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Helping the old folk at home Pride of place this month goes to a story from my Tasmanian colleague, J.L. Apart from technicalities, it has a high human interest, serving to remind us that, TV program knockers aside, there are many people for whom this is the only form of entertainment available. Technically, J.L. 's story emphasises the problems created by the need to expand an antenna distribution system as the demand for outlets grows. Anyway, this how J.L. tells it. "Got a light mate?" The sepulchral voice boomed out of the darkness in the ceiling space over the old people's home at Taroona, south of Hobart. I had been called in to add a new outlet to the 60-odd outlets already connected to the home's TV antenna system. The last thing I expected was to be asked for a light amid the dust and fluff between the rafters. The enquiry came from a Telecom technician who was looking for a phone line terminal board. I had often come across phone lines among the power and antenna NURSE'S LOUNGE KITCHEN CHAPEL ➔-~ o-4 ·nl ➔ OINING ROOM Basic layout RESIDENT'S LOUNGE 2 -WAY SPLITTER --<> TV OUTLETS 4- AY SPLITTER I <$>-o ~ I I D SPLITTER [> AMPLIFIER ◊ OROPTAPS ADMINISTRATION BLOCK HOSPITAL ANO RESIDENTIAL WEST WING Fig.1: general layout of the retirement home installation. It is not possible to show each individual T and outlet, but the order and value of T's shown is typical. Note the splitter feeding the nurses' lounge. 38 STLICON CHIP wires but I couldn't help regarding the terminal panels. The best I could do was lend him my lantern so that he could continue his search in greater comfort. He surprised me by revealing that Telecom employees don't rate a torch or lantern as part of their normal kit. They have to draw one from store and he had not done so because he didn't realise that he would be working in the ceiling. This is probably the most surprising encounter I have had since I inherited the installation. I have had many other encounters over the years, some uplifting and some tragic but all of them interesting. I was called to the home for the first time when the system consisted of about 40 outlets. Since then I have added another 20 and another 20 need to be added before every room has an outlet. It is quite an installation. The home is made up of two residential wings which form two parallel sides of a rectangle. What is called the west wing is a single storey building which houses the hospital and some 25 residential rooms. The east wing is a double storey building of about 46 rooms. An administration block forms the third side of the rectangle and a chapel, residents' lounge, dining room and kitchen, the fourth side. The system began soon after the home was completed, not long after TV started in southern Tasmania. But as with many new buildings, little thought was given to the provision of either telephone or TV services, and both facilities have had to be added piecemeal over the years. It seems that the story began when a local doctor's mother moved into the home, and found it impossible to receive satisfactory TV About this time the firm that had installed and maintained the system was sold. The new firm cut out all domestic service and the home authorities had to find a new firm to serve their needs. I was the third or fourth technician offered the contract but it appears that I was the only one game enough to take it on. (I sometimes wonder if "thick" should be substituted for "game"). Although I have cursed the job at times, particularly on hot summer days in the stifling atmosphere under the roof, I still think it is one of my more interesting occupations and I'll be very sorry when age makes me give it up. Anyway, that's the background to the story; now for some technical and physical details. ~ ~ <G>~~~ SOME OF -rH~ -RE.SIDE:"1\S WeRE. GE1\\NG VJORM'-t' 'P\G,u~e:.s •.... signals with an indoor antenna in her room. So her son had an antenna installed for her. When her fellow residents saw the lovely picture she was getting they all wanted an antenna. But the home authorities, quite understandably, didn't want a forest of antennas on the roof. So a well known and respected TV service firm was called in to install a system that would accommodate any resident prepared to pay for an outlet. The heart of the system was a distribution amplifier near the antenna, of about 25dB gain. From this a long trunk was run the length of the east wing with "T" taps for each subscriber. A "T" tap (or simply a "T") is designed to isolate each set from the line and minimise local oscillator and similar interference to other sets. It has a relatively high loss between the line and the set (side loss) and a minimum through loss to the remainder of