First check of each microwave connectors, door switches and circuit boards were not unusual . When a fault occurs in the measured voltage across the lamp lighting the furnace only 140V, measuring computer board U1 (MW09701G-V2) of {20 } { 21 } feet without +5 V voltage. Furnace lights, turntable motor , cooling fan is governed by the door control switches and relays RL1 , and RL1 is controlled by the foot of U1 {24 } . When not U1 +5 V operating voltage , it can not make RL1 work . +5 V voltage is provided by the Q1 (H945) ( local circuit see photo) . Measuring the input voltage of the collector of Q1 is 25V ( voltage measured after the normal circuit with a load of 19V), the base voltage of 5.2V, a problem obviously Q1 . Q1 measure under welding is good , but the fault itself is good times and bad , so replace the boot after Q1 , Q1 emitter measured +5 V output voltage is normal , microwave oven is working properly. Found that when very hot touch Q1 tube , the tube is to provide +5 V and other circuits U1 voltage regulator tube series regulator circuit , the voltage difference between the collector and emitter of 19V-5V = 14V, if Q1 is the load current 20mA, which consumes tube 0.28W, power circuit optional H945 only 0.25W, and did not add radiator, obviously insufficient headroom selected pipe , causing the pipe due to the long working temperature excessive damage , caused by the failure .KBMservice manual download site
Overhaul , the H945 can be jacketed pipe a heat sink , or use C8050, pipe 2N5551 to substitute other models in order to prevent damage to the tube again ; may also be a direct replacement for Q1 with a 78L05 three-terminal regulator circuit and external components .KBMservice manual download site