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, SERVICEMAN'S LOG A dogged approach is justified It's funny how the job you think is going to be a snack can turn out to be a right proper bomb, while something you are tempted to write off at the beginning can sometimes come good. But it does happen and my main story this month is an example of the latter. The story started when a new customer drove up to the shop with a very large TV set in a box trailer behind his car. It turned out that he had just moved into the area and one of my regular customers had directed him to me. I recognised the set at a glance - a Kriesler 5·9-1. Well, that was my first negative thought. This was the first Kriesler on the market when colour TV was introduced in Australia, back in 1975, making it anything from 15 to 17 years old. Granted, they were a good set in their day but spare parts can now be a problem. Then came the customer's comment: "It just stopped. And there was a smell of burning and some smoke out the back". Our loss is your gain Fracarro's ESVU2. The really low loss VHF/ UHF mixer with areasonable price tag. Peter C. Lacey Services Pty. Ltd. P.O. Box 676 (74Fulton Ad.) Mount Eliza 3930 Tel:03 767 2077 Fax:(03j 767 3460 ACN006893438 84 SILICON CHIP Well that didn't sound too good either. In any case, I didn't feel like manhandling the thing into the shop - even with the owner's help - only to find that it was a write-off and then have to manhandle it out again. Fortunately, the set was quite accessible on the trailer and it took only a few moments work with a screwdriver to get the back off. And while I was doing this, the owner nominated the lefthand side as the area from which the smoke had emerged. This suggested that the fault was somewhere on the deflection board which carries the vertical and horizontal circuits. And so it proved to be. With the back off, I could dearly see the source of the smoke and smell. It was roughly in the centre of the deflection board and involved two capacitors: C760 (0.33µF) and C761 (0.39µF). These are connected in parallel and are then wired in series with the horizontal linearity coil (T761) and the horizontal scan coils. This is not the first time I've encountered trouble in this area. It's not unusual for one of these capacitors to fail, either directly or due to a dry joint. The result is severe horizontal overscan; normally enough to bring a response from the customer - but not always. Some people will watch anything! . And if both capacitors fail? Ah well; then you can be in real trouble. Among other things, the EHT can rise to around 50kV, while excessive voltage on the scan coils can punch a hole through the neck of the picture tube. For now, it was obvious that one of the capacitors, C760, had a dry joint on one of its leads. But it was around C761 that the real damage had occurred. As with C760, it had a dry joint (as I confirmed later) but so much heat had been generated around it that the board had been charred and the track damaged. More importantly, both capacitors had been out of action, at least briefly, before the set was turned off. Had the picture tube survived? Again, I couldn't be sure from a casual inspection , although there were no obvious cracks and there was no discoloration inside the glass, or obvious damage to the screen, as often happens when air enters. I explained this risk to the customer, adding that I thought the picture tube was intact and that it was probably worth taking a punt on the other fault. I suggested I take a closer look and advise him if I thought the cost was going to exceed $100. He said up to $150, I said OK, and we carried the monster into the shop. Tube OK When I finally gained proper access to the works, I quickly confirmed that the picture tube was intact before tackling the two capacitors. C760 was easy - I simply resoldered its connections. C761 was another matter; not only had the board been damaged but the capacitor itself was also damaged, one of its leads having come adrift. Fortunately, I had a replacement capacitor on hand, in spite of the unusual value. I cleaned up the damaged board, fitted a wire link in place of the damaged track, and then refitted the capacitor. I then carefully examined the rest of the board, checking for dry joints or other obvious trouble spots , but found nothing suspicious. Dead as a dodo It was time to switch on. Ever the optimist, I thought the set might come I I I r-----' .. ,v, I ----- --, I : I .• ,_ I L-..4-______:.4[I}-4--_J ii ~, I I "I ~---+---.1------r-..---"' I ~I i ~I u, i;.i--m~;-i ~I ~I L....;n:llill!l-,1.