Arfa Daily wrote:



In short, the output electros in a SMPS are critical to it's operation and
in many cases there is nothing to prevent the output voltage going high
when they wear out.

I have seen serious damage done to 5V logic when this happens.



David - see Phil's answer regarding the choke being first. And +1 on his
observations about serious damage occurring when a cap goes high ESR and the
control loop 'lets go' in its efforts to correct for what it sees as the low
output voltage. Notable among these cases are cheap Chinese DVD players and
set-top boxes, but I have also seen it happen on equipment from what you
would normally consider to be 'reputable' manufacturers ...


** The examples I had in mind were Yamaha Multi FX processors like the SPX90 and a few relatives.

The SMPS was on its own PCB and there was enough heat to dry out the electros on the DC rails - particularly the 5V one. That rail could rise to nearly double voltage and take out a whole bunch of 74LS ICs and others that were strictly 5V.

The same PCB had the dreaded "yellow glue" dobbed all over the place eating the leads off small electros, zeners and resistors in the control loop.

After a full clean up and fitting all new electros, I added a SCR crow bar to the 5V output to protect the main board in future.



..... Phil