Electronics > Repair

First Step in Fault Finding - Observation and Confirmation

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Basic Principles of Fault Finding / Fault Diagnosis or if you like, Repairing Stuff -

First Steps: Observation > Confirmation > Disassembly > Re-confirmation


- Before plugging the unit into the Mains or fitting Batteries a few basic observations should be carried out.
Check for any physical damage, ingress of liquids, infestation of insects, dampness, corrosion, arcing and the general state of the unit. Check the power cord for any signs of damage, cuts, burns, arcing - Basically ask the Question "can it be safely used to power the unit" If the unit is Battery powered are the batteries in good condition, the right type, corroded, flat. Use a fresh set of Batteries if in doubt. Check the Battery compartment for signs of corrosion, leakage into the unit and the condition of each battery terminal.


- What is the fault, what should we be looking for.
Download or Consult the Instruction Manual and familiarise yourself with the correct operation of the unit. Confirm the description of the fault, is the fault related to the incorrect setting of the controls? Check other functions to see if they are also affected. If Mains and Battery powered check the operation on both power sources. Is there a Default mode, try defaulting the unit to factory settings. Basically try to gain an overall understanding of how the unit should correctly operate and what each control does and how it affects the fault you see. Write down what you see and what is faulty; if that helps you to remember later.


- Take your time and clear and prepare your work surface, find a couple of containers for screws.
Remove the Power cord and/or Batteries. If the unit contains high voltages it may pay to wait 5 to 30 minutes for the voltages inside to drop down to a safe level. Identify and remove screws noting where each one went, just in case they are different sizes, lengths or types (Self tapping or Machine screws). If the case won't open or the covers won't come off, don't force them! instead check for hidden screws under a label or clips that may need to be released first. Try and remove covers without jolting the unit severely as if the fault is intermittent you may inadvertently fix the fault, which could make the next steps harder.


- It is important at this stage to recheck what you observed in the original confirmation of the fault. Because the unit is now open there are some extra considerations.
If the unit only connects to the AC Mains, powering it from an Isolating Transformer or RCD would be recommended for your protection. If Battery power is available think about just powering it from the batteries. Basically think about what dangerous voltages could be inside, is it safe for you and others around you like curious children or pets. Know your limitations and the limitations of any safety devices you use, like Isolation Transformers and RCD's.
Position the unit so there is plenty of room around it. Check again that it is safe to apply Power. OK? Now apply power and recheck that the original fault is still there. Did removing the screws or covers change the fault or is there a different fault?

Any changes, additions are welcomed below

My Dim Bulb Tester

Just a incandescent bulb wired in series with the DUT, and good to have a small range of bulbs.

This one is switched SPST, but in countries without a polarized main plug I would recommend a double pole switch
As is evident it was knocked together at no cost in a dead PC ATX case, using the existing IEC mains socket and a few "bits and bobs".

On power-on the bulb should momentarily flash bright, then dim after inrush currents subside.(if all is well).
If the bulb remains bright and the bulb wattage is appropriate for the DUT then "Houston, we have/still have a problem.  :'(

If you have a DUT that has emitted the magic smoke or continually blows fuses this can be indispensable.

Yes a section for basic test equipment would be worthwhile, especially if it is something that can be easily made at home.

A few ideas to stimulate thought......

This post refers only to low frequency general fault finding and testing.  ;)

While scope hook probes are great, there will be times when they just won't do the job.  :rant:

Soldering test wires to the DUT can be OK occasionally, but time consuming and frustrating if there are several POI.
If you are working on SMD, then you must use other methods to connect.

I use Tek grabbers, relatively cheap and versatile, but be mindful of their 40 V max rating.
They seem fine for all the SOIC work I have done.  :-+
They have 25 thou pins and one can easily knock up a selection of leads, with a tinned loop.
Have a rummage through your junk pile for wire with the right connectors, knock the shells off and shrink sleeve.  :)
To have enough for a 4 Ch scope you would want 5 as a minimum IMO.

And if you have more POI's then you could knock up a little test probe header like this.

This one is from cheap grabbers off eBay, good quality wire and the header from a 10 mm thick mains switchboard off-cut. Each cable is finished in a loop and glued in a hole, and recessed for safety. The hole is large enough to accommodate a scope hook probe or the reference (GND) lead clip.
If I built another, I would use better grabbers and white nylon/plastic for the header to better display the grabber colors that were put on with a Sharpie or highlighter.  ;D


--- Quote from: tautech on June 30, 2014, 01:15:58 am ---My Dim Bulb Tester

If you have a DUT that has emitted the magic smoke or continually blows fuses this can be indispensable.

--- End quote ---

A slightly modified version of mine:


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