-.. I 1. ' ,_ - - - "'I i: ----- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - G. I I Fig.1: the switchmode power supply circuit in the Kriesler 59-1. The four rails generated by the supply are on the right, starting at the top right corner. The sub-board, or "power supply control unit", is at lower left. good. Alas for my hopes; it was as dead as the proverbial dodo. Back to square one. A little preliminary testing pointed squarely at the switchmode power supply; it simply wasn't working. And this was where my stockpile ofboards came to the rescue. By substituting a known good board one can quickly determine whether the original board is faulty or is being prevented from working due to a fault somewhere else. In this case, there was no doubt. I fitted a replacement board and the set came good. When I took a closer look at the power supply, it was immediately evident that someone had been there before me. And they'd been pretty heavy handed. In fact, they were probably looking for the fault I subsequently located. Minor components had been changed and various joints resoldered, and it all looked a bit messy. Worst of all, a 2.20 10W resistor (Rl 11) had been replaced with a couple of 3W units in parallel. This may have been adequate electrically but I had a spare on hand and this was fitted as a matter of course. I then went over the board, tidied things up a bit, remade a couple of joints which I thought looked suspicious, and put the board back into place. I didn't really expect it to work and it didn't. So I tried another tack. The power supply consists of two boards: (1) a main board; and (2) a sub-board called the "Power Supply Control Unit" which connects to the main board via an edge connector. So which one had the fault? I have a swag of sub-boards, so I simply fitted one in place of the original unit. And this time it did work, which meqnt that the fault was on the subboard. But before I could pursue that line, I realised that something else was wrong. The picture was displaying an effect sometimes known as "cogging" or "gear wheel effect". More exactly, it takes the form of horizontal displacement of vertical lines, usually in a square wave pattern. The term "gear wheel effect" comes from the effect it has on test pattern circles - it turns them into a crude imitation of a gear wheel. In this case, there were many such displacements although the amount of each displacement was quite small. In fact, at a normal viewing distance, on ordinary program material, the average viewer may well have not noticed it. But on a test pattern, it was clearly evident. More importantly, it decreased with time and after about 15 minutes, it had virtually disappeared. I didn't have many clues at that stage. All I knew for certain was that it appeared to be thermally sensitive and that it was in the power supply somewhere, because a replacement board cured it. Also, I tipped that it was probably a ripple problem in one of the supply rails, most likely due to a faulty capacitor. · Hula girl In broad terms, I likened it to a problem known as the "hula girl" effect. This was common in early TV sets and was caused by mains ripple in the horizontal system. However, most modern sets use switchmode supplies and so any ripple would be at much higher frequency, thus giving a far greater number of displacements. Partly due to pressure of other work, DECEMBER 1992 85 SERVICEMAN'S LOG - CTD and also to give me time to think, I put the set aside until the next day. I didn't have any inspiration in that time but, when I switched it on again, from dead cold, the effect was more pronounced than ever. So was it a thermal fault or a faulty electrolytic that was being partly reformed by applied voltage? I tended to favour this latter thought now, so I started looking at all likely capacitors in the power supply. The reader can best follow this by starting at the top right corner of the accompanying circuit diagram (Fig.1). First, a 205V rail is derived from pin 7 of the switchmode transformer (T131) via diode D131, capacitors C131 and C132, and inductor 1132. These two capacitors were measured for capacitance and checked OK. The next rail down the circuit is the main HT rail at 155V and the two capacitors involved, C137 and C138, are both in a single can. Initially, I thought I'd struck oil here because one unit measured zero capacitance. I was able to fit a replacement pair but it had no effect, at least not on this fault. Further down the circuit again is a 28V rail, involving C142 and C143, but I could find nothing wrong here either. That left only a very