First point A potential of 2.3 V in the map shows that fault is caused by A protection circuit action. Then c point potential mapping to 0 v, that rules out the 140 v, 10 v over-voltage and line or field scanning circuit over-current fault. Measured again point potential for o. B 2 v, that point A potential for high level is for point B. The potential for low level. And the cause for low level point B only three: one is the overpressure rectification after 300 v mains voltage is too high, make the light coupling device at a much lower internal resistance D8193, 4 feet light activated triode; Two control tube is 0856 c, e extremely asked breakdown, three is the light coupling between 3 and 4 feet breakdown, check stabilivolt D821 negative potential of 0 v. Obviously not mains overvoltage caused by. Finally found is light coupling device breakdown between 3 and 4 feet. After the replacement D819 troubleshooting.i4Nservice manual download site