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A woolly picture at Wollongong For those of us who service TV sets in a big city served by several transmitters, all carefully sited and delivering signals of about the same strength at any one location, life is (relatively) simple. But not so for our collegues who work in fringe areas, where customers seek the best of both worlds. This train of thought was started by a story from a colleague who works in and around the Wollongong area on the NSW south coast. The story itself, while interesting in its own right, was made even more interesting because of the way it highlighted the many problems encountered in this area. But before getting stuck into the technical problems, we first need to set the scene. For the benefit of those not familiar with this part of the world, the City of Wollongong is some 70km - as the crow flies - south of Sydney. (I've never been able to work out why the poor old crow has been given the job of making these measurements or, for that matter, how he goes about it. Perhaps he does it on the basis of so many wingbeats to the kilometre?) Anyway, the accompanying sim- ple map should help put the reader in the picture. Given reasonable terrain, 70km is not a great distance for a TV signal but, in this case, the terrain is anything but favourable. Wollongong city is at sea level, and sits immediately below an escarpment, some 300 metres high, which runs parallel to the coast and only a few kilometres inland. As a result, the advent of TV in Sydney (using channels 2, 7, 9 and later 10) saw a forest of tall masts, fitted with high gain antennas and amplifiers, which sprang up in Wollongong and the coastal suburbs to the north and south. As might be expected, the results were patchy and unpredicable. While Smith might receive excellent pictures, poor old Jones next door could often get little more than snow. \ 44 SILICON CHIP This situation prevailed for some years, during which time both the residents and the local TV sales and service industry learned to live with it. And, in many cases, they achieved results which, in theory, should not have been possible. In short, the population had become Sydney TV orientated. Eventually, the powers that be presented the area with its own TV channels: channel 4 (94-101MHz) as the commercial channel and channel 5A (137-144MHz) as the ABC channel. Both these bands, regardless of their geographical allocations, have been controversial; channel 4 because it occupied part of the "international" FM band, and channel 5A because it transgresses certain international satellite channels, and is adjacent to the amateur 2-metre band (144-148MHz). But controversies aside, the channels have in general served the area well. The two transmitters are located on Knight's Hill, some 700 metres above sea level and about 30km sou' sou' west of Wollongong. They also serve areas to the south, such as Nowra, Jervis Bay, Ulladulla and even further down the coast, as well as inland. In theory, of course, the locals should have been content with these two channels since, officially, the Sydney channels were never intended to serve that area. But human nature being what it is, and considering the history of Sydney reception, most wanted the best of both worlds. And very nice too, if you can get it; four commercial channels plus the ABC puts you one up on Sydney viewers. (By the same token, many Sydney clubs and hotels have channel 4 and 5A antennas pointed at Knight's Hill to take advantage of Sydney sporting events not broadcast locally). Knight's, Hill er~· eKiama Nowra /~~ . ateman's Bay Fig.1: this map shows the area, south of Sydney, involved in the accompanying story. The Sydney TV transmitters are at Gore Hill while the Wollongong transmitters are at Knight's Hill. But satisfying this desire wasn't always plain sailing. In some cases it worked out very well, often by default, but in others it presented a host of problems. Signal strength problems and around Wollongong itself, since the antenna is pointed towards Sydney and the local signals are approaching from the rear and the side. In addition, the antenna is not cut to favour the local signals, so there is enough natural attenuation to prevent problems. But not always. As we move south from Wollongong the distant signals get weaker but, because the local transmitters are moving around to the front of the antenna, their signals get stronger. And as we said earlier, there is conflict between channel 4 and channel 9. The main reason appears to be the nearly two-to-one frequency relationship between the two channels. While the second harmonic of channel 4's vision carrier falls short of channel 9's vision carrier, some of channel 4's video content would reach