22-12-2015, 03:31 PM
(21-12-2015, 04:01 PM)ppppenguin Wrote: Decouplers are not perfect. They have stray inductance and are not truly zero impedance. It's the designer's job to select suitable components. Some circuits can be very critical about their decoupling ...
Consider instability in a TV IF strip. There are a lot of decouplers in there and, on a wired chassis, may well have been fitted first to keep the wires as short as possible for low series inductance and, therefore, difficult to get at.
Yet it is unlikely that more than one has failed - but which one?
Easy - take another suitable cap - 1nF tubular ceramic, for example - and carefully hold it across each suspect in turn. But wait - you can't have long leads if it is going to be a guaranteed successful test and the wires are closer together than the width of your thumb and finger ...
So, what's 200V between friends?
I can't claim originality for this but here is something I made well over 50 years ago to solve the problem ...
All you need is a scrap of wood (mine was a glue block from an old cabinet), two panel pins and a 0.001µF ceramic capacitor - tubular for preference.
Knock the pins into the end of the wood block - the spacing should correspond to the PCB spacing* - and cut off the ends about 12mm with cutters to leave two points, then solder the cap between the two pins. Now you can shunt with immunity to shock!
* Of course, it works just as well in a wired chassis! IIRC in my original, the pins were arranged diagonally across the piece of wood to get sufficient spacing so, if you are using a smaller piece of wood than it shows in my drawing, that's the answer ...
