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SERVICEMAN'S LOG I did it; but it wasn't my fault That heading probably sounds like a half-baked admission by a mayhem suspect who maintains that he was influenced by an evil spirit. In fact, it is an admission by the author but I still maintain it wasn't my fault. This all started when I was conned into a job which, in the normal way, I wouldn't have touched with 12.192metre (40ft) barge pole. Which brings me to the real background for this month's notes - the problem facing servicemen when presented with faulty appliances which are completely out of their field. They are jobs which usually result from a plea by a relative or personal friend. It matters not that the particular appliance is one that you know nothing about; that you've never seen inside one and have absolutely no technical data or circuits for reference. If it "works by electricity", you're it mate! In this case, it was a telephone answering machine, owned by an inlaw and presented to me during a FRACARRO SAVES••• ... more than $10. from every UHF television antenna you buy. Fracarro's 4PUN for better performance from weak UHF signals. Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 20TT Fax:(03) 787°3460 ACN006893438 44 SILICON CHIP social visit. And the bloke concerned introduced it with a typical con µ1an's approach; the supposedly flattering, "You know all about these things". I made a feeble attempt to disclaim such expertise but in vain. So what was the problem? Having moved beyond the "it doesn't go" stage, I learned that it had failed to deliver its "no-one-at-home" spiel (or the outgoing message as it is described in the user's manual), leaving the caller with the impression of a lifted handset but no reply. Further study of the manual put me more in the picture, at least at a general level. It was a combined telephone and answering machine and used a single tape. This latter point was important, because it meant that the outgoing message was recorded digitally in a chip. In fact, the length of the message is limited to 16 seconds; enough, but only just. Volatile recording Naturally, this is a volatile recording; it needs to be continually powered. This prompted the obvious query; had the outgoing message been lost due to power failure? The unit is powered from the mains via a 9V AC plugpack, but also takes some power from the telephone line and is fitted with a backup battery. Had the unit been left disconnected from the phone line or mains? And how old was the battery? As far as we could determine, no such failures had occurred. The batteries were fairly new and, I assumed, would be limited by their shelf life. In any case, I realised that it was possible to check the message quite readily; pressing one button (START) replays it through the speaker for checking. And it did, very faintly; so faint that I had to put my ear down to the speaker to follow it. But it was quite clear. And when I pressed the button a second time, it came forth at full bore. The next time I tried, it was at mid level. And so it went on; each time I pressed the button, the level varied and sometimes it was so faint that there was little chance it would be heard at the end of a typical phone line. A bench job Well, that was enough on-the-spot diagnosis. To have any chance of real progress, I would have to tackle it on my own bench. Not that I was very confident. The best I could hope for was an obvious faulty connection - a dry joint, a faulty plug and socket assembly, or something similar. On the bench I established that I could take it apart by releasing four screws. But one of the screws was under the backup battery - four AA cells - so I had to plug it into the mains, remove the cells, undo the screw, then replace cells to restore the backup. That done I could remove the top of the case. This carried the tape deck, the dial pad and the control buttons, plus the handset receptacle and switch hook. It was connected to the main board in the lower half of the case via a short 18-conductor flat cable, which proved quite a nuisance at times. I was even less confident when I looked at the board. Oh, it was well enough put together but the vast array of components and the complex wiring pattern, for which I had no circuit, were very off-putting; it made me realise that I was flying completely blind. Anyway, I plugged it into the mains, hoping that the backup battery had held the fort while all this was going on, and pressed the START button again. As it turned out, the message was still there but it was very erratic. In one sense, that was the good news. If it was all that sensitive, some careful prodding and tapping might find the cause. And so I started. I didn't get far at first; the whole board was touchy. I found a couple of more sensitive areas and went over them with the jeweller's loupe, looking for dry joints. I picked a couple of suspects, which I resoldered, but it didn't do any good. Nor was the job made any easier by the need to activate the message continually, with only a 16-second burst each