it. Anyway, the practical result is that, with too much channel 4 signal, channel 9 becomes unwatcha ble. As these problems became apparent the TV antenna industry responded with a range of "traps" designed to control channels 4 and 5A, and having different orders of attenuation. The first one offered provided 30dB attenuation and, when this proved to be too severe for many localities, was followed by 20dB and lOdB models. These helped a lot but were not adequate in all localities. Which only serves to highlight the difficulties encountered in this and similar areas; each case has to be treated on its merits and installations tailored to suit. Which brings us to the specific situation that started this story, and which the reader will be better able to appreciate against the background we have just set out. In fact, from here on, the story is best handed over to my colleague. This is how he tells it. Colleague's story The story really started about 15 years ago, and concerns a customer who lives at Ulladulla, about 85km south of Knight's Hill and about 180km from the Sydney stations at Gore Hill. I helped him install a new TV set when he moved into the area at that time. His home is on an excellent site and I felt sure that Sydney's signals would be readily available. But he wasn't interested and, in fact, for a few years was content to watch the two local channels, using nothing more pretentious than a pair of rabbit ears. I don't like rabbit ears. Signal strength notwithstanding, they simply cannot provide a clean Broadly, the problem is one of having to cope with vastly differing signal strengths and the risk of cross modulation caused by powerful local signals mixing with weak distant ones. In particular, there is conflict between channel 4 in Wollongong and channel 9 in Sydney. A popular set-up for Sydney reception is a phased-array antenna, such as the Hills model CA16, feeding a suitable amplifier, usually masthead, on the tallest practical mast. These antennas are cut to resonate around channel 9, but are broad enough to cover 7, 8, 9, 10 and even 11 in some circumstances. This arrangement is often good enough to cope with both groups of signals. This is particularly so in OCT0BER1988 45 SERVICEMAN'S LOG signal in all circumstances. Movement of venetian blinds and other metal objects, or even people, can create all kinds of ghosting problems. As a result, I continued to urge him, over the years, to fit a decent antenna. He succumbed in the end and I fitted a simple 4/5A antenna on a "hockey stick" mast mounted on the bargeboard of the house. And even he had to admit that the improvement was worthwhile. He was happy enough with this for the next couple of years, then suddenly, about 10 years ago, expressed a desire to receive the Sydney channels. I suspect that the advent of some sporting events, available only on these channels, prompted this decision. Anyway, I was confident I could provide a suitable installation. I chose a Hills CA16 phasedarray antenna, mounted on a 5-metre length of masting, and feeding a Standard Components MH20 masthead amplifier. This produced excellent signals from the Sydney channels, apart from channel 9, which suffered severe cross modulation from channel 4. This came as no surprise, having come to grips with the problem previously in this area. The solution called for yet another trap filter which had been developed by the antenna manufacturers, following representations from myself and other servicemen. Initially, we had tackled the problem using the lOdB or 20dB 4/5A filters already mentioned. This was tricky enough in itself because the strength of the channel 4 signal as fed to the amplifier can be quite critical. Too little attenuation can wreck channel 9, while too much 46 SILICON CHIP can put channel 4 into the noise. And the gap between these limits can be quite narrow. But there was an even bigger problem, involving channel 5A. For a couple of reasons, channel 5A's signals are significantly weaker out of the antenna, to the point where they can be well into the noise by the time channel 4 is under control. One reason is that 5A's signals are naturally weaker in this area, an official DOTC survey listing them as lOdB down on channel 4. By my observations this figure may be a little high, but there is at least a 6dB difference. The second reason is that this phased-array antenna favours channel 4 rather than 5A; probably because, being cut for channel 9, it favours channel 4 on a harmonic basis. Anyway, whatever the reason, channel 5A can be down anything from 6dB to lOdB. So, on a worst case basis, channel 5A could be 20dB down on channel 4, and even in the most favourable circumstances is going to be 12dB down. By itself, this difference can usually be tolerated. But