low volt- age rail, designated as "Horizon. Osc. Quick Start". Apart from resistor R14 7, it involves only one small capacitor (C147) and diode D147. I didn't really expect to find anything wrong with the capacitor and a quick check confirmed this. I finally turned to the CRO and checked the ripple on each supply rail. This didn't reveal very much. Most of the rails showed some ripple but how much was acceptable? I tried comparing it with a known good board and while the faulty board seemed to be marginally higher, even after the fault had cleared itself, it was all very inconclusive. Where to now? So where to now? Apart from proving that a lot of things were not the cause, I hadn't made much progress. But I did have second thoughts about one thing. Whereas I had been visualising a sick electrolytic capacitor that was reforming itself, my failure to find such an electrolytic had swung me back to my original theory - a thermal fault. Perhaps this influenced what happened next. ...,. ~ ~~ -1-\E- HA.'D -:S-UST MOVE.t> IN'T"O -me:. A'REA ANO -ONE. OF M.'( ~E,G,ULA'R. cus,o~~~s HAO t:>l'R.E-C.Te:,p HlM -ro I\AE-oe, 86 SrucoN CHIP In any case, while I was going over all I'd done and seeking further inspiration, I noticed something different about the board. All the diodes in these rail circuits were similar Mullard types (mostly BY206). And they all looked the same: plastic encapsulated, green in colour, and about 5mm in diameter and 7mm long. At least they should have been and they all were on my own boards. But on this board, D147 in the "Quick Start" rail was a ring-in. There was no discernible type number on it and it was a small glass-encapsulated type (commonly called a low signal diode). How or why it had been fitted remains a mystery. Contrary to other service work on the board, it didn't look like a bodgie; it was as neatly fitted as any other component. Yet I couldn't imagine it being fitted at the factory. Acting on a sudden impulse, I reached for the freezer can and gave it a squirt. The set had well and truly warmed up and the fault had vanished by now but the effect of the freezer was dramatic; the fault was back in full force instantaneously. And that was it. I pulled the bodgie diode out, fitted the correct type, and that was the last of the problem. In fact, the set now delivered a firstclass picture. So, after all my hours of hard work and frustration, I had stumbled across the fault virtually by accident. It was all a bit of a blow to the ego but no less welcome for all that. I checked the diode after I pulled it out and the fault was quite clear cut; it had significant reverse current leakage at ambient temperature but this fell to virtually zero with moderate heating. That much established I decided to trace the rail and determine its function. In fact, it ran to the deflection board and to another subboard (or "module"), CU701, called the Line Control Unit which contains, among other things, the horizontal oscillator. At t);iat point, I reckoned I'd traced it far enough. But why the bodgie diode was fitted in the first place and why it had failed is anybody's guess. Perhaps the 25Vp-p applied to it was just too much. And what were its specifications anyway? We'll never know, of course. But at least the problem was solved and I ii EHT GENERA TOR Based on a brand new, very compact modern EHT flyback transformer which includes an EHT diode rectifier. We supply the transformer, a very simple circuit, and the components to suit. All you need to make a very simple EHT, DC or • pulsed AC supply. Powered from 12V DC supply. Great for EHT experiments, EHT testing, plasma displays, etc. Total price for the transformer and the components is ONLY: LASER POINTER VISIBLE LASER DIODE BARGAIN Small PCB Assembly based on a ULN2232 IC. This device has a built in light detector, filters, timer, narrow angle lens, and even a siren driver circuit that can drive an external speaker. 