Unlike PC system boards where any disasters are likely to only affect your pocketbook, power supplies, especially line connected switchmode power supplies (SMPSs) can be dangerous. A set of safety guidelines has been provided by Rectifier Technologies to be followed whenever one is working on line connected power supplies as well as TVs, monitors, or other similar high voltage equipment.
Having said that, repairing a power supply yourself may in fact be the only economical option. It is common for service centres to simply replace the entire power supply board or module even if the problem is a 25 cent capacitor.
It may simply not pay for them to take the bench time to diagnose down to the component level. Many problems with switchmode power supplies are easy to find and easy and inexpensive to fix. Not all, but surprisingly many.
This document by Rectifier Technologies will provide you with the knowledge to deal with a large percentage of the problems you are likely to encounter with the common small switchmode power supplies found in many types of consumer electronic equipment including PCs, printers, TVs, computer monitors, and laptop or camcorder power packs. It will enable you to diagnose problems and in many cases, correct them as well.
With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text.
Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair centre - assuming they would even bother.
Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions. It will also be easier to do further research using a repair text such as the ones listed at the end of this document.
In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.
Most common problems
The following probably account for 95% or more of the common SMPS ailments:
- Supply dead, fuse blown - shorted switchmode power transistor and other semiconductors, open fusable resistors, other bad parts. Note: actual cause of failure may be power surge/brownout/lightning strikes, random failure, or primary side electrolytic capacitor(s) with greatly reduced capacity or entirely open - test them before powering up the repaired unit.
- Supply dead, fuse not blown - bad startup circuit (open startup resistors), open fusable resistors (due to shorted semiconductors), bad controller components.
- One or more outputs out of tolerance or with excessive ripple at the line frequency (50/60 Hz) or twice the line frequency (100/120 Hz) - dried up main filter capacitor(s) on rectified AC input.
- One or more outputs out of tolerance or with excessive ripple at the switching frequency (10s of kHz typical) - dried up or leaky filter capacitors on affected outputs.
- Audible whine with low voltage on one or more outputs - shorted semiconductors, faulty regulator circuitry resulting in overvoltage crowbar kicking in, faulty overvoltage sensing circuit or SCR, faulty controller
- Periodic power cycling, tweet-tweet, flub-flub, blinking power light - shorted semiconductors, faulty over voltage or over current sensing components, bad controller
In all cases, bad solder connections are a possibility as well since there are usually large components in these supplies and soldering to their pins may not always be perfect.
An excessive load can also result in most of these symptoms or may be the original cause of the failure. And do not overlook the trivial: a line voltage select switch in the wrong position or between positions (possibly by accident when moving the supply, particularly with PCs), or damaged.