Some manufacturers have inexpensive flat rate service policies for power supplies. If you are not inclined or not interested in doing the diagnosis and repair yourself, it may be worthwhile to look into these. In some cases, £25 will get you a replacement supply regardless of original condition. However, this is probably the exception and replacements could run more than the total original cost of the equipment – especially as in the case of most TVs and many computer monitors, where the power supply is built onto the main circuit board.
Nothing really degrades in a switchmode power supply except possibly the electrolytic capacitors (unless a catastrophic failure resulted in a total meltdown) and these can usually be replaced for a total cost of a few pounds. Therefore, it usually makes sense to repair a faulty supply assuming it can be done reasonably quickly (depending on how much you value your time and the down time of the equipment) and, of course, assuming that the equipment it powers is worth the effort. Most replacement parts are readily available and kits containing common service components are also available for many popular power supplies (such as those found in some terminals, Macintosh and other Apple computers, various brands of video monitors, and some TVs and VCRs).
Where an exact replacement power supply is no longer available or excessively expensive, it may be possible to simply replace the guts if space allows and the mounting arrangement is compatible. For example, for an older full size PC tower, the original power supply may be in a non-standard box but the circuit board itself may use a standard hole configuration such that an inexpensive replacement may be installed in its place.
Alternatively, many surplus electronics distributors have a wide selection of power supplies of all shapes, sizes, output voltages, and current capacities. One of these may make a suitable replacement for your custom supply with a lot less hassle than attempting to repair your undocumented original. It will likely be much newer as well with no end-of-life issues like dried up electrolytic capacitors to worry about. Of course, you must know the voltage and current maximum current requirements of each of the outputs in order to make a selection.
For the specific case of SMPSs for standard computers (PC, Macs, workstations, servers), it often doesn’t make sense to spend much time or money on repair. The cost of replacement of power supplies for PCs in particular is so low, that just buying a new power supply may be the best course of action. Furthermore, the risk of a faulty repair causing expensive or fatal damage to the mainboard and peripherals including total loss of all data stored on disk, makes repair a risk unless thorough testing can be performed before installation. However, it won’t hurt to check for obvious problems like bad connections. Put the dead one aside and considering trying to repair it if there isn’t anything better to do. Realistically, this will be never. 🙂
Whatever your decision, if you’re not confident, always seek professional help!