Last issue we talked about getting your electronic repair and
installation toolkit together. Now that you have all the right
goodies, you're probably anxious to tear into that piece of
gear badly in need of repair. But hold on a second! For the safety
of yourself and your broken giz, it's best to take a few precautions
Let's start with clearing off that large area of that mess
you call your workbench. That's right, toss out all those
half-eaten Big Macs, broken guitar strings, etc. Next, make sure
you're working on a non-conductive surface like wood, indoor-outdoor
carpet, etc. It's also helpful to have your test equipment,
meters, and scopes on a shelf above the bench for ease of viewing
and general neatness.
Now, unplug the device to be repaired before attempting to open
it up. Once open, your first task will be locating the power supply
section. It can usually be found near the area where the power
cord enters the chassis. It might also be near the on-off switch
or fuse. The power supply is easy to spot because of its proximity
to the power transformer, filter capacitors, and bridge rectifiers.
The power supply area represents a shock hazard, so stay away
from this area with your test leads, etc., until you have discharged
the large electrolytic filter caps. This can be easily done by
using a test lead and a screwdriver with an insulated handle.
Simply clip one end of the test lead to the screwdriver shaft
and the other end of the test lead to one of the screws or terminals
on one filter cap. Now touch the tip of the screwdriver on the
other terminal on the filter cap, which will short the cap and
discharge it with a slight spark. Repeat this procedure for each
power supply filter capacitor.
Now let's check out some power supply basics. Most electronic
circuits run on DC (direct current). However, AC (alternating
current) is what flows out of the wall. After the AC leaves the
wall socket and flows through the power cord, AC switch, and fuse
or circuit breaker, it then arrives at the power transformer.
Through inductance, the transformer supplies the approximate voltages
that the circuit will require via its multiple taps or outputs.
However, the output of the transformer is still AC. That's
where the bridge rectifier comes in. The bridge rectifier is a
configuration of 4 diodes that converts AC to pulsating DC.