© Mike Konopka

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 first.

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.

The pulses in the "raw" DC are called ripple, which are responsible for power supply hum. The smoothing of this ripple is the job of the filter capacitors. These large caps are typically large electrolytic cans. Some power supplies also feature voltage regulators and voltage trackers and many other circuit variations to provide for bi-polar supplies, etc. Again, power supplies present a shock hazard, so unplug the unit, discharge the filter caps as detailed above before replacing any components or measuring their respective values.

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