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A few people have had problems with digital terrestrial
transmission - I mean apart from the problems of insufficient signal
strength, missing bouquets and general "teething troubles" which are
only to be expected in such a new venture. The problem in mind is
that of picture break-up caused when an electrical appliance is
switched on or off. The symptom seems to be worse in some flats,
which makes me wonder if the TV aerial coax has been run alongside
the mains power cables. Since these cables are often installed
simultaneously by electricians before completion of building work, it
seems quite common for them to be run together in conduit, trunking
or embedded in concrete! Of course, "this has nothing to do with
satellite TV repairs", I hear you say? Quite right but try telling my
customers that. They expect me to solve ALL of their problems and are
quite put out when I suggest that the answer is to demolish the
building and get the cables installed by someone more competent!
However, it's strange that the digital terrestrial transmission
standard has been designed to be extremely robust in the face of
"ghost" signal reception, yet "falls over" when somebody switches a
light on. Didn't we learn *anything* from the early days of 405 line
MTV destined for Europe is not scrambled but it carries Videocrypt data that tells a Sky decoder that it is! The result is that a UK satellite receiver which contains a decoder will actually scramble the picture by routing the unencrypted video signal through the decoder circuit.
I've had lots of requests from people who want to watch this programme. The simplest answer is to disable the sync-separator circuit so that the decoder no longer works. In an Amstrad SRD510, for example, you can use a switch to short-circuit capacitor C292 on the little daughter board. This is a 3300pF connected to pin 5 of the TEA2029C.
In most receivers that use a PTV111 sync-separator you can use a switch to disconnect the 1uF capacitor next to it (Pace MSS variants, Nokia SAT1700mk2 etc). In Pace PRD models, connect one side of a switch to the central test pin labelled "TST2". Connect the other side of the switch to a 1k resistor. Connect the free end of the resistor to Pin 1 of U2 microcontroller (5 volt supply). That should do the trick!
The customer complained of a "Blank screen but audio OK". My trusty hair dryer soon traced the problem to C166 220uF/25v on the main PCB. This capacitor is actually included in RELKIT 17 from SatCure and it's a good idea to replace all the capacitors listed in the kit instructions if you don't want the receiver to "bounce back" with other symptoms.
Frank was frantic. His customers at the "Lion and Swan" were expecting to watch the big match this evening but the receiver displayed no decoder messages. However, two grey bars appeared at the top left corner of all scrambled pictures.
Luckily, I recalled seeing this symptom about three years ago. My notes suggested U28 and, sure enough, after this 40-pin I.C. was replaced with one from a scrap SRD400, the PRD800 worked beautifully. As a precaution, I measured the ESR of the power supply capacitors. Lucky that I did because the 47uF/400v (C2) was way too high and wouldn't have lasted a week. I also replaced C5, C7 and C8 as these sit next to Q1 which runs rather warm. If Frank had been a bit less impatient I would have fitted the full RELKIT 1. Never mind; he'll be back again in six months!
No sooner had Frank left than a van pulled up outside. The driver unloaded ten receivers, grinned and drove off, leaving me to pick them up off the pavement. I do contract work for a brewery chain and they deliver a load of satellite receivers like this each month. They allow the pile to grow in order to keep costs low so, by the time I get them, they are all "ultra urgent". This wouldn't be so bad if the company would pay me sooner!
The first one off the pile was a PRD800. A note taped to it said "repaired last munth. wurked one day then picter went off. sownd OK".
Oops! If my translation was correct, this could be a "bouncer". What had I missed? I soon found out: the regulator REG1 must have been knocked because one wire was loose in the board. I resoldered it and the picture was restored. Since I'd already fitted the reliability kit there was nothing more to do except screw it back together and write "Sorry, my fault - no charge" on the label. I'm always honest.
The second one was also a PRD800. The note said "picter off. sownd OK". Could this be the same problem? Huh, no chance. It displayed a blue screen when I plugged it in and disabling the blue screen allowed me to see that the picture was rolling vertically. A pity nobody thought to fit satellite receivers with a "vertical hold"!
The picture contrast appeared to be OK so I suspected a sync separator fault. Disconnecting and reconnecting the power several times produced a good, stable picture which remained perfect on all channels until the power was removed and reconnected. I'd seen this before!
Sure enough, on page 107 of "The Professional 'Screwdriver Experts' Guide" was a description of the problem and a diagram indicating where to solder an 82uH inductor. Once I'd fitted this part, the receiver worked perfectly all the time.
It was almost closing time but I thought I could manage one more repair. This one was an SR950 and the attached note said "won't encrypt sky sports other sky's OK. card test OK in another reciver". I gleaned from this literary masterpiece the fact that the receiver was fussy about which channels it would descramble. My first check was to ensure that Video Bandwidth was set to narrow in the setup menu. However, there was no picture whatsoever. Clearly something had died after the receiver had cooled down. I removed the cover and lifted out the decoder board. Using the sketch in Relkit 16 to guide me, I connected capacitors in parallel with C66 (22uF), C41, C68 and C69 (10uF) in the tuner/demodulator unit. This unit is a swine to remove so it's much easier to leave it in place and simply tack new capacitors to the rear (top) face of the board.
My repair was successful and the receiver now gave good pictures on all encrypted channels. I think I deserve a drink after that. Hmm, Frank from the "Lion and Swan" hasn't paid me for that repair yet...