the line. Typical figures would range from 30dB side loss with 0.2dB through loss to 12dB side loss for 1.5dB through loss. Knowing what value of T to use in each part of the system is a vital aspect of distribution engineering. The original system worked well for the first 12 to 15 residents in the east wing but more outlets were soon needed and the hospital and west wing residents also wanted outlets. The much longer run was clearly beyond the capacity of the existing amplifier. The solution was to fit a larger, 30dB, amplifier at the antenna end and move the existing amplifier to the other end of the east wing trunk. From here another trunk was taken through the administration block to the west wing and a new network begun. Wormy pictures The 30dB amplifier had been fitted at the antenna end not long before I took over the system, and some of the residents close to it were getting wormy pictures. This was not surprising, since some of them had 80dBu or more at their outlets. At this point it might be wise to digress briefly to discuss distribution network terms and standards. TV signal strengths can be most conveniently expressed directly in decibels (dB), provided this is done relative to some accepted reference. By using decibels, gains and losses can be easily added and subtracted, rather than resorting to the complicated multiplication and division needed if other values are used. The reference used in this application is 1 microvolt (lµ V), expressed as dBu. Most modern TV sets require 1 millivolt (lmV) or 1000µ V for a good stable, snow free picture. On the basis of the lµ V reference, lmV is + 60dBu. Most sets would work, after a fashion, down to 50dBu, but 40dBu would result in a very snowy picture. At 30dBu there would be more snow than picture and a 20dBu signal would probably be unrecognisable as a TV picture. In the other direction, most sets can cope with 70 or 75dBu. Their AGC (automatic gain control) SEPTEMBER 1988 39 SERVICEMAN'S LOG should be able to take care of such levels. But at 80dBu and above the AGC cannot always cope and wriggly patterns on the screen are the result. My aim was to provide 60dBu at each outlet. This was most conveniently done by starting with a high level signal and reducing it as required at each outlet with an appropriately rated T. In this installation the antenna was delivering about 65dBu to the amplifier, which was raising the level by 30dB to around 95dBu. Thus, sets close to the amplifier were being overloaded, even after the losses in the first string of T's. The original system had used 16dB and 12dB side loss T's, which had been OK with the old 25dB amplifier, but were not good enough with the new, more powerful amplifier. My problem was how to reduce the level to the first 20 rooms without the expense of replacing all the T's down the line with ones having higher side loss values. The first step was to split the amplifier output into two trunks, using a four way splitter with the two unused splits terminated. A twoway splitter causes a 3.5dB loss in each leg and a four-way splitter about 7dB. Thus I was able to in- troduce a 7dB loss into each trunk before I had to worry about changing any of the T's. In addition, I have 4dB in reserve which I can reclaim at any time by changing to a two-way splitter. Then I took about 10 rooms closest to the amplifier and connected them through new 30dB and 26dB T's. This solved the overload problem for this group and left only a few troublesome sets on the other trunk running to the second amplifier. These were cured on a temporary basis by fitting attenuators at the outlets. Later, a more permanent cure was made . when new outlets nearby justified the installation of more appropriately rated T's. Nurses' lounge I was handed a sticky problem at one stage when I was asked to extend the system out of the west wing, along a covered walkway and into a nurses' lounge some 40 metres beyond the end of the west wing trunk. I removed the termination from the last T (the last T must always be terminated), and ran an extension to the lounge, feeding the new outlet from a terminated 12dB T. But when I connected a TV set to this outlet there wasn't enough signal for a satisfactory picture. It was too long a run from the last amplifier, there were too many T's in the cable, and the 12dB in the final T was the last straw. To solve this problem I tried a different approach. I went back to the start of the new run and replaced the last T with a 2-way splitter, and removed the Tat the end of the new run; ie, in the nurses' lounge. This meant that there was now a 3dB loss to the new run, but no 12dB loss at its far end, since the T had been removed from this position. This meant a gain of at least 9dB, but actually a bit