Switch Mode Power Supply Capacitor Replacement

Here is a classic fault, a switchmode PSU that has stopped working.

What is the correct way to repair this PSU?

Easy…

Clean and a “cap change”, or changing the electrolytic capacitors.

Over time the electrolyte dries up in capacitors, and their value in Microfarads can go up or down, and most importantly their ESR (effective series resistance) increases.

Excess heat, and failed cooling fans all contribute to failed capacitors because of overheating of the unit.

The ESR going up, means that when running at high frequencies the capacitor isn’t doing its job, and in some cases may as well not be there.

Any ripple, or other undesirable waveforms will not be smoothed out or changed, as the capacitor cannot react to the rapid changes in waveform, because the internal resistance has gone up due to the electrolyte drying out, leaking, or breaking down.

This effects the characteristics of a circuit, and can cause the switching transistors to stick on and short out, stopping the power supply from oscillating, and running.

What is the result? Failed equipment that will not start up, or it blows up while in use.

Firstly fit a nice new fan to the casing, because the old one had failed, and this had helped the unit to overheat, which would cook an dry out the capacitors.

The main driver transistors, and IC’s were checked with a curve tracer, tracker, or Huntron equivalent component tester, where a trace of a right angle usually indicates a good semiconductor junction.

A multimeter proves the justions are OK with a reading of around .3 to .7 depending upon the semiconductor junction type.

Having proved in this case that the semiconductors appear to be OK, we can carry on with the repair, and change the capacitors.
The old capacitors shown here removed, showing how they have leaked out electrolyte, and the ESR measures low on some of them because they have dried out. The ESR meter hardly registers on some, showing they were “worn out”.

The PCB has leaked electrolyte on it from the blown capacitors; this will need cleaning off before we continue.

I like to kink the legs on the capacitors with a kinking tool; this helps them grip the PCB when turning it over for resoldering, and it also lifts the capacitor off the PCB’s surface to allow more airflow, and to help them run cooler, and last longer.

We also fit 105 degree C caps instead of 85 degree C, because they last longer. Low Impedance, low Z, or low ESR capacitors have been used as well, because these are better quality, and last a lot longer.

A lot of capacitors on the market, or already in equipment aren’t very well made, and are cheapened to keep down costs.

We think this is false economy, so put the best quality in that you can get, and you’ll get the best performance and life as a payback!

The new capacitors have now been fitted, raised off the PCB by leg kinking.

See the one capacitor lifted up for better airflow, and the PSU is all cleaned up nice, and as new.

Another angle showing the mounting of the capacitors, and how clean and nice the PSU is inside the casing, to maximise airflow for cooling and efficiency.

The main DC bus capacitors have been replaced as well, because these had got very hot, and the bleed resistors changed from 25K to 33K as they had overheated and cracked. They are only to discharge the high voltage from the bus caps when the unit is switched off, so 33k will be fine.

A close-up showing a small electrolytic capacitor mounted proud from the PCB, this looks better, and protect the PCB from leaking electrolyte, besides helping the capacitor to last longer by creating a heat break between the PCB and itself, and so it runs cooler, and the life is dramatically increased.