Teardown, receive dummy load boot or have a "squeak" sound, the primary voltage output to 0 v. Disconnect L823, the main voltage output voltage is 101 v, standby voltage control circuit is normal. By pass circuit, images, sound all normal, just D815 protect damage of SCR. This machine D815 USES is FD312, but don't have FD312 replaceable, so choose a breakdown voltage value is 135 v SR2M change after users. Note that when using SR2M FD312 instead, must choose low breakdown voltage SR2M, otherwise no overvoltage protection.U7sservice manual download site