more because the T at the end of the old run would have introduced at least ldB through loss into the new run. Thus the nurses' set should have been at least 10dB better off and the end result, a first class picture, would seem to confirm this. But was I l 11 40 SILICON CHIP I tempting fate in terms of isolation, particularly from either of the sets on the splitter back into previous sets on the line? Yes, I was breaking the rules. A splitter typically provides around 22dB of isolation between the two sets it is feeding, but only about 3.5dB between either of these sets and the line into the splitter. So, while there was little chance of interference between the two sets, there was a risk that either one could interfere with other sets on the main line. But I was lucky; I have had no problems or complaints. Putting new outlets into an old building often presents real difficulties and taxes both my ingenuity and physical endurance. I often need to be in the roof and on an outside wall at the same time. And you can't nail a cable to an aluminium window frame. Most of the cables are brought out under the eaves and down the outside walls. This isn't an ideal situation - I would prefer to run them inside the wall cavity. Unfortunately this is impractical in most old buildings and impossible in twostorey ones. So I run them as inconspicuously as possible on the outside walls. Civil engineering One particular outlet will stay long in my memory. It was into a downstairs room of the two-storey block, with steeply sloping ground outside the window. The first problem was to provide a stable, level base for the ladder. This took over half an hour of civil engineering, involving quite a bit earth moving and the use of bricks and planks to create a retaining wall. (This ladder base is now a garden bed; another feather in my multitalented cap!) When I put my ladder against the wall I found that, fully extended to eight metres, it only just reached the top of the upstairs window. If it slipped, I would go through the glass. So I had to modify the top of the ladder with a plank held in place with G-clamps. This worked, but I felt decidedly unsafe perched so far above ground. After all this preparation it was up the ladder, drill a hole, down the ladder, grab the coax, up the ladder, push the coax through the hole, down the ladder, into the building, up the stairs, into the ceiling, and search for the end of the cable. Alas, the low pitch of the roof prevented me from getting closer than three metres from the coil of cable, clearly visible under the eaves. I had the idea that moving a few tiles might give me enough freedom to push the cable into a more accessible position. But my ladder wasn't long enough and I could only stand on the top rung, with my chin on the guttering, wondering how I might move those tiles only inches from my nose. Then I had the first really good idea I'd had all day. I came down the ladder (again), got into the van and drove into town to buy a 4-metre length of 3mm mild steel rod. With the end of the rod bent into a small hook I effectively had an arm four metres long and could easily reach the cable - without having to move tiles or buy a longer ladder! The rest of that job was an anticlimax, although there was still a lot of up and down the ladder while I fixed the cable to the wall. But my elderly client was ecstatic when she saw the "lovely pictures" and her delight made all the hassles seem worthwhile. As I mentioned at the beginning I've had both happy and sad contacts on this job. One of the sad ones was a dear old lady who was so crippled with arthritis that she couldn't press the buttons on her TV remote control unit. And I complain because it sometimes hurts me to hold a screwdriver! Other clients have died before they got full value from my work. Then I have had others (metaphorically) leaping over chairs in delight at the birth of a great-greatgrandchild. Although most of my clients at the home are, in a sense, really there just waiting to die, they are almost all a happy and nice-tobe-with crowd. One old chap likes to help me, even though he has trouble picking up cable clips, one at a time. Well, that's J.L.'s story, and I found it most interesting. The only comment I would offer concerns the lack of problems with the splitter. The most likely cause of interference between sets on the same antenna system is where the local oscillator frequency of a set on a lower frequency channel falls within the video band of a higher channel. Granted, there are other possibilities, involving harmonics etc, but this is the major problem. I'm not sure about all the channels in this area. The best reference I can find lists channels 2 and 6 in Hobart, and a channel 8 translator in