Electronic Repair

Discussion in 'General Electronics Chat' started by Johndon2000, Jun 12, 2013.

  1. Johndon2000

    Thread Starter New Member

    Jun 12, 2013
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    Hi, if you have a circuit board that you know is faulty but you have no other diagnostics, and the only way of repairing it was to swap out devices from a good board until it eventually works, would there be a specific order of devices to swap out first... i.e, in what order would you swap out the following devices going on prior knowledge of device failure rates:

    Processor/ DRAMs, SRAMs, PROMS, ASICs, FPGA's, resistors, capacitors etc etc???

    Many thanks,

    John.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Not a good way to do it because if you swap a good component and there still a faulty one, it might damage the new one.
     
  3. Johndon2000

    Thread Starter New Member

    Jun 12, 2013
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    I agree it's not the best method but it usually works... just very time consuming & I guess I need a quicker, more technical method! Any suggestions?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Inspect the board for any burnt components and replace them first.
     
  5. Johndon2000

    Thread Starter New Member

    Jun 12, 2013
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    Visual inspection all ok, nothing burnt!
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
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    If you can't operate both boards & compare the voltages at various points,because the fault is likely to damage the rest of the equipment,or some other valid reason,your other alternative is to remove both boards from the equipment,place them on the bench side by side,& compare the resistance between various points on each,comparing good board to bad.

    If you measure across a resistor for instance,you may not read the value of that component,because there are other devices attached,but if the reading is the same on both boards,that resistor & associated devices are probably OK.

    By doing this,you should be able to isolate the fault to a particular area on the board----then you could try removing bits.

    Before you do this,however,it would help if you knew what the symptoms of the failure were,as this might lead you directly to the problem.
     
  7. Johndon2000

    Thread Starter New Member

    Jun 12, 2013
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    Hi, thanks for that. The issue is there are 200+ IC's on the board, 200 Resistor networks, a few hundred capacitors & resitors so checking resitances at various parts of the board could be quite problematic.

    Unfortunately, there are no other diagnostics - it was taken out of a telecomms system and the fault was simply flagged up as "Faulty"!
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,783
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    I have had a fair amount of luck locating bad IC's using the 'how hot is it' method.

    I place the tip of my little finger on the IC's one by one while it is powered up in circuit and if I find one that feels very hot I swap out that IC. Sometimes it works. Depends mostly on what kind of IC it is. As a rule, most logic gate types may get warm but will seldom ever feel 'hot'. Op amp types may get very warm depending on the current demands of their output side. Complex large logic types, including micro processing dies can run cool to very hot. They will usually have a heat sink applied if they normally run hot, so...

    good luck
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,440
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    What happens if you have two or more faults on the board at the same time?
    Swapping one component at a time is not going to find the faults.

    If all of the ICs are in sockets your chances are improved.

    Personally, I usually diagnose a problem by looking for functionality, not by looking for a faulty component. For a complex board marked as "faulty" your options are very limited, i.e. it is destined to the trash heap or for salvageable components.
     
  10. Johndon2000

    Thread Starter New Member

    Jun 12, 2013
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    Thanks guys, I've used the "finger on IC's" method for finding faults with limited sucess!


    Unfortunately, all I have is "complex boards marked as faulty" but it can't be scrapped!

    Thanks agaiin :)
     
  11. Shagas

    Active Member

    May 13, 2013
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    Just out of curiosity , what is this ... board that you are testing?
     
  12. bug13

    Well-Known Member

    Feb 13, 2012
    1,208
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    for the 'how hot is it' method, you can replace your finger with compressed air spray, spray the board until it's frosted, the hot chip will de-frost first
     
  13. absf

    Senior Member

    Dec 29, 2010
    1,492
    372
    Do you have a schematic for this board that you're repairing?

    Is this board able to work off system? I mean just supplying power to it and have all the bus signals to be tested on a scope?

    I have repaired a digital board outside a Yamaha Electone just supplying +5 and 0V to the board. The Address bus and data bus are all dead and the IRQ pin on the Z80 is permanent Low. The fault was traced to a faulty 74HC367 and a bad tone operator chip.

    Allen
     
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