add 10 to 20dB attenuation caused by a filter needed to control channel 4, and 5A is well and truly into the noise. The new filter, designated FL300-4, was designed to attenuate channel 4 only, by 18dB. I have no details of its response curve, but it is reasonable to assume that it would be level within a few dB over the bandpass. It was a simple device, on a printed board measuring about 25 x 60mm and consisting of four resistors, three fixed capacitors, a trimmer and three printed inductors. It was enclosed in shrink plastic for external moun- ting, but I chose to mount it in the amplifier housing. And it worked. Channel 4 was eliminated from channel 9, and channel 5A was left in the clear to provide a clean picture. So everyone was happy. Lost colour End of story? No, the beginning really. As I mentioned, all that happened about 10 years ago and was virtually forgotten until recently. Then the customer was on the phone with the complaint that he was losing colour on channel 4. My first reaction was to enquire whether it was only channel 4 that was faulty. (Some customers will insist that a fault is on one channel only, for the simple reason that they only ever watch that channel!) On being assured that channel 5A and all the Sydney channels were OK, my next reaction was to suspect faulty fine tuning for channel 4. On this basis, I told the customer to check the fine tuning and see if this made any difference. If that didn't work, then I would have to make a call. Well, it didn't work and he was on the phone the next day to report this. So I headed for the hills. I'd taken care to pack the field strength meter because, even at that early stage, I had a feeling that the fault was likely to be associated with the antenna. I found the situation essentially as the customer had described it, and quickly convinced myself that there was nothing wrong with the set. So I disconnected the coax from the set and made some field strength measurements on the channel 4 signals. The vision carrier (95.25MHz) was running at 54dBu (0dBu = 1µ V) which was possibly a trifle low, but not ELECTRONIC COMPONENTS +5 I -5 ; ill -10 ,_ ~ "' " \ - 15 -zo 80 We stock a wide range of electronic parls / '- / V • For service • For Hobby • For Transmitters • For Receivers \ I '- 85 95 90 100 Also in stock: 105 110 Valves for Transmitters - 6146, 8950, 4X150, 6JS6, 811 and many others. FREQUENCY (MHz) Fig.2: response curve of the channel 4 filter. As can be seen, it gives a pronounced dip at 100MHz. seriously so. But the sound carrier (101.75MHz) was down to 35.5dB and, while it is normally down significantly on the vision carrier, I felt that this was more than it should be. And if something was lopping off the top of the response curve it would show up as a weak sound carrier, but could also mean a weak chominance sub-carrier. So it was up the ladder and onto the roof to lower the antenna for a thorough inspection. This revealed nothing significant. Nor were there any other visible faults , such as bad coax connections, or the like. So it looked like something funny in the ampiifier or the filter. The filter was the easiest to check, by the simple process of removing it. This brought the vision carrier to 73.5dBu (a jump of just on 20dB) and the sound carrier to 59.5dBu (a jump of 24dB). Of course, channel 4 was now back to full colour. But poor old channel 9 was the loser with cross modulation from channel 4 very much in evidence. The vision carrier increase was not unreasonable for an 18dB filter, but I was suspicious about the sound carrier which had recovered an extra 4.5dB gain compared to the vision carrier gain. This seemed to confirm that there could be something amiss at the high frequency end of the filter bandpass, as I had already half-suspected. As far as the immediate situation was concerned, the easiest thing to do was leave things as they were, while I took the filter back to the workshop for a more detailed check and consideration of what best to do. Back at the bench I fed signals from the generator through the filter and into a suitable level meter. Selecting an arbitary zero reference at B0Mflz I took a series of readings across the bandpass. The results are shown in the accompanying graph and the point that stands out is a marked dip at 100MHz which is virtually the chrominance sub-carrier frequency (99.7MHz) and which is no less than 16dB down from reference. Just why the filter had developed this dip was a mystery. A visual examination revealed nothing and a detailed component-by-component check would have taken more time than I could spare at that stage. So I simply pushed it to one side. Antenna trap The important thing was to decide on the best solution. Simply fitting a new filter was one option, but I