The symptom is can power on, display show LOW RAIL but no sound. LED on main board not flash. +rail at heat sink at 89V (normal at +30V with no signal) and -rail at heatsink -30V (normal). The cause of defect is due to probe around fan control circuit and suddenly power off and then show defect symptom above. The fan noise is really too loud for home use and but if not set the fan on, the drop down resistors are too hot and also the +/- 15 regulator is also hot and cause the pcb to be brownish color.
The failure is due to U17 LM358 defective (body very hot) and thus replace a similar op-amp M5238. Reboot the set and it is working again. The +rail voltage will only drop to +30V after the set is fully boot up. Tested for 2 hrs output and confirm the set is fully functional again. See some AP measurement.
Note that with no input, the output is quite noisy, may be due to the switch mode supply. Also at 1kHz, the harmonics distortion is quite a lot. S/N is about 95dB, while the spec quoted for 100dB. Thus this amp does not sound very quiet at background but good for high power drive for PA and Cinema use.