Taroona. By my calculations, SEPTEMBER1988 41 SERVICEMAN'S LOG ~ ~ -- · ~7 MRS '-Nl>l'Altf' ( DOt.S~'i ~NOIN WHA1 Mt.iAfftORIG more likely to be a component fault, probably the dew sensor itself. I suggested he bring it in for me to check. So he turned up a couple of days later with the machine and I connected it to a monitor and turned it on while he was there. The dew light came on as predicted and, since the weather had been fine and dry during those few days, there seemed little doubt that it was a false alarm. I warned him that if it was a faulty dew sensor there might be some delay, since I had no replacements in stock. He indicated that there was no great hurry. At the first opportunity I pulled the covers off to have a closer look. And it was then that I observed the second fault; as soon as power was applied the video head drum commenced turning, which was definitely not normal. I pushed a cassette into the carrier and it accepted it and put it down on the deck. But that was as far as it would go; pressing the play button .produced no response - which wasn't really surprising. Two faults or one? none of these would qualify as likely to either cause or suffer from such interference. But other localities may not be so fortunate , particularly those within range of two different TV transmission centres. For example, a combination of channels 2 and 5 would be suspect, while channels 6 and 10 clashed badly on the NSW north coast some years ago, to the point where channel 10 had to be changed to channel 11. And these are only two possibilities. Faulty dew sensor From my own bench this month I have a story about a puzzling dew sensor fault in a National NV-370 video recorder. In fact, there were two faults, seemingly unrelated, but 42 SILICON CHIP the dew sensor fault was the one which alerted the owner, and the only one that he knew about. The story started with a phone call from the owner - a new customer - who explained that he had been away on holidays for a few weeks and that, on switching on the recorder when he returned home, the dew light warning appeared. Since there had been a bout of wet weather while he was away, he simply left the machine turned on for a few hours, expecting that the condition would correct itself. When this did not happen, he rang me to ask whether it was reasonable to expect that there would be any moisture left in the machine after this treatment. I said it seemed unlikely and that it was So, did I have two separate faults or was it one fault producing two apparently unrelated symptoms? I decided to pursue the dew sensor fault first, clarify that situation, then take it from there. The dew sensor in this machine is mounted on a small sub-assembly bracket mounted on the rear right hand corner of the deck. The accompanying exploded view of this corner shows the bracket (item 69), the dew sensor (67), the loading motor connection board, VJB00A54, to which the dew sensor is connected, and sundry minor components. Dew sensors are strange beasts. As I have found on previous occasions, they are basically resistors which increase in value in humid conditions. (Don't ask me how they work!) On this basis, an open circuit or high resistance dew sensor would simulate a high humidity situation. And the logical way to test for such a condition would be to short out the dew sensor, at least in theory. In practice, in this machine, it y-/ _/ ✓ 68 ELECTRONIC COM ONENTS We stock a wide range of electronic parts '7 .. • 1 ~· 11 • For service • For Hobby • For Transmitters • For Receivers Also in stock: ~D/:}__·1! Valves for Transmitters - 6146, 8950, 4X150, 6JS6, 811 and many others. Valves for receivers, made by Rhode & Schwarz, Siemens & Collins. R-388, R-390(A}, R392 and more . Fig.2: exploded view showing the dew sensor bracket (69), the dew sensor (67), and the loading motor connection board, VJB00A54. The relevant corner of the main chassis is also shown. wasn't all that simple. While the exploded view (Fig.2) may give the impression that it should be easy to get at, this is not so when everything is in its proper place. The best I could do was release the bracket and tilt it at an angle which allowed me to get a pair of sharp pointed test prods onto the sensor terminals to provide a patch. This had no effect, which seemed to rule out the sensor as the faulty component and to support the idea that it was a common fault creating both symptoms. At this stage, pressure of more urgent jobs and the need to think about the problem prompted me to put the job aside for a few hours. In fact, it was the next day before I turned it on again, only to find that the fault had vanished; the dew 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 8 ~ cs-~Awo,, - · WC\RN\NG AP~RE.D. $2.00 