Recommended logic analyzer/scope tools? (was: RL02 write faults,

Dave Dunfield dave04a at dunfield.com
Sun Jan 8 03:51:05 CST 2006


> 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




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