NotTaR of Television Sets : Missing or noisy channel or block of cha..  
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Missing or noisy channel or block of channels

If you are unable to receive certain channels or blocks of channels, this is a tuner problem - could be as simple as bad connections - or even simpler:.

First, check to see that the tuning mode is correct - TV, CATV, as this is the most common cause of channels 'disappearing'.

TV channels are assigned frequencies ranging from 72 to almost 800 MHz depending on broadcast or cable channel assignment. To tune over such a wide range requires splitting it up into various bands even if these are not actually defined. If you have a varactor tuned set, then you already know about the Vl, Vh, and U bands which may use separate front-end components. Even modern quartz PLL synthesized tuners need to allocate circuitry depending on frequency range. Therefore, if a block channels is not working, it could be due to a failure of some component related to that frequency range. Aside from looking for bad connections, resoldering the shields and connector pins, prodding, pressing, praying, etc. you will need a schematic to have any chance of finding such a fault.

There is another slight possibility. Some TVs have a parental lockout capability (pre V-chip) to prevent kids or other unauthorized access to selected channels. The channel selections may have been accidentally altered. Check your user manual for instructions on programming this feature. Even on models without this option, the same internal circuitry could be present but not normally accessible. A power surge or stray cosmic ray could have put the set in a screwy mode. Unplugging power for a minute or probably a much longer time might possibly reset such an anomaly.