SERVICING NICAM DECODERS
Because Nicam circuits are largely digital in operation, and because they dissipate little power, they are among the most reliable sections of TVs and videorecorders.
Alignment
Some early DQSPK demodulator ICs have external trimmers for adjustment. The type shown in Fig. 9.6 has one associated with pin 6 for setting the carrier clock. It should be adjusted for 6.55185 MHz ±50 Hz with an accurate frequency counter and no Nicam signal applied. Alternatively an oscilloscope can be used, synchronised externally from IC pin 22 and displaying the waveform at pin 20. With a Nicam signal present adjust for maximum eye-height in the pattern shown in Fig. 9.9.
The data clock frequency also has an external adjustment in this type of chip, in the form of a trimmer at pin 22. Adjust for zero volts ±30 mV on a digital voltmeter connected between pins 12 and 21
(Nicam signal present) or alternatively for 5.824 MHz ±20 Hz at pin 26 with no Nicam signal present.
In the demux chip illustrated in Fig. 9.7 provision is made for set- ting the DACLK frequency: adjust the trimmer at pin 12 for a frequency of 16.384 MHz at pin 11. Other types of demux IC have no need for manual adjustment.
Some decoder designs have an adjustable bandpass filter in the
6.552 MHz feed to the DQSPK demodulator IC. It is set for best eye-pattern at the spectrum-shaping filter, e.g. pin 20 of the IC in Fig. 9.6. Setmakers usually give specific alignment instructions for this filter.
Fault tracing
Seldom do Nicam decoder faults give rise to ‘borderline’ symptoms. As with most digital processors the device tends to either work perfectly or not at all, due to the various mute systems which come into operation when a PLL unlocks or in the presence of heavy data corruption. In that event the receiver will generally fall back to conventional f.m. (mono) operation, signalled by front-panel or on-screen indications.
If there is no Nicam reception first ensure that the TV transmission is in fact Nicam-encoded, and that any system switch or installation/user software is correctly set. After checking that the ICs are getting correct Vcc supplies, examine the level of the signal at the mute pin of the first IC: in Fig. 9.6 this is no. 18. If it is low (mute on) check that the Nicam carrier is reaching the chip input, and then that the PLLs are locked, here indicated by correct clock rates as described above. If the mute line is high (mute off) the fault lies further downstream in the decoder, probably around the demux chip.
In these circumstances the first tests should be at the data and clock signal inputs to the demux chip, and the outputs from the IC to the D−A converter: clock, data and ident. If the latter are missing or incorrect, check that the PLL within the IC is working and locked up before suspecting the peripheral components, and then the chip itself, in that order. Most Nicam ICs have test/switching pins which can be identified from the IC manufacturer’s (or setmaker’s) data and used in fault diagnosis.
If the Nicam sound is distorted the cause is unlikely to lie in the digital sections of the decoder: as a general rule they will automatically mute before operating conditions deteriorate to the point where sound reproduction is impaired. For distortion, then, the starting point for tests should be the D−A converter IC if both channels are affected and supply voltage levels are correct. Any fault which is confined to one of the L/R sound channels will not arise from the digital section of the decoder because both are handled together there. Using an oscilloscope, check the output signals from the low- pass filters immediately following the D−A converter, and then continue downstream in the faulty channel until the trouble is located. The fact that two identical channels, one working correctly, are present is a great help in diagnosis because comparison tests can easily be made.