Very often when I attempt to repair electronic devices, either mine or from friends, I find out the problem is with the power supply which these days is most often a Switched Mode Power Supply (SMPS). Most of them are flyback, regulated, SMPS (but not all!). In this post I will be discussing the repair of flyback SMPS and a basic understanding of how flyback power supplies work is assumed. In my experience, most of the time the output electrolytic capacitors failed and if you are lucky this is all that needs to be replaced but sometimes this has lead to other components also failing. Electrolytic output capacitors fail a lot and SMPS designers need to catch up with this problem. These capacitors are being subjected to high currents of high frequency and are stressed beyond their designed capabilities. Not to mention that there are cheap, bad quality, caps being used out there. This has lead to the so-called capacitor plague. A very useful tool in checking and diagnosing electrolytic capacitors is an ESR meter. This tool allows for a quick check of all electrolytic capacitors without removing them from the circuit. I built myself an ESR meter based on this article which is itself a practical building of a design published by the Italian magazine Nuova Elettronica N212 as kit 1518. I made some modifications (which I think are improvements) to that design and the device has been very useful to me in repairing power supplies. An ESR meter does not measure capacitance but ESR (duh!) which is a very good indication of a capacitor's health and well-being. I recommend this tool and understanding how it works to anyone who will be repairing power supplies. Besides this one there are plenty of designs out there which can be easily and cheaply built by the amateur. One thing I like about this one is that it signals if the cap is shorted. This is very important because a shorted cap looks ideal with regards to ESR. The building of such an ESR meter would be a worthwhile project for anyone who does not own one. So, when confronted with a non-working SMPS the first step is checking all output capacitors and replacing any defective ones. Replace them with good quality, low ESR capacitors. As the hobbyist may not have access to high quality capacitors ad, especially, to ESR information, my advice is to generally use higher capacity than originally installed which means lower ESR and the cap generally being less stressed. Bigger capacitors will have smaller ESR than smaller capacitors. Two 1000 uF caps in parallel will have lower ESR than one 2000 uF cap. The (generally schottky) output diodes should also be checked and replaced if necessary. Up to here it is all very straightforward. At this point, if we are very lucky, the SMPS might work again but it may be that other components were damaged. The first one to look for is the switching element on the primary side of the transformer. Here things can start getting tricky because it will be mounted on a heat sink, it may be difficult or impossible to find a replacement, etc. Other components, including the control IC may be damaged as well. Sometimes the switching element and the control circuit are on the same IC like the STRT-G6153. If the switching element shorted then the fuse blew and maybe the diodes of the rectifier bridge also blew. These elements are easy to check and replace if necessary. If the switching transistor and/or bridge diodes were blown it makes sense to insert a lightbulb in series with the mains when testing because that limits the current and the damage should anything else still be wrong. I remember in my first attempts to repair blowing one transistor after another... So sometimes we get to the point where we have checked the output side and the input rectifier bridge and capacitor but the SMPS is still not working due to the switching element and/or other feedback/control parts. Now things can get tricky. We can test all components individually one by one but this can get tiring and may not be possible with all components. Many would have to be removed from the circuit. And the problem is that there is no way to test parts of the power supply because it need the feedback of the regulation. if something, anything, is not working, then with no feedback the power supply will just blow. So I am considering building a tool which would have certain parts of a power supply and which could be connected to a power supply under test. This tool would provide some of the functions like the feedback and the switching element so that it could temporarily replace those parts of the SMPS being repaired. Maybe even provide manually adjustable duty cycle switching. The idea is to provide external, protected, DC voltage to the primary side and a switching element which can be adjusted in duty cycle (I can see a 555 coming). Plus a load on the secondary side. The idea is something like this: with voltage on the primary and load on the output side we slowly start switching pulses which start from zero duty cycle very slowly up and see if power is being transferred to the load. Feedback could be used to adjust the duty cycle. An isolation transformer would isolate from the mains and a lightbulb in series would limit the current (and the damage). This tool would be designed for SMPS which have mains voltage input and low voltage output (say anywhere between 5 to 25) which seems to cover most cases. I am working on this idea which is intended to be extremely simple and, hopefully, useful. All comments are welcome.