Beovision MX 3500
15th February 2015
This set worked normally except that it couldn’t be switched completely off, pressing stand by only muted the sound and picture, a dull grey glow still being visible on the screen with the tube heaters on and the EHT still present. When first connected to the mains supply the set showed a narrow white line across the screen but gave a normal picture when the <TV> key of the Beolink 1000 was pressed.
With these sets the power supply circuit is switched between normal and stand by operation by the presence or absence of the ‘power fail’ signal at 4P79 and 4P81. This is processed by 4TR35 (BC548B) and voltage checks showed the correct change in voltage was occurring at the emitter of this transistor. 4TR6 (BC548B) also had a changing voltage at its collector, although it was only 9V instead of the correct 19V when stand by was selected. This brought suspicion on 4R87 (680k) and 4D27 (19V zener) but both measured normally. MOSFET transistor 4IC5 (IRF624) was therefore suspect and removing it resulted in the correct voltage change across 4D27. Replacing 4IC5 cleared the fault, slight leakage between the gate and source terminals was suspected although none could be measured. [Beovision MX 3500]