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STEPPING MOTORS & sgItem ea. or 5 for $35 No. 0133 ' - - - - - - GREEN LASER HEADS Two used 3mW Siemens He-Ne visible red laser tubes, and one of our efficient 12V universal laser supply kits. · ~ sgg Item No. 0105X The second tube is a Christmas Bonus till end of January 1993 only. Brand new 1-1 .5 mW laser heads that produce a brilliant green output beam. Because of the relative human response these appear as bright as about 5 mW red He-Nes. Very large: 500mm long by 40mm diameter. Lead and high voltage connecto r attached . Very limited quantities. " Ring " to check availability. $590 Item No. 0104 u Brand new units. Diameter 58mm, height 25mm , 5V operation, simple to drive (two phase - 6 wires}, 7.5 degree ~~~s, coil;~;ance 6 6 ~ Item No. 0140 Q 6V GEL BATTERIES s2s ~ Item No. 0161 · per pack ELECTRONIC KEY KIT Use them to activate door strikers for entering buildings, car alarms, cental locking, the most secure key ever (see E.A. July 92). ON SPECIAL <at> $49.90 For two keys, and one decoder kit. For the Argon head only. We also provide a circuit for a power supply. Item No. 0109 DIVERGING LENS Stock up on this LIMITED STOCK surplus item . Circuit supplied. Brand new, 6-12V (6V/50mA, 12V/150mA}. GIVEAWAY at: $6 ea. or 5 for $25 <at> • • A high quality laser beam diverging (beam expander) glass lens, mounted on an aluminium plate, with mounting screws provided. Dimensions: 25 X 25 X 6mm . Use it to expand the laser beam for Holog°raphy, Special Effects, or one of the two lenses required to fine focus a laser beam, for Surveying and Bar Code Reading. sg 90 ,. .. .___ _ _• _ _ __ RING OR WRITE TO OBTAIN A BROCHURE LISTING THESE AND OTHER PRODUCTS AND KITS OATLEY ·ELECTRONICS Brand new Japanese 6V-500 mA Hr. Gel batteries. Fresh stock, charged , and very compact. 57 X 50 X 13mm. Limited quantity at PO BOX 89 , OATLEY , NSW 2223 Telephone: (02) 579 4985 Fax: (02) 570 7910 MAJOR CARDS ACCEPTED WITH PHONE AND FAX ORDERS $7 per pack ~ c__o_r_5_;p_a_cks_f_or_S_3_ 0_ ONLY ......,.. $17 S75 ssoo s10 ea for 10 for $85 Item No. 0130 Telephone display based on an NEC uPD7503 single chip microcomputer and an LCD display. Has ALU, 8 bit serial interface, ROM, RAM , 1/0 lines, time and date display, Xtal. oscillator, and a dedicated LCD display. Features extremely low power consumption at 3.5-6V operation . A connection diagram and microprocessor data is included . CLEARANCE AT: . power pack -that was intended to be used in emergency lighting applications. Contains 17 "D" size 1.2V/4 AHr batteries which can be easily separated into individual cells. New packs in their original packing. Used air cooled Argon laser heads that produce a BLUE beam! Power output is in the 30-100mW range. Limited stock, at a fraction of their real value: Brand new large precision projection lenses, which were originally intended for big screen TV projection systems. Will proiect images at close proximity, onto walls and screens and it has adjustable focussing. Main body has a diameter of 117mm and is 107mm long. The whole assembly can be easily unscrewed to obtain three very large lenses: two plastic and ·one glass. The basis of the cheapest large magnifier or projection system? Experimenters delight at .,:ir·,.... r_ r .,S.~ A complete nicad battery s279 Brand new 6 volt 1 watt amorphous solar panels. Two of these in series make a great 12V battery maintainer/ charger. Incredible pricing. ,- - ._, ._ , 1J , "' NICAD BATTERY PACK Item No. 0121A Item No. 0108 Includes one "Free" rifle or pistol mount. " sggo A very small but compl IA telescope which only needs an EHT power pply, which is provide kit form . New 12V/1 .9 Hr. German Gel batteries. Fresh stock, charged . Dimensions are 178 X 60 X 33mm. Limited quantities at • We should have suggested power supply circuit to suit, at the time of publication. 12V GEL BATTERIES A 5mW gunsight at an unbeatable price. .. New 40mW helium neon (visible red) laser tubes: One metre long! Need approximately 3KV at 20 mA to operate. For discos, displays, holography, etc. Incredible introductory price: a small fraction of their helmet mount binocular real price! ~viewer. Limited stock at sgg If• ~ BIG LASER TUBES Brand new regulated power supplies. 240V mains in - 13.6V or 14.2V at 1.8A continuous output. Less than 2mV Pk-Pk ripple at 1.7A. Has additional output terminals for battery charging. 450 mA maximum . Approved Australian made units that employ a low profile Ferguson PL18 - 40VA transformer, and a regulator circuit (easily modified}, in a very compact , all metal housing: 170 X 95 X 50mm. Mains lead plug attached: 1.8M long. Information/specifications provided . Limited quantities at: High quality ex-military Item No. 0166 •• • • Item No. 0125 An LSI surface mount Includes one "Free" controller IC for this 130mm glass IA filter. display is available elsewhere for about $35. ~ , <', : __, Certified p&p : S6 in Aust. NZ (Airmail) : S10. j SERVICEMAN'S LOG - CTD oo•WOU\•.'t> \-\E: GO OFF H\'S TUC.KE.~ A~t> 'r\AV£ '1"0 'S~ ,-A"'-E-~ TO A ·-poG ?"5Y'CH\A'1'"~\S,-.