Phil Allison wrote:

"D Yuniskis"

I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

** So this fuse was not being stressed all the time - ie bending or faintly
glowing ?


The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.


** So ONE of these loads was intermittently drawing high current ?


After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?


** The good, old fashioned way is to use a larger fuse and wait for
something to get hot or start smoking.

Dunno about you, but I use a current meter in the AC supply for all bench
service jobs - it is sensitive enough ( 1mA resolution) to observe small
changes in the AC current draw and indicate if the current is creeping up.
I also keep a book recording the normal current draws ( of and on load for
amplifiers ) of everything I see.

this is an interesting practice- logging such info.