First check the mainboard CON5 + 5 v socket end and CON4 + 8 v end of power supply is normal, no problem with the power supply circuit. According to the features of no screen and no boot screen, for machine adopts servo and extract the integration motherboard, extract failure parts in parts, so in the case of a power supply is normal, should check the system clock oscillation circuit and reset circuit mainly. 6Q8service manual download site
 6Q8service manual download site
 As shown in figure 1, measuring vibration Y2 (27 MHZ) both pin voltage is about 0.25 V (normal one feet is 1.65 V, the other foot 1.87 V). Oscillating voltage on the low side, the reason may be supply U12 (SPCA702A) voltage is not normal. Measured, found U12 + 3.5 V voltage supply is only about 1.8 V. Measuring D2 negative side of resistance is about 3 k Ω about normal, excluding + 3.5 V load circuit short circuit, leakage. 6Q8service manual download site
5 V normal measuring D1 is end, the negative side of about 1.8 V, bad judgment D1. Negative terminal voltage rise to normal after replacing D1, D2 + 3.5 V, the machine back to normal work.6Q8service manual download site