time. Still, I persevered. Eventually I found myself in one corner of the board, near a 16-pin IC mounted in a socket. I had no idea what this IC did but it was very sensitive to even the slightest touch. Naturally, my first suspect was the IC socket - after all, I've seen enough faulty sockets in TV sets. So, as a first step, I pulled the IC out, squirted the socket with contact cleaner, and put the IC back in. Result - the unit went completely dead, at least initially. It did show some life after some more prodding but it still wouldn't deliver the message. Had the IC or the socket finally failed for good? And if the IC had failed, could I get a replacement? It was identified simply as a type 250K. I rang around my most likely suppliers but no-one could p1ace it. And the situation wasn't helped by the fact that I didn't have a clue as to its function. It was time to ask for help. I don't know where the machine is made but it is handled by a local company, which I tracked down via the phone book. I rang them and asked for the service department: No joy; the service technician was out for the day. But the young lady took my name, phone ½. Price PCBs& Panels! We have over 1,000 different printed circuit boards and screen printed front panels for kitsets from EA, ETI, AEM, HE and Silicon Chip. On special this month only for half normal price! For a complete list of available PCBs and front panels, their date of issue and price, simply send your name, address and stamped self addressed envelope. Note: Due to copyright laws we cannot supply copies of magazine articles. These are available from libraries or the magazine companies (by arrangement) ALL ELECTRONIC COMPONENTS l!l 118-122 Lonsdale St. Melbourne 3000. Ph: (03) 662 3506. Fax: (03) 663 3822. -~- ---- ------ Name Address 0 ~9:z •... l: WA.S FL-"<ING COW\~L-E.'T"E-\...Y ~L-\Nt> Postcode L-------------.J NOVEMBER 1992 45 SERVICEMAN'S LOG - CTD number, and brief details of the unit, and promised he would call nie the next day. Surprise, surprise! He did. And he was most helpful. When I related my story, he immediately identified the IC. It was the outgoing message memory chip-which explained a lot. Was a replacement chip available? Yes, it was worth only a few dollars. Crook socket But his bet was the socket. He suggested that I replace it first to see if that cured the problem and then, if that failed, replace the chip. But he ruled out a suggestion that I solder the chip directly into the board. These chips are quite temperature sensitive and don't like being soldered. He also promised to send me a circuit, which turned up in the mail the following day. Full marks all round! And so, from my local supplier, I purchased a machine pin type IC socket; beautifully made, with solid, gold-plated, contacts. This was duly fitted and bingo , that was it. I recorded a new message and it played perfectly. It was almost an anti-climax. Normally I would have returned the unit at that stage but it wasn't convenient for either of us to make the journey. So I took it home for the weekend, plugged it into a convenient extension outlet, and set it up to say its piece. Of course, no-one rang, not even a cladding salesman. At least not until late Sunday. Unfortunately, when that call did come through, the machine ignored it but not as it had done before. This time, the machine didn't respond at all; it simply didn't know that the call was there. Well, that was a real setback. As I have remarked before, there is nothing more frustrating than to cure one fault and then discover that you have apparently created another. So what had· I done? And how should I tackle it? Most importantly, I needed a ringing signal to work with. I considered a number of possibilities but settled, initially, for Telecom's own ring-back facility, as used by their technicians 46 SILICON CHIP to check ring functions (don't ask me how I know this). Back on the workbench on Monday motning, I first used the ring-back to confirm that the failure was not just a one off - which it wasn't. This also revealed something else; something which had been masked in my home set-up by the nearby regular phone. There was no ring tone being given out by the machine. At this point I fished out the circuit and began analysing it in detail. I started with the incoming phone line a standard arrangement in handset circuits; it takes power from the phone line to drive the handset. And it is necessary because although the supply is DC, its polarity reverses with certain functions, such as answering an incoming call. So I mentally wrote that one off for the moment. The other bridge would ·also have its counterpart in a handset. It can respond only to the AC ringing pulse and drives the ringing tone generator. It obviously performed this function in this circuit but did other things as well, such as instructing the answering circuit to (electrically) lift the handset and then initiate the outgoing message. So, since I was getting neither a ring tone nor the answering function, it was the obvious circuit to check. I checked the bridge diodes first but they were OK. I then went back to the incoming line and worked towards the bridge. It went through one of the hook switch contacts, then the lµF capacitor and a 4.7kn resistor. The bridge output circuit involved several more components, including a zener diode and an electrolytic capacitor. Crook joint so FO'R --root< ,.,.. 