On Saturday 22 Aug 2015 12:47:52 Alan McKinnon wrote: > On 22/08/2015 13:25, Mick wrote:
> > Then try a replacement PSU if you have a spare one, or take your > > multimeter and measure the output, checking for lower voltage values and > > fluctuations. If you get bad measurements, then take your soldering iron > > out and for a few pence inspect and replace any domed, or all capacitors > > on the secondary (output) side. > > <nitpick> > A multimeter is not really a valid test. If say the 5V rail is dodgy, > then the output will still be a solid 5V. What's happening is that the > PSU regulator circuitry can't keep up so the output averages 5V (that's > what the transformer gives out) with large amounts of high-frequency > ripple superimposed. Your multimeter average's that out and displays ... > 5V! When things get really bad the output may dip momentarily when load > is drawn, but by that stage the PSU has been struggling for a long time > already. > > Use an oscilloscope instead, and you see immediately what condition the > output is in. > </nitpick> Valid nitpick, esp. if an oscilloscope is available. Anecdotally, I have seen the amplitude of the ripple almost double *after* a repair than before. Admittedly, I think I used a capacitor with higher voltage rating, because that's all I could find at the time. Nevertheless, the PC worked fine after the repair, because the voltage output was at the right value and would hold steady under load. BTW, I've seen voltage values look reasonable when not connected to a load and collapse when load is applied. > Few IT techs just happen to have an expensive oscilloscope just lying > around, so a good recommendation is to replace the PSU anyway every 2 > years or so - more if the thing runs hot. I consider these as wearing > items, sorta like oil filters Yes, most electrolytic capacitors 'wear out' as time passes and drift from their original tolerance, which is quite wide to start with. I have repaired half a dozen of PSUs over the years with good results and unless I have a spare PSU available I resort to replacing the capacitors. It used to be the case that PSUs with (Japanese made) Hitachi caps could be relied upon for a build, but I don't know what comes out of China today. -- Regards, Mick
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