Beolit De Luxe 611-FMT

29th August 2008

This set had worse than usual background noise on all wavebands. As the noise continued even with the volume reduced to the minimum setting it was clear that the fault must be in the audio amplifier stages. Shorting the signal path to ground using a 100nF capacitor confirmed that the fault was in the region of the second transistor AC151-2 but a replacement transistor made no difference. The noise even continued with the transistor removed, pinpointing resistor 145 (4K7) as the defective component.

The amplifier of these sets is not particularly well designed and has rather too much gain. This tends to lead to excessive background hiss and a poor usable range on the volume control. These problems can be mitigated by disconnecting resistor 145 from the supply rail and putting an extra 1K resistor in series with it. If capacitor 150 (10uF) is then connected to the junction of the two resistors instead of the collector of the transistor the gain (and noise) will be reduced to a more usable level. Along with this change, adding a 10K resistor in parallel with resistor 149 (22K) improves the effectiveness of the negative feedback and results in better sound quality. [Beolit De Luxe 611-FMT]