SYSTEMS CONVERSION
On occasion it is required of a service engineer to convert receivers, be they part of TV or videorecorder or separate, to different reception standards. Where different mains supply voltages, encoding systems and r.f. bands are encountered, economic and practical limitations seldom render the project worthwhile unless the power supply system is switchable, the decoder has provision for alternate modes (see Chapter 7) and the r.f. tuner is easily replaced by a suit- able type.
The easiest conversion project is that involving a receiver with different sound carrier characteristics but otherwise similar parameters, and typical of these are receivers designed for CCIR system PAL G imported to the UK from continental Europe. This category of receiver can be made to work with UK system I transmissions merely by retuning the intercarrier sound channel from 5.5 to 6 MHz; this may involve adjustment of filter coils and quad-f.m. detector coil, or replacement of fix-tuned ceramic filters. In the latter case it is important to order the correct filter type – those used for bandpass tuning have different characteristics to those used as resonators in quadrature demodulators. In videorecorders it is also necessary to retune to 6 MHz the sound generator coil in the r.f. output modulator (sometimes called converter) module.
Merely retuning the sound circuits is not sufficient for correct performance, however. To avoid buzz on sound, vision beat patterns and vulnerability to adjacent-channel interference, the i.f. response curve must also be changed to conform to the 6 MHz sound-vision carrier spacing, with regard to co-sound and adjacent-sound traps, see Fig. 3.5, so the SAW filter must be replaced with a system I type. This can often be ordered from importer or manufacturer as a standard spare part for the UK (etc.) version of the equipment. A final point concerns the sound trap in series with the vision/luminance channel, which may be in ceramic or LC form. Unless it is replaced or retuned, a fine dot pattern and tonal distortion of the picture highlights will result.
TV and videorecorder equipment marketed in the Middle East is very often triple-standard (PAL, SECAM, NTSC) and in spite of the basic PAL-B (VHF) specification for local broadcasts, is usually capable of both VHF and UHF reception. Such receivers can easily be converted for system I as described above, but will then be incompatible with broadcasts and interfacing equipment if returned to their country of origin.
CCIR system PAL-B receivers (Australia, New Zealand etc.) also require sole-VHF tuners (and/or r.f. modulators for videocassette recorders) to be replaced by their UHF equivalents – viable where pin-compatible units are available as spares, time-consuming otherwise.