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SERVICE 'SLOG I was only a little hit careless In the context of my main story this month, that of safety, the above heading says a whole lot. The most important point is that it was said at all, because being a little bit careless can sometimes mean being a little bit dead - if you'll pardon the superfluous qualification. It is a story about a microwave oven and, in that sense, relates to the story and comments about a fatality as set out in the January 1991 issue. I will have more to say about that later. But, in terms of safety, the story I'm about to relate should never have hap pened. I am no stranger to high voltage situations; and I don't mean the nominal high voltages one encounters in ignition systems and TV sets. I mean the fair dinkum variety which mean what they say. Some of this has come from servicing microwave ovens, which I have been doing for several years. And before that I had the opportunity to work in the broadcasting field, helping to service radio transmitters; typically the ZkW types used in country commercial stations. Such transmitters would feature a valve type final stage, running at around 5000V and drawing at' least 1.5A; not the kind of power supply one would wish to tangle with but, in truth, only marginally worse than the average microwave oven. Yet consider the rules and rituals associated with servicing these transmitters. First, all power had to be disconnected from the transmitter. And in case this was overlooked, the protective panels carried interlock switches; remove any one of them and the system would shut down anyway. Then, hanging inside the cabinet, permanently connected to chassis, was an adequately insulated probe with which one was required to check 70 SILICON CHIP any high voltage point, partly to guard against a failed interlock but mainly to ensure that all capacitors in the system were discharged. Only then was the equipment regarded as safe to work on. But that was only part of the ritual. Inevitably there would come a time when measurements would have to be made with the transmitter fired up but with the panels removed and the interlocks bypassed. The first rule here was that there should always be two people present, both conversant with the appropriate switching plus proper emergency procedures. TETIA TV TIP National TC2001 A Symptom: dark band down centre of screen and small ripple on vertical lines. The band is stationary but the ripple moves in time with the video content. Cure: C533, a 3.3µF 250V electrolytic capacitor, open circuit. This capacitor bypasses the 180V supply to the video output transistors and explains the shaded picture. What it doesn't explain is the vertical wriggle but this cleared up with the new capacitor. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. In addition, it was expected that everyone would exercise normal care and follow commonsense precautions; rubber soled shoes, one hand in the pocket, and so on. All of which was good training for handling microwave ovens. In fact, I have tried to follow these rules as far as practical, although the "two persons" rule is usually impractical for me, as it is for most one-man shows. But I do use a shorting probe to take care of capacitors, etc. Nor would I ever contemplate checking the power supply using the "size of spark" technique. (Quite apart from the danger, it's not very accurate). For this job, I use a professional high voltage probe, made by Fluke, which is rated at 40kV. After that, all one can do is be extra careful. As I mentioned earlier, a microwave oven power supply is only marginally less dangerous than a transmitter power supply. They typically operate at 4000V, deliver 650W of RF and, on the basis of something approaching 50% efficiency, deliver around 0.3 to 0.5A (they can deliver more than that on demand) . Slow microwaves In this case, the particular microwave oven was a commercial model, a Sharp R2340E, from a local restaurant. And as might be imagined, when commercial units like this fail, there is a fair amount of pressure to get them working again as quickly as possible. This oven is about four years old and I have serviced it several times during this period. On one occasion, the high voltage power supply capacitor failed and on another, the high voltage rectifier failed. The third fault was more unusual. It involved the connections to the filament pins of the magnetron, which are push-on clips similar to those used in the automotive industry. One of these had failed, probably because of sw, ,--------7 SW4 THERMAL CUTOUT (MAG) LATCH SWITCH A (PRIMARY) LATCH SWITCH 8 (SECONDARY) SHORT SWITCH ...L GROUNDING m- CHASSIS GROUND MAGNETRON NOTE: Door is closed. Unit is not operated. BL 220/240V SOHz SW2 r---------., I F I J. T J. RY2 FA; :::-r:: I I I I SW3 :I J/ SW1 -cn..o- FUSE 8A MF FAN MOTOR CH HIGH VO LTAGE CAPACITOR I L - - r - - SW3 I l. ___ J ~220/ I P240 I ---------I _J -MTH DIODE HIGH VOLTAGE TRANSFORMER ....... VARISTOR -.- rlrl TL TEMP SENSOR (THERMAL PROTECTOR) ~ RY1 POWER RELAY A RY2 POWER RELAY 8 TL LOW VOLTAGE TRANSFORMER DIGITAL PROGRAMMER CIRCUIT HUMIDITY SENSOR (NN-7807) -tt-14- PROTECTOR DIODE L IMPORTANT SAFE1Y NOTICE: POTENTIALS ABOVE 250V IS PRESENT ON THE PARTS AND WIRING IN SECONDARY CIRCUIT OF HIGH VOLTAGE TRANSFORMER, WHEN THE OVEN IS ENERGIZED. EXTREMELY CARE SHOULD BE TAKEN DURING REPAIR. MTT OVEN LAMP TURNTABLE MOTOR Fig.I: this circuit of the National Panasonic NN-7807 microwave oven, while differing from the Sharp model, is typical & will help you follow the story. It uses a 4000V power transformer to drive the magnetron, plus a 1.14µF high voltage capacitor, a protective diode & sundry other components. the quite heavy current involved. But this latest fault was different again. The owner complained that, while it still worked, its cooking times had increased markedly, thus largely negating the whole purpose of the device. To understand the story better, a brief description of the oven may help. It is a rather ingenious arrangement