Page 22 - Troubleshooting Analog Circuits

Troubleshooting by Phone-A Tough Challenge 9

ilar projects. Sometimes I hang a copy on the wall, to warn all my friends. Some-
times I send a copy to the manufacturer of a component that was involved. If you
communicate properly, you can work to avoid similar problems in the future.
Then there are other things you can do in the course of your investigation. When
you find a bad component, don’t just throw it in a wastebasket. Sometimes people
call me and say, “Your ICs have been giving me this failure problem for quite a
while.” I ask, “Can you send me some of the allegedly bad parts?’ And they reply,
“Naw, we always throw them in the wastebasket . . .” Please don’t do that, because
often the ability to troubleshoot a component depends on having several of them to
study. Sometimes it’s even a case of “NTF”-“No Trouble Found.” That happens
more aften than not. So if you tell me, “Pease, your lousy op amps are failing in my
circuit,” and there’s actually nothing wrong with the op amps, but it’s really a misap-
plication problem-I can’t help you very well if the parts all went in the trash. Please
save them, at least for a while. Label them, too.
Another thing you can do with these bad parts is to open them up and see what you
can see inside. Sometimes on a metal-can IC, after a few minutes with a hacksaw, it’s
just as plain as day. For example, your technician says, “This op amp failed, all by
itself, and I was just sitting there, watching it, not doing anything.” But when you
look inside, one of the input’s lead-bond wires has blown out, evaporated, and in the
usage circuit, there are only a couple 10 krR resistors connected to it. Well, you can’t
blow a lead bond with less than 300 mA. Something must have bumped against that
input lead and shorted it to a source that could supply half an ampere. There are many
cases where looking inside the part is very educational. When a capacitor fails, or a
trim-pot, I get my hammer and pliers and cutters and hack-saw and look inside just to
see how nicely it was (or wasn’t) built. To see if I can spot a failure mechanism-or a
bad design. I’m just curious. But sometimes I learn a lot.
Now, when I have finished my inspection, and I am still mad as hell because I have
wasted a lot of time being fooled by a bad component-what do I do? I usually WID-
LARIZE it, and it makes me feel a lot better. How do you WIDLARIZE something?
You take it over to the anvil part of the vice, and you beat on it with a hammer, until
it is all crunched down to tiny little pieces, so small that you don’t even have to
sweep it off the floor. It sure makes you feel better. And you know that that compo-
nent will never vex you again. That’s not a joke, because sometimes if you have a
bad pot or a bad capacitor, and you just set it aside, a few months later you find it
slipped back into your new circuit and is wasting your time again. When you WID-
LARIZE something, that is not going to happen. And the late Bob Widlar is the guy
who showed me how to do it.

Troubleshooting by Phone-A Tough Challenge
These days, I do quite a bit of troubleshooting by telephone. When my phone rings, I
never know if a customer will be asking for simple information or submitting a rou-
tine application problem, a tough problem, or an insoluble problem. Often I can give
advice just off the top of my head because I know how to fix what is wrong. At other
times, I have to study for a while before I call back. Sometimes, the circuit is so com-
plicated that I tell the customer to mail or transmit the schematic to me. On rare occa-
sions, the situation is so hard to analyze that I tell the customer to put the circuit in a
box wiah the schematic and a list of the symptoms and ship it to me. Or, if the guy is
working just a few miles up the road, I will sometimes drop in on my way home, to
look at the actual problem.
Sometimes the problem is just a misapplication. Sometimes parts have been blown
out and I have to guess what situation caused the overstress. Here’s an example: In