decided to try another approach which I had developed more recently. In many cases, it has proved to be perfectly adequate. This involves making a simple trap from a length of coax; a quarter wavelength long at the appropriate frequency [allowing for the coax velocity factor) and which is connected across the amplifier input terminals. The other end is Valves for receivers , made by Rhode & Schwarz, Siemens & Collins. R-388, R-390(A), R392 and more. Mail order welcome D. Dauner Electronics 51 Georges Crescent, Georges Hall 2198, NSW Phone: (02) 724 6982 Telex 178 401 TV TECHNICIANS & SERVICE COMPANYS Have your PHILIPS VARICAP TUNERS (ELC2060) Repaired or Exchanged ONLY $17.oo ea TEN or MORE $15.oo Quick service 3 Months Warranty Send Faulty Tuners Cheques and money orders + P&P $2 .00 to: -:T.V.TuNERS) 216 Canterbury Road, Revesby 2212, Sydney Australia Phone: (02) 77 4 1154 OCT0BER1988 47 SERVICEMAN'S LOG left open and, by reason of the quarter wavelength dimension, makes the device look like a short circuit at the chosen frequency. The "Q" of such a trap is quite high - too high for this application - so the trick is to cut it for a lower frequency, about 88MHz in this case . This normally provides enough attenuation to prevent cross modulation but has little effect on frequencies above the channel. This offers another advantage. There are several Sydney FM stations in this segment, and these are well received in good local positions. The ability to use the TV antenna for these signals is a bonus which customers appreciate. Happily, this approach worked in this case and I had the customer's system back to normal in a couple of days. It is not, of course, a panacea, but when it works it has the advantage of low cost, simplicity, and reliability. A convenient form of coax is the very thin Teflon dielectric type, the higher cost being of little consequence in this application. End of story Well, that's my colleague's story and I think the lesson to be learned is that there is no one answer to the problems encountered in such areas. The successful serviceman or technician is the one who has a whole range of options available, most born of long experience in the area, and a good understanding of the basic principles involved. From my own bench For a change of scene, here is a short report from my own bench about a puzzling case of sound distortion in a TV receiver. The first one occurred several years ago and I have had about eight since then. Naturally, after the first one, they presented no problem. I had intended to write about this on many occasions but, for one reason or another, never managed it. It was only when another such case landed on the bench a few days ago that I was reminded of my good intentions and resolved to procrastinate no longer. That first case involved a Sanyo 6603 colour TV set but I hasten to add that the fault is not confined to any one brand. The lady who owned it was basically an ABC viewer but occasionally chose a commercial program if it appealed to her. And this was the first vital clue because her complaint was that, FOR A Ct\~~GE OF see.NE HE'RE. IS ~ SHOR, 'R£POR.T' FROM W\'1 OWN 'gE.NCH•o• 48 SILICON CHIP while the sound was perfect on the ABC, it was distorted on one of the commercial channels. (I can't be certain which one; most likely channel 9, but I would stand corrected on that. In any case, it doesn't alter the sense of the story}. It was convenient to make a house call on that occasion, and I f?und that the customer's description of the fault was quite accurate, except that the distortion was far worse than she had led me to believe. But why distortion on that channel only? The picture was first class but I checked the fine tuning anyway, just in case of something silly in that department. All I learned was that, by detuning the channel to the point where the picture had virtually vanished, the sound was significantly improved, although it was still a long way from perfect. Then the penny dropped. That particular channel had just enjoyed a blaze of publicity as a result of its being the first to introduce a stereo sound system. So that was it; I'd have been prepared to wager a new picture tube to a dud diode that for some reason, yet to be determined, this set didn't like the stereo sound signal. Well, that was a start, but there was still a -way to go. What would cause a set to respond to a stereo signal in this way? With the stereo system only recently introduced, it was a real brain teaser to try to visualise how this might happen. The stereo signal I had followed the technical discussions which had preceded the selection of the system, so I was familiar with its basic theory. And to refr_e sh the reader's memory, our TV stereo system uses a second sound carrier