Service Manual Part 5

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Overvoltage protection is provided by diode D2 and fuse F2. D2 is a 16V zener transient

suppression diode that reacts instantly to overvoltage DC or spikes. If an overvoltage

condition persists, causing excessive power dissipation in D2, it will become short cir-

cuit, causing F2 to blow. F2 will need to be replaced before operation of the power sup-

ply is possible. D2 will need to be replaced to restore overvoltage protection.

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Transistor Q3 is mounted on the heatsink and its junction temperature therefore closely

follows that of both the heatsink and Q1. Q3 is biased by R12 and RV2. As the tempera-

ture of the heatsink increases, the temperature of the base- emitter junction of Q3 also

increases, which results in the V

of Q3 decreasing. At 95°C, the base-emitter voltage is

exceeded by the biasing voltage, and Q3 switches on. As a result, Q4 turns off, and the

‘deadtime pin’ (pin 4 of IC1) is pulled high. The ‘deadtime’ is now 100%, which means

Q1 is completely turned off.