This machine volume, although no sound but there is noise. Shutdown cooling later on and return to normal, but shorter working hours and repeat the above phenomenon, each time I work time is short, finally will not lead to boot the grating. Grating disappear when measuring IC501 5 feet to greater than 0.7 V, 0.75 V shows that action of the internal protection circuit has, then measured voltage is 15 V has Q801 collector conduction, this means that protection circuit is started by Q801 action. At this point, the IC501 6 feet to 0 v and cut off the line drive signal. Check line output stage, promoted the level, + 12 v IC802 and tube circuit, all did not find out the problem. Lines measuring the output pipe and tube also is normal, replace the IC802 and line output transformer also of no help. Later, Q801 welding, cold boot still repeat the failure phenomenon, failure occurs when the measured again IC501 to 0 v, 5 foot 6 feet to 2 v, no action shows that internal protection circuit, line has been driving signal output. Measuring line drive pipe Q501 base voltage of 0.4 V, the collector 80 V, working properly. Measuring pipe Q551 base of 0 v, the manager has not plus the driving signal. Before measuring R560 to 0 v, also specify root cause to promote the interior of the transformer secondary line, occurred during start up time temperature hot short circuit phenomenon, make the line drive signal cannot be added to the manager, she had stopped work. At the same time produce circuit IC802 no 12 v + 12 v output, cause breakdown makes Q801 D502 conduction, start IC501 driven by protecting circuit and cut off the line signal. After replacement of line drive transformer, troubleshooting.