to: -:r.v.TUNERs) 216 Canterbury Road, Revesby 2212, Sydney Australia Phone: (02) 77 4 1154 SEPTEMBER 1988 43 SERVICEMAN'S LOG P1512 1 2 3 4 5 6 CAPSTAN FG LED GND LOADING <at> t- 1 LOADING (f;l) l+I SENSOR LED DEW SENSOR BP1512 BP1512 BP1512 BP1512 BP1512 BP1512 - I 2 3 4 5 6 Pl515 I TO LOADING MOTDRI I 2 LOADING LOADING<at> Fig.3: interconnection board, VJB00A54, and the dew sensor. Note the panels indicating the various plug connections. light was out and the drum had stopped turning. I loaded a tape and it played without hesitation; everything was back to normal. So there was the further complication that the fault was intermittent. And, as if to prove the point, the next time I turned it on, the fault had returned. Racking my brains for an inspiration I remembered that the mode select switch is something of a problem child on the National NV-450 and, while I had never had any such problems with the NV-370, it uses the same switch. I had such a switch on hand and, since there are only four leads and a minimum of mechanical work involved, it was a fairly simple job to swap them. Unfortunately it proved to be a false trail; it made no difference. So where to now? Delving back into the manual I began concentrating on the previously mentioned loading motor connection board to which the dew sensor is connected. The pattern of it is reproduced here and, as can be seen, it is little more than an interconnection device between various sections of the machine, which are fed into it via four plugs, P1512, 1513, 1514 and 1515. The sections connected to three of the plugs are shown in the panels alongside the board pattern; P1513 goes to the sensor LED, Pl514 to the 44 SILICON CHIP capstan motor, and P1515 to the loading motor, all three then being re-routed via P1512. (Incidentally, notice how the common Asian confusion between the letters "L" and "R" has crept into the designation of pin 1 in the P1513 panel). It was while I was studying this board pattern and the various interconnections, seeking inspiration, that I noticed a possible common factor. The wide copper track, surrounding a blank circle near the centre of the board and branching in two directions, is a chassis connection using the mounting screw (413 on the exploded diagram Fig.2) to complete the c.onnection. This pattern provides two chassis connections; one for the TETIA TV TIP General Electric TC53L2. Symptoms: Horizontal shakes of a few lines at a time . Occasionally shuts down with a loud screech . If kept running with an external power supply, the screech can be modified by tapping the line output board. Cure: TR704 (2SC1 722) line driver transistor intermittent internal open circuit. The transistor seems to be insensitive to direct tapping but responds to vibration through its connecting pins. dew sensor and one for the capstan motor. Could this be the common factor, assuming that a malfunction in the capstan motor circuit could ultimately be reflected in the false operation of the drum motor? While such interaction between the two motor circuits seemed somewhat tenuous, I was clutching at straws. But there was one other factor. Somewhere in my memory chip something stirred; a vague recollection of a "shop talk" session with a group of colleagues during which someone had mentioned a set of weird symptoms created by a faulty chassis connection in a National machine. I hadn't paid much attention at the time - not enough, obviously - but then, someone else was telling me about another equally interesting fault. All these thoughts went through my mind in much less time than it takes to tell - and they all pointed in the one direction; the mounting screw, 413. I reached over and applied gentle pressure to the board. And that was it; the dew light went out and the drum stopped turning. I reached for a Phillips screwdriver and tried tightening the offending screw. It was very tight and moved only a fraction of a turn, but that was enough to effect a more permanent cure; enough to let me play a tape and generally confirm that everything was working as it should. But I wasn't prepared to trust that screw. I pulled the board out and soldered a length of lead to the copper pattern near the mounting screw. Then I replaced the board and anchored the lead securely under a nearby screw. Since then I have learned that a mod sheet has recently been issued concerning this problem, and recommending that a separate lead be fitted along the lines I have just described. Now they tell me! One other point. I realise now that all my effort to short out the dew sensor was wasted; it was already disconnected at the chassis end, so the test was meaningless. Had I shorted it to chassis I would probably have solved the problem a lot quicker. Now I tell me! •~