This failure is caused by multi-S4 bad damage. Open the case, you can see the fire selector assembly timer control panel. First check whether the plug terminal S4 oxidation or off. If so, clean-oxide, with a sharp nose pliers to adjust the plug wire terminal crimping force posted after re-seated plug wire terminals, namely failure to exclude. If there is a problem internal switch S4, respectively, with a multimeter two pen touch timer switch terminals, turn the timer knob several times, the pointer

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