~ was able to return the set to its owner within the agreed price structure. Pongo's TV set And now, in lighter vein, here is a story about a TV set and a dog. It was related to me by a colleague who has his own shop in a nearby suburb. And because it is secondhand, I cannot answer all the questions which the story prompts. But I will vouch for my colleague's integrity. This is how he tells it. It started with a telephone call from a stranger. Would I come out and have a look at his TV set? Naturally, I .wanted a few more details; I try to avoid house calls if the set is small enough for the customer to bring it in. In any case, I need to know the make and model before making a house call. The upshot was that the set was a Rank Arena model 2601; too big to bring in and, at 17 or so years, probably too old to be worth repairing. The fault was a complete failure, preceded by loud sizzling noises from inside the cabinet. I had a fair idea as to what I'd find and, among the items that I chucked into the van, I included a spare tripler that I had on hand. Original Rank triplers are like hen's teeth these days 88 SILICON CHIP and about as expensive if you can find one. I now use a modified version of another brand but they're still pretty pricey. When I got there, I found that the Rank was obviously a second set, having been relegated to a rumpus room which, by all appearances, was well used. I turned the set on briefly and, yes, there was a series of crackles from inside the cabinet. I didn't really have to guess what the trouble was. The main questions were how bad was it, could the original tripler be salvaged, and had other damage resulted from the tripler failure? I pulled the back off and switched it on again. Talk about fireworks; it put New Year's eve to shame. The tripler was a write-off and I explained the situation to the customer, adding that I could fit a replacement (which I just happened to have on hand). I also told him that I didn't think it was worthwhile. In particular, I pointed out the age of the set, the risk that the tube may be near the end of its life, and that I couldn't guarantee that there was no other damage or that something else might not fail in a couple of months. The money would be better spent on a new set. The owner shook his head. "No, don't want to do that. How much would it cost to fit this triple thing?" I did a couple of quick calculations and suggested a figure of $175, all inclusive. Privately, I felt sure that that would change his mind. But he barely hesitated. "Could you do it now?" It was my turn to hesitate. Did I have everything on hand? "Yeah, all right; I could do it now if you like". Then came the punch line. "Well. I've got to have it fixed. The dog'll go mad ifI don't. You'd better go ahead". Well, I've heard plenty of blokes trying to justify an unjustifiable repair in similar circumstances. It normally goes something like this: "It's not for me y'understand - I don't watch it much. But the missus - and the kids - they get a lot of fun out of it". With minor variations, its a standard routine. But the dog! I've never heard that one before. In fact, I tended to write it off as a bit of leg-pull. Maybe Pongo liked the atmosphere of the rumpus room when others were watching a program but as for taking serious notice of the images, well, you've got to be kidding. In fact, it has always been my impression that animals take little notice of TV or film images. Some people have insisted that their eyesight is such that they cannot see them as the complete images that we do; that they don't have adequate persistence of vision and/ or that they lack colour perception. Others insist that, even if a dog can see the images, the lack of smell tells him that it is not the real thing. And so on. Wild thoughts On the other hand, I must confess that a lot of wild thoughts passed through my brain. Did Pongo show any marked preference for particular programs. "Lassie" perhaps? And how was he expected to express his distress (ie, "go mad") if the set was not fixed? Would he go off his tucker and have to be taken to a dog's psychologist - complete with canine couch - to straighten him out? The thoughts became wilder the more I thought about it. But all I could say was, "The dog? Surely he doesn't watch it seriously". "Oh yes", said the customer. "He really does watch it. Doesn't take his eyes off it and gets quite excited about some scenes. I'll show you when its working". I mentally shrugged my shoulders and went to work. After all, Pongo's money was as good as anybody else's and I'd be rid of the tripler which I had feared I would never use after all this time. It was a straightforward exercise, with no hassles, and in no time at all I was . ready to switch on. The set worked first go and produced