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replacement procedures for stud-mount and flat-pack thyristors and diodes are virtually identical, with the difference being that thyristors will have two addi- tional 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 follow- ing 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. Stud- mount 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 (thyris- tor). 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. 5. Spread a small amount of thermal compound on the new thyristor, tak- ing care not to get any compound on the thyristor threads. 6. Insert the stud in the heat sink, reassemble the flat washer and the star washer, and then tighten the retaining nut. 7. Attach all leads to the proper locations, being sure that all connections are clean and tight. Flat-pack thyristors and diodes, sometimes referred to as "hockey pucks," are used in higher power rectifiers. They range from 2 through 4 in. in diameter. As with the stud-mount devices, the only difference between a flat-pack thyris- tor and diode is the presence of gate and cathode leads on the device. A flat-pack device is secured between two current-carrying bus bars by a clamping mechanism. Some clamps have indicators built in, whereas others do not. When replacing a device secured with a gauged clamp, note the reading before removing the device. The other type of clamps used are either 5,000- or 10,000-lb clamps. These sys- tems consist of a pair of clamping bars, connected by two studs, between which is sandwiched the bus bars, a Belville washer system, and the semiconductor device. Replacement of thyristors or diodes utilizing these types of clamps requires the use of measuring devices. The following steps should be taken to replace a flat- pack thyristor or diode (refer to Fig. 8). 1. Note the clamping arrangement being used. If a gauge is present on the lamp, record the indication. Mark and remove the gauge and cathode leads if replacing a thyristor. 2. Uniformly and slowly loosen the nuts on the clamp studs. Remove the 647