SERVICING SATELLITE EQUIPMENT
With no high voltages or currents, no thermionic devices and no moving parts, fixed-dish satellite systems should be much more reliable than the VCRs and TVs they sit alongside. In some cases they are, but many designs of sat-box run much hotter than they need to, basically because of cost-cutting designs and inadequately rated components, especially within the PSU section. Indeed it is the power supply section which is most vulnerable to electrical breakdown. Most other faults are associated with the outdoor unit, and many of them arise through poor practice or false economy at the time of installation.The majority of user problems, however, stem not from these, but from what is known in the trade as ‘finger trouble’ (mistuning, incorrect hook-up, random key-selection or twiddling of presets), or a lack of understanding of the operation of the equipment by its owner/user.
One of the most common troubles with a satellite home unit is sparklies, as shown in Fig. 4.8. This is due basically to a poor C/N ratio, and can stem from many causes, most of them associated with the outdoor unit. First check that the tuning is spot on: when it is, there should be an equal mix of black and white sparklies. Next see whether both polarisation groups are affected (unless the system is a DBS type). If only one polarisation group is in trouble, check the polarisation-change arrangements in the receiver: generally a change of LNB supply voltage between 13 V and 17 V, or a change of ±35 mA through a magnetic polariser. In the former case, if the volt- age is changing the LNB unit is suspect. With magnetic polarisers a marked change in sparklie-count should be seen as the current is altered by whatever means the receiver affords, or as the polariser cable connections are broken or reversed.
If the sparklies invade both polarities of signal, first carefully examine the whole outdoor unit and downlead for signs of damage, corrosion or ingress of water, foreign bodies etc.; then if necessary check the accuracy of the dish-pointing. With an accurately aligned dish the problem will be due to a faulty LNB, downlead or receiver unit. The most practical course now is a substitution test on each in turn.
Faulty LNBs cannot generally be repaired except by a specialist workshop with precision test equipment, some of which offer a repair/exchange facility to the trade, a cheaper solution than discard- ing the faulty unit and fitting a new one. Magnetic polarisers, together with the feedhorn of which they are a part, can be replaced separately from the LNB in their rare cases of failure.
Like the LNB, the tuner heart of the receiver, pre-aligned and enclosed in a screening can, is not usually regarded as a serviceable item and must be treated as a ‘module’ for service and repair purposes, though experience has shown that a careful examination inside (for dry joints and the like) can be worthwhile. Before condemning a suspect unit, carefully check its peripheral circuits components and pin voltages/waveforms.
Coming now to the internal electronics of a satellite receiver (Fig. 4.22), it is not usually difficult for an experienced TV engineer to
discriminate between ‘r.f.’ (pre-demodulator) and baseband (post- demodulator) faults in both sound and vision sections of a satellite receiver. Such symptoms as sparklies and instability (vision), and hiss and sibilant distortion (sound), come into the first category, while in the second are low contrast, flicker (faulty clamp?) for vision, and incorrect level, hum or one stereo channel missing in sound. With an oscilloscope, multimeter (see Chapter 23) and the manufacturer’s service manual such faults are easily tracked down. Very often what seems to be an internal fault has its origins in the connections, tuning or protocol of the associated TV, VCR or terrestrial aerial system. Control systems rarely fail: where this appears to be the case the key checks, as with TV and VCR equipment, are for supply voltage (Vcc generally +5 V), clock oscillator operation, and reset pulse presence. If all these are correct, confirm that the data lines are not ‘stuck’ by some external cause, and that command data is reaching the microprocessor chip, before condemning it.
It has been suggested earlier that the power-supply section is the most prolific source of true electrical faults within a satellite receiver, and in the case of failure of any sort it is wise to start by checking all internal supply lines. PSUs are fully covered, with trouble-shooting advice, in Chapter 11.
There are now available power-supply repair/overhaul kits for many models of satellite receiver, including replacements for the components which actually fail and those which are likely to have caused their failure, also those (mainly electrolytic capacitors) which are subject to deterioration in use. Use of these kits is recommended for reliable service work. They are available from manufacturers and component distributors.