Beocenter 9500

27th August 2008

All the functions of this complex machine worked correctly except for the FM radio which gave noise but no stations, even when the manual tuning mode was tried. As the AM section was working this cleared the tuning voltage supply and much of the microcomputer from suspicion, so what next? Pin 8 of the tuner module (the tuning voltage input) was stuck at 29V regardless of what the frequency readout was set to so it was disconnected and a 0 > 30V variable bench supply connected in its place. Altering the voltage allowed stations to be tuned so the FM front end, IF amplifier and stereo decoder were deemed to be in order and attention turned instead to the pre-scaler circuit. Sure enough there was no divided-down output at pin 4 of 10IC6 and so 10IC4 (SP8529), which has not proved to be especially reliable, was suspected. Before replacing it however more oscilloscope checks showed that there was no input signal to 10IC4 either and 10TR3 (BF240) was found to have 5V on both its collector and base with nothing at its emitter. The transistor was open circuit of course and a replacement brought all the FM stations back with no further complications. The same fault could also occur in the Beomaster 5500 and its derivatives as they use the same circuit for the radio section. [Beocenter 9500]