D Yuniskis wrote in message
...
Hi Greg,

GregS wrote:
In article , D Yuniskis
wrote:
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".

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.

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?

I often use light bulbs on the mains to limit current.

Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)


I happened to be playing with the innards of a domestic PIR unit for hotspot
"divining". Not too sure whether it would be useable. Uses RPY96 6 to 14
micron pyro and LM324 (.8 micron is red light) . Monitoring an lm324 output
for a small component 8 degree C over ambient, gave about 50mV swing up and
down of a nominal not too steady quiescent long term level. So if static at
2.3V would swing up to about 2.35 and then down to about 2.25 with a time
constant of a second or so , so again inconvenient. You have to scan across
as not a static process of generation of signal. There is a piezo effect as
well with the sensor so you would have to be very gentle in movement to
monitor 1 or 2 degree C over ambient. I shrouded the TO5 sensor with a 1.5
inch long dense foam rubber tube but matt metal maybe better, any thoughts?