Metal Finishing Guide Book


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the firing circuit. Then measure the signals at the gate outputs to the thyristors with the DVM. They should typically be about 1 V DC. Perform these measurements with great care against shorting any of the leads to ground or to another pair of terminals, as there may be line voltages of up to 600 V AC between these terminals and ground. As with the drive circuit, if any signals are missing or incorrect, remove the board and bench repair using the procedures outlined in the operators handbook. BASIC REPAIRS Once a defective component has been located, it should be replaced with a part of comparable quality and ratings. It is especially important when replacing temperature sensors that the replacement have the same temperature rating as the original. Caution: Before attempting replacement of any component, ensure that the power is removed from the rectifier and that the capacitors are discharged. Electrical and Electronics Components Replacement of electrical components, such as push buttons, thermal switches, relays, and switches, as well as electronic PC boards, is relatively straightforward. Carefully mark all connections to the defective device before removal, replace with the correct item, and reattach the wires. It is also advisable to check the rest of the rectifier for clean and correct connections at this time. Thyristors and Diodes Thyristors are typically found in modular, stud-mount, and flat-pack configurations, whereas diodes are usually the stud-mount or flat-pack style. The replacement procedures for stud-mount and flat-pack thyristors and diodes are virtually identical, with the difference being that thyristors will have two additional small leads to be attached. The modular thyristor is the smallest of the three types and is typically found in lower power systems. The module contains two thyristors and has terminals for connecting gate and input/output leads. Mounting holes in the base allow attachment to the bus bar. To replace a modular thyristor, perform the following steps: 1. Note where the gate and input/output leads are attached. 2. Mark the leads and remove the thyristor. 3. Clean the bus bar surface and the new thyristor surface. 4. Apply heat sink compound sparingly to both surfaces. 5. Fasten the new thyristor to the bus bar. 6. Reattach the leads. Replacement is now complete. Stud-mount thyristors and diodes are no more difficult to replace. Studmount devices can be mounted on either air- or water-cooled heat sinks and are typically found with or in. diameter studs. Replacement of stud-mount devices is the same for both air- and water-cooled systems, following the steps below: 1. Mark and remove the two signal leads from the terminal blocks (thyristor). One of these is the gate lead, and the other is the cathode signal lead. 2. Remove the large braided cable. 3. Remove the nut and washers, and remove the device from the heat sink. 4. Clean the bus bar and new thyristor surfaces. 750

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