alongside the main one, approximately 242kHz higher in frequency; more exactly, 242 .1875kHz ± 1Hz, if my memory serves me correctly. (All right, all right, I looked it up}. To maintain compatibility, the main channel carries a left plus right (L + R} signal (mono receivers thu_s receive a normal mono signal} while the second channel carries a right only (2R} signal. In a stereo receiver, these two signals are SIF AMP . & DEMOD . IC151 AN340 or LA1320 SYNC. DRl\11 0202 2SC5: 18'- ., """ fl90 1' 3300 FRO'"4 SW90 18 l TO A UOIO OUTPUT Uh' fll:('11.' Fig.3: portion of the sound IF channel of the Sanyo 6603 showing the IF amplifier and demodulator chip (IC151), the sound IF transformer (T151), and the quadrature coil (T152). matrixed together to recover the separate left and right channels. Which is all very well as basic theory, but was not much practical help right then. For one thing, I couldn't be sure whether I had a one-off situation - ie, a fault in this particular set - or whether there was something in the design of this model which made it incompatible with the new system; something which nobody had foreseen when the set was designed. I finally decided that the faulty design theory was the lesser possibility, particularly as I had heard nothing of any such probl~m elswhere. But, if it was this particular set, where did I start looking? I fished out the manual and turned to the sound IF section of the circuit, the relevant portion of which is reproduced here. The main portion is the sound IF amplifier and demodulator chip, IC151 (AN340/ LA1320}, and I had to consider the possibility that the fault could be in the chip itself, or in any one of a dozen or so associated minor components. But my strongest hunch was that it was simply an adjustment problem, if only because such maladjustments, while rare, are not unknown in the mono scene, where they cause varying degrees of distortion. And, while in theory at least, such adjustments should be made using a sweep generator and CRO, experience has taught me that it is usually possible to make a perfectly acceptable adjustment, in the field, with nothing more than a keen ear and a little care. The two adjustments I had in mind involved the sound IF transformer, T151 , which feeds the sound IF to pins 1 and 2 of IC151 , and the sound detector transformer (quadrature coil) between pins 9 and 10. Based on experience I tackled the latter first. TETIA TV TIPS National TC1401 A Symptom: Bottom foldup . Linearity can only be corrected if height is reduced almost to minimum. Voltage on TR454 collector low at 80V instead of 91 V. Cure: C-463 , a 22µF 250V electro , open circuit. This capacitor is the bypass on the main rail to the vertical output and its loss drops the rail voltage , as well as introducing excessive negative feedback. A vital precaution in making such adjustments is to first note and mark the position of the core or slug, then move no more than one turn initially to judge the effect. Further adjustments can then be made if necessary but one should always be in a position to return to the original setting. In this case, one turn was enough to produce a dramatic improvement, so I kept going until I passed the obvious peak, then returned to that point. And for the moment, that seemed to be the answer. But had I found the right peak? Was this the main sound channel or the second channel? Confident that I could return to this setting when I needed to, I turned the adjustment back to where it had been, then beyond this until I found another sound channel. Subjectively, it was hard to pick the difference between the two, but I reasoned that the first was more likely to be the right one, so I reset the core to this position. Then I moved to T151 and gave it a slight tweak, which was all it seemed to need. Finally, I switched the set to the ABC, which I knew was not even experimenting with stereo at that time, and was happy to note that the sound was perfect here also. Undoubtedly, had I picked the second channel instead of the main channel, this test would have revealed it. So that solved that problem and, as I said earlier, I have had about eight such cases since._ All have responded completely to this simple adjustment routine. As to why the fault occurred in the first place, I can only guess. One possibility is that the adjustments simply drifted, and it is true that most of the sets had been in use for several years. On the other hand I am more inclined to the idea that the adjustment was out from day one; not far enough out to upset a mono signal, and therefore capable of passing final test, but far enough out to allow a distorted version of the second channel to be heard. That's all we have room for this time around; see you next month.~ OCT0BER1988 49