1-\0M~ 11-\e:. WG:~i<: ~NP..... ~ which is marked "R" and "T" for some reason which escapes me. Slowly it began to make sense. The first thing I realised was that the telephone circuitry, which is normally contained within the handset, formed part of the main board. This is natural enough,Isuppose,butitaddedtothe complexity of both the board and the circuit. However, I soon had the circuit mentally divided into three sections; the aforesaid telephone section, the tape recorder section, and the answering section proper with its control switches and housekeeping circuitry. So back to the incoming line. This goes to two bridge rectifiers - directly to one in the DC sense and indirectly to the other via a lµF polyester capacitor. The direct-coupled bridge is The soldered connections to the switch lugs checked out OK visually and the ohmmeter confirmed the switch function. Next was the lµF capacitor and this was where things began to happen. One soldered joint was clearly OK but the other one looked suspicious. About half of the joint was covered with flux (not unusual on this board), while the visible solder that remained just didn't look right. Although the solder extended over the track, it was tucked under, rather than flowing over the surface. Naturally, I wanted to see the rest of the joint to be sure it had flowed onto the pad proper. So I started picking away at the flux, which turned out to be a substantial lump. In fact, it was all flux; I finished up right down on the board and still no solder. More importantly, there was no copper either; I was looking at bare board. I reached for the solder wick and soaked up the remaining solder. And what a sight! The copper pad was completely missing; it had broken off at the end of the track. And the only indication of a soldered connection was a barely visible run of solder along the broken edge of the copper track, so faint as to need the jeweller's loupe to confirm it. There was no solder on the surface of the track. How this had happened I can only guess. It seems likely that the original joint had been faulty and someone had tried to repair it, but they had been too heavy handed with the iron and the copper pad had lifted and broken off. They had then tried to run the solder onto the track but it had taken only to the bare copper along the edge of the break. This had made a genuine soldered joint but a very fragile one. It may have lasted indefinitely had I not gone through my prodding and tapping routine, looking for the first fault. And I'm quite sure that the 1µF capacitor was one of the components that I had wiggled and pushed in the process. So, yes, I did it but it wasn't my fault. The repair was simple. A short length of fine tinned copper was wrapped around the capacitor pigtail TETIA TV TIP Akai VS-4E Video Recorder Symptom: can 't remember channels after power failure. The set works quite normally and remembers all settings, so long as power is kept up. And it wasn't the backup battery! Cure: With the power off, check for 3.5V on pin 41 of the "syscon" microprocessor. If the voltage is present, replace this chip. If not, replace switching transistors TR5 and TR6. 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. and soldered to it. This was then run directly to the switch contact at the other end of the track, only about 20mm away, and soldered to that. And from somewhere a voice muttered, "muck about with that if you can." EmonaLAB4 Four instruments in one package! The LAB4 has four full function test instruments: ■ DC POWER SU PPL V Triple output; SV, 15V, 0-S0V ■ DIGITAL MULTIMETER ■ FUNCTION GENERATOR with neatly laid out and labelled front panel controls. Also, the LAB4 is compact and on ly takes up a small space, compared to four bench-top instruments. Full function, auto-ranging 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 Computer illiterate As a serviceman, I long ago swore I would have nothing to do with them - computers that is. In my opinion, they were only ever invented for one reason: to provide incompetent clerical staff with an irrefutable excuse when they foul up one's account, or order, or payment. And the computer can't defend itself. 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 0.02Hz-2MHz, with SWEEP ■ That fixed it, of course. I ran it on a workshop extension for a couple days, allowing it to answer calls, then overriding it manually. It never faltered. So it was duly returned to the inlaws. They were very grateful but obviously had no idea of the problems the monster had created. But I've no doubt I'll get the choice cut of the turkey at the next Christmas get-together. Oh well. (Just one final thought. Once it was established that the type 250K IC was a memory chip, someone made the smart suggestion that 250K meant just that: the storage capacity of the device. Why didn't I think of that?) So what was the next unfamiliar territory in which I found myself. EMONA INSTRUMENTS NSW ......................... 