NICAM decoders

11th November 2008

B&O nicam decoders have proved fairly reliable but when faults do occur they tend to worry engineers as the circuitry is not generally well understood.

The earliest design, as used in the Unity I TV chassis and the VX 5000/VX 5500 VTRs, has proved reliable but one common fault is uneven channel balance of the audio output, either from NICAM or from the FM intercarrier sound that is also routed through the NICAM decoder. The faulty components are usually the 10uF coupling capacitors around the switching stage, replace them all if one is found to be faulty.

The later type of decoder, as used in the Unity II chassis and early variants of the Beovision Avant, is of a simpler design. Its main fault symptom is spluttering or intemittent NICAM sound, the defective component being the VCO trimmer. Cleaning it is a waste of time, fit a new one and set it up carefully as per the manual. Another weak component in some variants of this design is the 220uf 16V capacitor that smoothes the 5V output of the switching regulator. Despite being a “long life” type after a few years use its contents can leak out and make a mess of the board. Fit a replacement and clean away any contamination, it is corrosive to the copper trackwork in the long term.