8 Jan
2006
8 Jan
'06
1:51 a.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.
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.
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
when I realized the circuit wasn't supposed to work quite the way I had thought,
but even then I learned something I needed to know.
Dave
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.parse.com/~ddunfield/museum/index.html