by Bryan Bergeron, Editor by Bryan Bergeron, Editor
Good Things Come With
A good friend drops by your place and, knowing that
you’re an electronics enthusiast, asks for help with a new
handheld gadget that suddenly stopped working. Eager to
lend a hand, you check the batteries and try the reset
button, but the device fails to respond. Next, you expertly
pop the case and find a tiny circuit board populated with
a dozen solid-state components. However, most of these
components are unmarked. Moreover, a thorough search
on the web fails to reveal a schematic. With your friend
looking on, you suddenly feel dread and powerless to
help. Sound familiar? It shouldn’t and needn’t be.
If we’re talking about a $5 device, then the practical
solution is to simply buy a new one. But sometimes there
are impractical issues involved: the challenge of
diagnosing and repairing a piece of electronics; the
sentimental value of a device; or the need to validate your
expertise in electronics to others. Whatever the reason, if
you’re faced with the task of repairing the circuit, don’t
forget to leverage the signposts staring you in the face —
namely the component packaging.
Gnd Out In
E B C
B C E
Fortunately, many leaded and SMT components use
standard packaging. For example, in the diagram, you can
see that SMT transistors in the Small Outline Transistor
(SOT)- 23 packaging have standard lead designations.
Facing the top of the package (with the single lead on
top), the base lead is in the lower left lead; the emitter is
the right lower lead. The collector is the lone top lead.
If you need more information — such as whether the
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device is PNP or NPN — it’s a simple matter of using your
ohmmeter to identify the polarity of the individual
junctions. Recall that with an NPN transistor, the emitter-base junction conducts when the emitter is negative,
relative to the base. Alternatively, you can monitor the
voltages on each lead, looking for forward and reverse
bias conditions to identify transistor types.
The SOT-89 package is often used with voltage
regulators. As shown in the figure, ground (Gnd), input
(In), and output (Out) are standard leads, from left to
right. The tab on top is connected to the input lead. It’s a
simple matter to check the voltage input and voltages of
the device to help determine whether it’s working. If
there’s no output with an input, then the component may
have failed or there may be a short in the output circuit.
Many voltage regulators are short proof.
The same diagnostic walkthrough applies to
transistors in leaded TO-92 and TO-220 packaging. Armed
with lead designation and transistor type, a board full of
components suddenly takes on new meaning. You should
be able to sketch out a simplified schematic and have a
good idea the area of the board responsible for the
failure. All is not rosy in the land of SMT components, and
there are some formidable challenges associated with the
latest SMT devices, namely lack of markings because
there simply isn’t space on the device to write anything
legible. As it is, I’m forced to rely on a 10X scope when I
work with a board populated with SMT components.
Another challenge of working with SMT components
is simply getting a probe on a specific lead without short-circuiting the device. To this end, I recently picked up a
pair of needle-tipped probes by Fluke on eBay. I’m
dreading the day when I impale myself with one of the
probes, but they allow me to make measurements of SMT
components that would have been impossible with
Assuming you’re successful in identifying the
defective transistor or other component(s), the next step is
to replace the component. For this, I rely on Digi-Key
( www.digikey.com) and Mouser Electronics
( www.mouser.com). A short blast of air from a hot air
pencil removes the defective component from the board.
If you prefer, a standard soldering iron with solder wick
works just about as well.
Taking the time for shipping the replacement
components into account, give yourself about a week to
repair anything substantial. Not bad, when you consider
most electronics devices are simply tossed in the trash at
the first sign of trouble — to the detriment of the