Slightly OT: Old CRT displays vs. new LCD

Tony Duell ard at p850ug1.demon.co.uk
Thu Dec 6 15:19:16 CST 2007


> 
> 
> On Wed, 2007-12-05 at 23:25 +0000, Tony Duell wrote:
> 
> > The fualt I was thinking of was actually a leaky capacitor, part of the 
> > RC filter network on the A1 supply. It was pulling the supply down, of 
> > course. The series reisstor was fine in this case, and yes I have had 
> > high-value resistorrs go even hihgher, or open.
> 
> It seems particularly common with highish value resistors (more than a
> few hundred k) used at high voltages, even if the current through them
> is fairly tiny.

Indeed it is. Another example is the startup resistor in many small 
SMPSUs. Typically a few hunderd k in value, and may be 2 or more 
resistors in series, it goes from the +ve side of the rectified mains 
smoothing caapacitor to a point in the chopper control circuit, and 
provides current to get the thing started when you turn it on. It often 
fails, resulting in a supply that runs fine, but when you turn it off (or 
the mains fails), it won't re-start. I've fixed _dozens_ of such supplies 
by lookking for a reissotr matching the description and replaicnig it if 
open-circuit.

-tony



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