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the transformer side of the thyristors, you can determine if the thyristor regulator part of the system is feeding balanced voltages to the main transformer. The oscilloscope is valuable when performing fast maintenance checks on a number of power supplies. The scope should be connected to the back of the out- put DC panel voltmeter. As the voltage control on the panel is increased, a waveform will appear that has six peaks and valleys for each cycle of the line fre- quency. Each period is 16.6 milliseconds long. If any of the six major peaks is miss- ing or the valleys are too wide, there is a serious problem in the power circuit that must be investigated further. BASIC TROUBLESHOOTING This section briefly describes some basic diagnostics to determine why a power supply is not operating properly. Before starting any diagnostic test on a power supply, you should obtain a copy of the electrical schematic drawings for the par- ticular equipment you are working on. On these drawings, you should be able to identify the basic functional areas that make up virtually any rectifier. The four basic building blocks of a power supply are the following: 1. Electrical controls 2. AC power circuits 3. DC power circuits 4. Electronic controls CAUTION: Only qualified personnel should attempt to service power supply equipment. Dangerous and lethal voltages may be present. The electrical controls provide simple low-power functions for the power supply. You will notice such items as push buttons (stop, start), pilot lights, relays, timers, limit switches, flow switches, thermal switches, thermal overlay relays (heaters), and other 120 V AC protective devices. These items are typically drawn in the familiar ladder diagram format. Diagnostics in this area will usually require the DVM to measure continuity or the presence of control voltages at var- ious components. To check for proper voltages at the low-power components, find the common on the ladder diagram and attach the voltmeter to it in the actual circuit. With the con- trol power energized, you will be able to check the AC controls on the ladder diagram and measure for the presence of an AC voltage at the corresponding point in the actu- al circuit. This method is most useful when there is a loss of control circuit voltage that prevents a portion of the controls from working properly. When the missing volt- age returns at a particular point in the circuit, this indicates you have just moved past the defective component, such as a contact, a terminal, an interlock, or a thermal switch. The faulty component can then be repaired or replaced. You may find there is more than one bad part; so be sure to test all of the low-power components. The AC power circuit is the portion of the power supply located between the AC input power terminals and the regulation thyristors at the primary of the three- phase power transformer (assuming a primary thyristor/secondary diode configuration). The components representing this AC power section are usually found near the center of the electrical schematic. The clamp-on ammeter is the diagnostic tool used in the AC power circuit. Place the ammeter around one of the incoming AC conductors. Operate the power sup- ply with no load and check that the magnetizing current of the main transformer 644