8 Jan
2006
8 Jan
'06
1:08 p.m.
Incidetnally, on most DMMs (including my Fluke),
the fuse is for the
current ranges only. The voltage and resistance ranges work fine with the
fuse blown. But of course there are plenty of other faults that can cause
a zero reading with a voltage on the input. Heck, even test leads can go
open-circuit.
Never assume this is the case - I've got a couple of rat-shack DMMs
where the fuse is simply in the positive meter lead - pop it and you lose
all functionality.
I never trust a meter to be working until I see it read something I expect
(I think this comes from the early days when all I had was a rat-shack
meter which used to get dirty contacts causing failures from time to time).
First thing I do when "entering" equipment is measure the power supply.
Yep, I always check the PSU (prefereably on dummy load) first, too. So
many problems are traced to PSU problems. I once spent a long time doing
battle with the Unibus logic in a PDP11/45, the fault turned out to be
that the PSU was sitting at 4.2V( should have been 5V, of course). The
darn machine would work most of the time, and then do some really odd things.
This both tells me that the supply is
"reasonable" and that my meter is
working (granted a double-failure could occur which happens to give me
a incorrect by reasonable reading, however this is not all that likely so I
usually don't get a second optinion unless I have cause to suspect the
PS).
At the beginning of a session measuring resistance, I touch the probes
together to be sure "zero is zero" - just habits I have gotten into, but they
have turned up problems on more than one occation.
Having been brought up on analogue meters where you have to set the zero
on the resistance range by hand, I do this too. Force of habit.
Just as, if I'm using the continuity beeper in my DMM, I tap the probes
together when I've selected it (just to check I've pressed the right
buttons). And then tap them together every so often thereafter, just to
make sure it's still working.
Something I was taught early on (in a different
context, but it applies
to electronics too). 'It is much easier to make measurements than to know
what you are measuring'. With most 'socpes, if you touch the probe on a
point in your circuit and twiddle the knobs you will get _some kind of
trace_. It may well mot be any use in actually tracking down the fault,
though. It may not even be a true representation of the signal at that point.
Not sure if this is a statement for or against, but my point is that by knowing what you are looking for, you have a better idea of
"if it looks right" or not.
Many times I found trouble because a signal was present but didn't match
what I expected. And lots of times I investigated and had a "eureka" moment
Oh sure. Many times there's _something_ on the testpoint, bnt it's not
what it should be.
-tony