In R. C. White wrote Tue, 24 Apr 2012 13:20:46 -0500:
So that is why they started to make them keyed so you couldn't plug them
in wrong. ;-)
I remember a hardware course where we had a final practical where
things might go wrong. I think the instructor probably gave every
team a defective item. Well, except for us. We got two of them. Just
a bit of excitement until we figured out what was going on.
Same here. And the instructors were very creative. Removed pins from
sockets, clipped components from circuit boards, etc. And once we were
paired off in twos. And one of the DC voltages was missing this time.
First thing my partner checked was the fuse. He pulled it out looked at
it and put it back in.
And I asked him if it was good and he said yeah, it said so. I said
what? I pulled it out and there was a piece of paper inside of the glass
tube that said GOOD. He said see! I said that doesn't mean anything and
I ohmed it and it was wide open. ;-)
I also knew this one guy in the military that couldn't troubleshoot very
well at all. But he did have an unique way to troubleshoot something
that blows fuses. As he would jumper around the fuse and then watch it
smoke. Then change all of the burnt components. And I'll be darn, that
did work most of the time. Not the way I troubleshoot, but it worked for
him. ;-)
Once I stopped by my dad's house for something and I found out he bought
a pinball machine that one score counter wouldn't update. I was thinking
why on earth would anybody buy such a worthless arcade machine? Anyway
there must have been six boards in all and he had them hooked up
altogether on the workbench and standing there with a voltmeter. He
didn't know what to check and was very glad to see me. He asked if I
could check it. I really didn't want to spend the time at the moment and
there wasn't even a schematic, block diagram, or anything. And I thought
I had a good excuse to get out of it.
I told him they were TTL logic chips and without a schematic, block
diagram, or even a reference manual handy, I wouldn't know if the
signals on the pins were right or wrong. He asked if the electronic
store down the street would have TTL reference manual? I said maybe and
they aren't cheap. So he asked me to go check and they did have one left
for about 30 bucks.
So I checked signals and I really didn't want to mess with this now. But
it seemed important to my dad. And I quickly found one output stuck and
not changing. He said what does that mean? I told him either this chip
is bad or the input of the next chip was holding it there. And I would
have to disconnect the line between the two to know for sure.
He asked how much do they cost? I didn't know for sure but I said a buck
or two. He said just buy both and replace them both. Back to the store
again and both were under a buck each (I probably bought chip sockets or
used my own that I had in the trunk of my car) and now I didn't care
which one was bad (very unlike me). I changed both and it worked fine
ever since. It made him happy and while I really didn't want to get
involved at the moment, it really wasn't that hard to figure out.
Running back and forth to the store was the hardest part of the whole
job. ;-)