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(02) 519 3933 VIC ............................ (03) 889 0427 OLD .......................... (07) 397 7427 DISTRIBUTORS Perth .................. (09) 244 2777 Adelaide ........................ (08) 362 7548 Hobart .......... .... (002) 34 2233 NOVEMBER 1992 47 SERVICEMAN'S LOG - CTD ,-..,\,-\\CS F~e.:R AA"'D C?I\IE.N 1-\W\ "PgRlll\\'Ss,ol'\\ -ro us~ "™e: C.Otv\?o'f"6.R. ......_, 'c.V"\ W\-\E-N \\ E. SW rt""C\-\E-"D \T <,N ON i\i\S oC(AS\ON, ,.,WE:.N1"" ''A\.-\- FUNN'<:~-. More to the point, in the practical sense, I know nothing about them. I don't know a bit from a byte, or a ROM from a RAM, although I remember the hydraulic ram from my boyhood days in the country. (My dictionary tells me that I am "incomputerate; without a working knowledge of computers or their technology". Couldn't have put it better myself). So this is a story about my being forced into a computer service, job, and breaking my solemn oath. But, in defence, I must claim that it was only a little job, so perhaps my oath was only a little bit broken. And again, the job resulted from a plea based on personal friendship, coming from the 14-year old boy who lives next door. His father had given him permission to use the computer to write his school assignments but, when he switched it on on this occasion, it went "all funny". I started to present my stock reply; that I knew nothing about computers and could easily do more harm than good. But curiosity overcame my caution and I made the fatal mistake of asking in what manner it had failed. 48 SILICON CHIP And apparently it had presented a message saying "insert disc" - which I am given to understand is quite normal - but this message was rolling and also jittering sideways. I was caught. The assignment was fairly urgent and getting a repair through normal channels would take time. I simply didn't have the heart not to try. I said I'd take a look and see if there was anything I could do. More to the point, I had gained the impression that it was simply a monitor fault and, I told myself, a monitor is only another type of TV set. Granted, there was probably a lot of signal decoding circuitry in it which would be double Dutch to me but I was hoping for a simple power supply fault. The hula girl My first look at the screen pattern seemed to support this. When it wasn't rolling, I gained the impression of a typical hula girl fault; hum in the horizontal scan. But the rolling was less easily explained. I decided to take everything along to the workshop, where I could use the CRO an<l have time to think. The monitor carried the "Magic" brand, was made in Taiwan, and was fairly old. I opened it, found the video input, connected the CRO to it, and powered up the whole system. This seemed to clear the computer, because the video signal, with horizontal and vertical pulses, appeared normal. But further investigation was hampered by the physical layout. The power transformer was mounted on a separate rear panel and connected to the main board via a pair of very short leads and a 2-pin plug. This made it very difficult to access the main board, so I unplugged the transformer and put it and the panel to one side. Examination of the board revealed a very conventional supply; the transformer fed a bridge rectifier, which in turn fed a conventional 12V regulator IC, plus the usual filter capacitors. The next step was to restore power and check the supply. I had two options: (1) extend the transformer leads; (2) or feed in power from another source. The latter was the easiest and it so happened that a low voltage DC supply was the most convenient source. OK, so that should go straight through the rectifier and into the voltage regulator. As soon as I connected the supply, the monitor came good, with no rolling and no hula dancing. So what was the problem? I found out by accident. In order to gain better access to the circuit, I disconnected the DC supply leads, moved things around and then reconnected them. And at that point, everything went dead. I then realised that I had probably transposed the power supply leads but, of course, that shouldn't matter when feeding a bridge rectifier. If it was a bridge rectifier, that is. Well, it was supposed to be but it wasn't; one of the diodes had gone open circuit. Yes, it was as simple as that. A few cents worth of power diode and it was going like a bought one. I took it back home at lunch time and delivered it to a very grateful neigh):mur. And my fee? I didn't have the heart to charge him. He is obviously a diligent student, prepared to work hard; and youngsters like this need a break now and again. (Must be getting soft in my old age). So I wrote it off to experience. At least I can now claim - albeit tongue in cheek - that I have had some experience in servicing computers. SC