a surprisingly good picture, indicating that the tube should last for at least a few more years. I gave it the usual routine onceover for height, width, focus, and colour balance, though I did wonder if Pongo would appreciate these subtle improvements. And as I stood back to admire my handiwork, I couldn't resist a dig at the owner. "Well, do you think the dog'll approve?" He took the hint, opened the back door and whistled. The next minute Pongo bounded in through the door. He was a fairly large dog, seemingly a mixture ofblue cattle dog, kelpie, and a lesser amount of indeterminate breeds. TETIA TV TIP Sanyo 83P chassis Symptom: dark picture. All voltages around the tube appear correct but the beam current is very low. Bears a resemblance to a faulty tube. Cure: R234, an 8.2kQ resistor between the brightness control circuit and the 130V rail, had gone high. This resistor biases the brightness circuit and insufficient voltage throttles the picture. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay 7015. As he came in the door he was just a big playful dog, ready to frolic with anyone. But the moment he realised that the TV set was running, everything changed. He sat bolt upright in front of it, his gaze never wavering, his eyes following every move on the screen. Electronics Workbench® The electronics lab in a computer! A simple, intuitive and very powerful teaching tool, Electronics Workbench lets students and hobbyists design and test both analog and digital electronic circuits, without the delays and expense of a laboratory . FEATURES • Quick and simple circuit entry • Digital and Analog Modules _included, complete with all components • Simulated instruments: dual trace scope, spectrum analyser, function generator, multimeter, digital word generator and logic analyser • Complete control over all component values and parameters • Print: circuit schematics, parts list, instrument readings, macros • Logic conversion - truth table to Boolean formula to logic gates • Customisable hypertext help system THREE VERSIONS • Professional Version: EGANGA colour display; unlimited components • Personal Plus Version: Monochrome display; unlimited components • Personal Version: Monochrome display; limited to 20 components per ....,. EMONA INSTRUMENTS NSW ......................... (02) 519 3933 VIC ............................ (03) 889 0427 QLD .......................... (07) 397 7427 DISTRIBUTORS Perth .............................. (09) 244 2777 Adelaide ........................ (08) 362 7548 Hoban .... ........ ..... .. ... ..... . (002) 34 2233 In fact, he seemed to be totally mesmerised but the best was yet to come. The program was suddenly interrupted by an ad extolling the virtues of "Doggie Do-Nuts" in which · immaculately groomed canines performed all kinds of acts to demonstrate their preference for this product over all rivals. And now Pongo really did go mad. He was up against the screen , pawing at it, whining, growling, even barking; totally frustrated at not being able to get at the dogs behind the glass. Don't ask me what his motives were but at least I had an explanation for the grubby marks I'd wiped off the bottom of the tube face, and for the scratches on the plastic surround below it. Now I've seen everything. Well, that's my mate's story and we both had a good laugh about it and indulged in a lot of speculation. Why, of all the dogs in the street, was it Pongo? Higher intelligence? Better eyesight? Do all other dogs need glasses?. And how would he react to his master's image (a new trademark?) or to his own image? The possibilities are endless. I'll leave it to your imagination. SC EmonaLAB4 Four instruments in one package! The LAB4 has four full function test instruments: ■ DC POWER SUPPLY Triple output; 5V, 15V, 0-50V with neatly laid out and labelled front panel controls. Also, the LAB4 is compact and only takes up a small space, compared to four bench-top instruments. ■ DIGITAL MULTIMETER Full function, auto-ranging ■ FUNCTION GENERATOR 0.02Hz-2MHz, with SWEEP ■ FREQUENCY COUNTER 1Hz-100MHz input range INDEPENDENT INSTRUMENTS Each instrument in the LAB4 is completely independent and can be switched ON or OFF as required. The LAB4 has only one 240V mains input, saving on multiple power points. · Each instrument is clearly defined EMONA INSTRUMENTS NSW ......................... (02) 519 3933 VIC ............................ (03) 889 0427 QLD .......................... (07) 397 7427 DISTRIBUTORS Perth ...... ...... (09) 244 2777 ........ (08) 362 7548 Hoban . ...................... .. (002) 34 2233 Adelaide DECEMBER 1992 89