Most function key failure, a variety of fault, fault in the CPU. As CPU working conditions are normal, check matrix circuit is no exception. Test the I2C bus voltage 4.8 V, normal, analysis all sorts of function setting and data are stored in memory, disconnect the memory and bus interface, the function key and a corresponding character display returns to normal, but searching and various states cannot store. Doubt memory damage or data error. So the first adjustments entered into a state of repair. Recovery circuit, first of all at the same time reduce the volume according to the panel and shown on the handheld remote control keys, make the machine instantly appear white raster after enter a state of repair, with a function key F option, available volume increase, decreases key data to adjust, but this is not adjustable, do only a data record, press the normal N exit status of maintenance, the machine back to normal.8j7service manual download site