Estimation is caused by the TV/AV conversion circuit fault. The machine microprocessor is IC101 (M37211M2-704 - sp), TV/AV signal conversion is IC305 (TC4053BP), related circuit as shown. Press the AV key, IC101 (48) foot switch side about 5 v high level pulse, make dry pole switch tube Q302 (C1815) conduction, IC305 control foot ((9), (10), (11)) voltage lower, F8TV into AV state. After check carefully, and found that the current limiting resistor 302 welding cracks have appeared on foot pulse electric transmission. The R302 after repair welding is firm, TV/AV switch back to normal, the fault cut bottom out.hcqservice manual download site