure, this will show if it is the disk or enclosure
-if it is the disk, it can be the logic board, or the motor -  you find which one by replacing the logic board( if you can find the correct one; note that it must be the same model and firmware level)
-if it is the enclosure, there is only a small power supply section, and 1 chip for the usb bridge; you can easily measure if the Power is coming to the disk
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by:Reece Dodds
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sorush... we're needing more information from you to be able to guide you in a possible fix.

1.  So the hard drive works OK when connected directly to a PC mainboard, so, it is an issue with the enclosure... does the enclosure ONLY have a usb port or does it have a USB and DC port?  is it for a laptop hard drive or 3.5" PC hard drive?

2.  You mention that when the hdd is in the enclosure, you're connecting it to the PC via a powered USB hub?  does it still exhibit the same problem if you connect it to the USB ports on the back of the PC?

If the enclosure is only USB powered, then it may need more power (by using two usb ports) to start the control board.  Or, the board could be faulty.
If it is DC powered, the DC power supply may be faulty.

Like i suggested in the very first post on this case, start with the power source...
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by:Reece Dodds
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Can you give me a photo of the enclosure's circuit board?
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by:sorush
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I have just done a very simple thing. And that is to test the power supply. From the looks of it The 12V out put reads as 10.38 and the 5v output is working fine with 5.34. Could this be the problem?
On the power supply I get less than 1amp for the 12v terminal and about 2-3 amps for the 5v terminal. Could this be the fault?
Front.jpg
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by:sorush
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In regards to the back image. I have tested the voltage and power supply to the sata hdd. In the image the pins are marked 1 to 4. I assume that pin 1 is positive and thus I tested the rest of the pins. With pin 2 I get 0.7amps same with pin 3. I get 17v with pin 2 and 3. But in pin 4 I get 13v and 0.2 amps. I have looked at the power consumption on the sata hdd and there are two power figures a 5v 0.65a and 12v 0.6a. Could this be the power problem?
Back2.jpg
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by:nobus
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>>  From the looks of it The 12V out put reads as 10.38   <<  that is too low, all voltages must be within 5% range
here the sata pinout :  http://pinouts.ru/Power/sata-power_pinout.shtml

it can be a power problem - but i think your measurements are not ok - remeasure plse !
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by:dbrunton
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For figure 2 you should be getting 2 readings, a 12 volt supply and a 5 volt supply.

You haven't quite identified the pins correctly.  Two of those pins should be grounds and there shoud be no voltage difference between them.  Check the voltage between pins 2 and 3 again.
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by:sorush
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nobus I wanted to say that the the DC power supply is as shown in this image. I have tested them using a multimeter. This image is printed on the back of the transformer part of the power supply. The pins that you have shown are too small for me to use my multimeter on them but I can do that at the board rather than the connector.

Also when I connect the power supply to the board and turn the power switch on, there is like a hissing sound that comes from the transformer.
P1010314.JPG
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by:sorush
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I have tested pins 2 and 3 and there is no Voltage between them. What can I do next. When I tested the power supply I meant the pins in the above diagram the transformer box.
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by:sorush
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Pins 1 and 2 show 16.65v and pins 3 and 4 show 3.76 volts.
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by:dbrunton
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Pins 2 and 3 are your ground wires.

Pins 1 and 2 should be your 12 volt line.  Pin 1 should be positive.
Pins 4 and 3 should be your 3.3 volt line.  Pin 4 should be positive.

Assuming your mulitmeter is accurate your voltages are out of specifcation.  The standard allows plus or minus 5% from these ratings.

So your 12 volts should lie within 11.4~12.6 and your 3.3 should lie within 3.1~3.5
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by:dbrunton
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Correction.

Pins 4 and 3 should be your 5 volt line.  Pin 4 should be positive.

And the 5 volt should lie between 4.75~5.25.
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by:dbrunton
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Oh, and test the voltages while under load, that is with a hard disk connected.
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by:nobus
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a hissing transformer is also a bad indication - but it may still work
however, if the voltages are wrong, it's clearly bad - without or with load
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by:sorush
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I've ordered a new power supply and hopefully I will find out what the problem is.
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by:sorush
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Would it be possible to actually repair the transformer, has anyone done anything like that. I don't think I will because of the lack of electrical knowledge that I have and there are higher voltages involved.
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by:CompProbSolv
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I've got to toss a comment in here about the general question of fault finding.  A critical question is what are you really trying to accomplish?  For example, if someone brought me the external drive you are describing I would follow the suggestions that have been posted here about pulling the drive out and testing it in a computer.  If it worked I could fairly conclusively say that the problem is either in the drive housing circuitry or in the external power supply.

This runs us right up to the question of what you are trying to accomplish.  I would be trying to save the client's data (which is likely intact since the drive is working) and get them back up and running with an external hard drive.  I'd either purchase an external enclosure (around $30) and install their drive in it or I would purchase a new external drive ($50-100, depending on size) and copy the data over.

What I would be very unlikely to do is to try repairing the failed circuitry.  Time and reliability are worth much more than the cost of the new hardware.

My point is that we've moved beyond component (i.e. resistors, capacitors, ICs, etc.) repair for computers in most (certainly not all) cases.  Hardware is inexpensive and skilled labor is not.

I don't like the fact that this turns hardware into trash (I do try to have all of it recycled), but one has to be realistic.

Otherwise, you've gotten some excellent suggestions about troubleshooting.  Isolation (checking the output voltage of the external supply, then the output of the internal board to the HD) and substitution (trying the bare drive in a computer) are often very effective troubleshooting methods.
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by:dbrunton
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Isn't necessarily the transformer.  Could be supporting components such as resistors, capacitors, transistors.  I'd suspect a capacitor for choice of culprit.

I'm presuming this is one of those devices that has an external black box converter that changes the AC to DC.  And that this is the device that is making the hissing sound.
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by:sorush
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dbrunton, that's right, that is the power supply that converts ac to dc.  
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by:dbrunton
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There probably won't be a transformer as you think of a transformer inside the box.

Look at the article here http://www.articlesbase.com/laptops-articles/analysis-and-maintenance-of-laptop-power-adapters-4093792.html and double click on the second picture.  That is probably an example of the interior of the power supply.
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by:nobus
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the hissing sound may come from a loose transformer winding, or a bad soldering contact
i would not try to repair it - difficult to find the parts to start with..and costly
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by:Reece Dodds
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New PSU is needed.  If you cracked open the plastic you'd likely find a bulging or leaking capacitor (hissing noise you refer to) and one or 2 near burnt resistors.  You could replace these components, but if your not confident with a soldering iron and working with mains power (even though you're working on the DC side) then you'd be best of replacing the PSU.
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by:Reece Dodds
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Where are you located mate?  I have a possible replacement for that unit (slightly difference pin out configuration) from one of our decomissioned LACIE external HDD's.
 PSU
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by:sorush
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, Yes Hi, I've ordered on now and its working fine. I can get access to the data. Thanks for all your help.
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