Mark Lawton's Generalist's Repository

This is a basic guide to repairing an electronic PCB; they come in all shapes and sizes, but many faults are basically the same.

I take no responsibility for injuries and errors from people ”having a go themselves”. Remember electrical safety when working on more than 30V AC or DC, and certainly when working on line voltages or mains voltages. Be careful on Switch-mode power supplies,  inverters for laptop/TV display back-lights, and TV line output circuits etc.

Always know what you are doing first, and isolate circuits for safety when testing and repairing them.

Visually inspect the PCB for any carbonization, or blackened burnt parts on it, also burnt areas, and areas that have been getting hot. This is an indication of bad or loose connections, solder joints, or failing overheated components.

The trouble on the above picture is; “how will I identify those resistors?” Well, sometimes it is a matter of comparing one area with another doing the same job; sometimes the components still measure sort of “OK”, and sometimes it has to be an “intelligent guess”, based on what circuits doing similar jobs you can find online. Obviously if you have the drawings, then it is easy, but if not, and the resistors are truly burnt out, then you need a comparison PCB, or you will be stuck! One way, (a last resort if it is safe to do) is to fit a couple of variable resistors, and start very high, and then work your way down, checking the current drawn, and the values at which the PCB bursts into life.

Any carbonised tracks will have to be drawn out on paper, and reverse engineered first to get it right. Then the carbon is drilled out, and the missing area can be made good by using epoxy resin, and/or a piece of blank (no tracking) PCB.

Either with bare wires, or a track repair kit (preferred), the PCB can be re-drilled, and rewired. Make sure the soldering is good, and soldered on both sides, especially if there are no “PCB Vias” or “lace-holes” to go through with the components.

Kink the legs of the components to stay proud of the PCB, but not too high up, just enough to allow good soldering on both sides of the PCB, and to help in heat dissapation, because air can no flow around the component better.

The lead forming pliers shown above with the formed component are ideal to form components to be inserted. They stay in place when the PCB is inverted for soldering the component into place.

On a “standard repair” I would firstly clean up the PCB by washing it in safewash,, or solvent cleaner. Then after drying it I would change all of the small electrolytic capacitors. Next I would replace any “tired” or old looking saggy fuses and fuseholders if required. Next I would remove and test any relays and transformers for correct operation. Moving on I would then use a tracker or “curve tracer” to check the transistors, FET’s, IGBT’s, diodes and Zener diodes etc. After this then measure any doubtful looking resistors, and check all of the soldering.

Finally I would check what IC’s I could check, mounting them on sockets if practical, and any opto-isolators (opto-couplers could be tested on some ”breadboard”.

After this the PCB can be cleaned of flux residue, and powered up if this is possible for functional testing to a greater of lesser degree.

The most common PCB faults are loose joints, bad soldering, and bad connectors, followed by poor capacitors that have dried out.

It is worth buying an ESR meter to test capacitors “in-circuit”, (with the power off!), and you’ll be surprised at how many are duff, or “high ESR” and no good, not even reading at all!

An ESR meter is well worth buying or making (project kit)

Change them EVERY time! and TEST large “bus” or PSU capacitors “to be sure”. One common fault on switch-mode power supplies is that the “47UF” “startup” capacitor is faulty, CHANGE this and you’ll find a surprising number of power supplies will burst into life!

However, I recommend changing ALL of the capacitors in a power supply “to be sure” you have nailed it!

Have a look at my PSU repair link below, showing a “classic cap change”, or a capacitor replacement on a switch-mode power supply.

http://www.mark-lawton.com/marklaw1/switch-mode-power-supply-capacitor-replacement/

Good luck with the repairs! Change those caps as well; don’t forget to ESR check them to show the problem up on some!

A high reading on a 47UF capacitor for example shows trouble.

Look out for my next post about the ESR meter and how to use it. It will be your “right arm” in repairs to PCBs, and other equipment. “don’t go without it!”

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