Page 73 - Troubleshooting Analog Circuits

60 5. Preventing Material and Assembly Problems

occurs only rarely but is driving them to distraction. One of the managers stands
thoughtfully for a while and then reaches over and warps the main board: Scrunch,
scrunch. To the horror and shock of the project engineers, the scrunching correlates
with the temble intermittent problem. When the main board’s DIP sockets were
replaced, the problem disappeared.
Like a faithful dog, a socket or connector is expected to do its job without ques-
tion, and it usually does. However, on the rare occasion when one does go bad, the
connector usually becomes intermittent before it fails utterly. Fortunately, many
engineers and technicians learn early on that the way to check an intermittent
problem is to make it reveal itself when the board’s connector is wiggled and jiggled
and plugged in and out while the power is on.
But don’t all the instruction books say that you shouldn’t plug in the board with the
power on? Sure, a lot of them do. But I’ve never gotten into more trouble plugging a
board into a hot connector than the trouble I’ve gotten out of. There may be some
boards that are destroyed or damaged by this method, but they are in the distinct mi-
nority and should be studied. One way to help avoid problems is to make the ground
fingers on a printed-circuit edge connector stick out longer than the other fingers.
Thus, ground will be established before any other connection. Still, if you have a
board that tends to latch up because the power-supply sequencing may be improper,
you have to be prepared to stop plugging the board into that hot socket-fast.

Learn by Fiddling and Tweaking
There are many situations that can foul things up, but we all tend to learn more from
fiddling around with things, tweaking and unplugging, than by purely cerebral pro-
cesses. Once I had a technician who thought that DIP sockets should not be secured
in place by tack soldering, but by glue. This technique worked fine for a while, but
occasionally the sockets would act like an open circuit on one pin or another. To
solve the problem, we used an old technique: We traced the circuit coming into the
IC, and it was fine. We traced the signal coming out of the IC; nothing. Then, we
traced the signals on the pins of the DIP itself; the signals were not the same as the
signals on the socket, not at all. I finally realized that the glue was getting into the in-
ternal voids of the socket and preventing the IC’s pin from making a true connection.
We banned the glue from that task, and the problem went away, mostly. Still, both
before and after that time, we have seen sockets that just failed to connect to an IC’s
pin. You merely have to probe to the pin of the IC itself, not just to the socket, to nail
down this possibility. Sometimes, the pin goes into the socket and actually fails to
connect; but, more often than not, the pin is simply bent under the package.
There is one other kind of problem you can have with a socket, as a friend of mine
recounted. He was trying to troubleshoot a very basic op-amp circuit, but its wave-
forms did not make sense. After several minutes, he turned his circuit over and real-
ized he had forgotten to plug an op amp into the socket. This example leads us to
McKenna’s Law (named after an old friend, Dan McKenna): “You can’t see it if you
don’t look at it.” We invoke this law when we discover that we forgot to plug in a
line cord or connect something. A vital part of troubleshooting is realizing that we are
all at the mercy of McKenna’s Law when we get absent-minded.
Connectors and sockets usually do more good than harm. They permit you to
check options and perform experiments that may seem absurd and preposterous, yet
are Instructional and life-saving. Once a friend was in the throes of a knock-down-
drag-out struggle to troubleshoot a fast AD converter. He had tried many experiments,
but a speed problem eluded him. He asked me if he should try a socket for a critical