which is really two domestic oven systems combined into one, giving a rated power output of 1300W. It has two magnetrons, two power transformers, two capacitors, two stack rectifiers, two lots of plumbing and two fans. The main common item is the microprocessor controller. As I said, it's an ingenious arrangement , using existing domestic technology and parts to produce a larger unit and provide some user flexibility at the same time. For example, an economical low power mode is available by simply switching off one complete system. It also provides a degree of redundancy; even if a component fails in one system, the other system will still provide a limited service. Unfortunately, I cannot provide a circuit diagram. I have no manual and the only circuit is pasted inside the main cover. And, apart from the problem of trying to photostat it in that situation, it's showing its age somewhat. It was good enough for me to follow but a bit too grotty to reproduce. The best I can do is to present another circuit which is at least typical. It is of the National Panasonic NN7807 and variations (Fig.1). As can be seen, there is not a great deal to the magnetron circuit: a 4000V power transformer, a 1.14µF high voltage capacitor (from the magnetron filament terminal FA to chassis), a protective diode and sundry operational switches, relays and protective devices . The real complexity is in the microprocessor control system but that does not concern us here. To digress for a moment, that capacitor value - 1.14µF in this case - is worth commenting upon. It is quite critical; much more so than its superficial role - that of a smothing capacitor - would suggest. In fact, it appar- ently also forms part ofthe magnetron resonant circuit and a wrong value here, even if the capacitor is perfectly good, will result in poor performance or failure to perform at all. So back to the job at hand. Naturally, the owner was anxious to get things fixed as quickly as possible and I had promised to look at it immediately ifhe brought it around. In fact, he'd picked a bad time; I was flat to the boards with TV sets everywhere and had even shanghaied a colleague to help me out. This was a help but it also meant I was distracted from time to time when he needed to clarify a technical point or locate a spare part. Routine tests Anyway, I set it up on the bench, switched it on and made a routine preliminary check. The check I use is a well established one, issued by Sharp. There are other procedures, mostly more complicated and probably marginally more accurate, but I have found that this one is perfectly adequate at a practical level. The procedure involves measuring MAY 1991 71 SERVICEMAN'S LOG - CTD 'l)OU"&.E INSIJLATE.D CRP.SI-I 1-\tL!V\ET WIT 1-1 l-\EP,.,.-SINK RUBB,E:R ORG>A('l TRIPL.£ INSVLP..TED BIFOCAL'S DONOR CA!<:'i) QVAP 1N5VLA GA'5 /V\&\SK WI 'SNP.CI<:; 'FOO COM?P\R:Tfll\8') \.-\AND tN DovSL-E tNSV<-A,~ . POC-1<£°1* A lump of 4 x 2 Pc.e, The oven was still where I had parked it, cover off, mains plug lying on the floor, and I simply reached inside to the filament pin, pulled back the insulation over the clip, and felt for the clip with my finger. Exactly what happend next is a little unclear and difficult to describe. Being hit between the eyes with a lump of four by two would be one way to describe it; there was a violent physical reaction and I certainly saw stars. And in addition to the physical shock, there was the mental shock, the surprise and the fright. Many thoughts raced through my mind. What had I done wrong? Had I only imagined I had pulled the plug from the mains? How else could I get a shock? I had one hand in my pocket, I was wearing rubber soled shoes, and I was standing on a carpet. After a few seconds, when I had collected my thoughts, I confirmed that the mains plug really was on the floor. That meant that the shock could have come from only one source - the capacitor. But how had I completed a circuit with only one hand? The only explanation is through my arm which must have touched the chassis. I had a burn mark on my finger but nothing similar on my arm. Next question; why was the capacitor still charged? Most systems, including the one illustrated in the circuit, feature a bleed resistor (9MQ in this case) across the capacitor to ensure that it discharges. SUPPOR'fS OP•tONf\L- t::::XtAA: MICROWAV£ 'POW£R.E.{) ''I-IE.L\> N\E,I. P..M LIVE:" FLP--Sl-\lNGNf.ON SIGN 1-\EA'"\SI-\RtNK ------.SOU<-'S A.DE.QVA·H-L'-1' RU~~~R------(~~• INSVl.-AT~ LAC.ES 1-\lG,~ VOl-"TA.GE:. r"R.O&E. ,...._ EXP6:c't"W ~A"t" £VE.R.'<ONE: S\-\OuL..t> EXE.RC\ S~ NOP.M~L- C~RE: & FOU...-0\N COH\f'I\ONSENS€ ~RE"C-AvTIOI\JSo•• the rise in temperature of a quantity of water, then applying a simple formula. More specifically, I use 500ml of water, in a glass container, and measure its temperature in degrees C. This is then heated in the oven for 60 seconds and its temperature measured again: f>. typical rise would be between 15°C and 1s c. From this, the cooking power in watts can be calculated from the following formula: W = 4, 2 x ml x Cr / S where ml = quantity of water in ml; Cr = temperature rise in °C; and S = heating time in seconds. As an example, 500ml of water 0 72 SILICON CHIP anything. It couldn't raise the water temperature by even a fraction of a degree. Well that explained the owner's complaint; all I had to do was find out why. I progressed as far as getting the cover off when I had to clarify a problem my colleague had raised. It meant that I had to leave it there, at least briefly. I pulled the mains plug and put it to one side. It was a good half hour before I could turn my attention to it again. By that time, I had recalled the abovementioned fault involving a faulty clip to the magnetron filament pin. Superficially, the symptom was the same; total failure. There are other causes of total failure of course, but I just had a feeling that this might be all that was wrong. heated for 60 seconds and showing a rise of 18°C gives a figure of 630W, which is typical for a domestic type magnetron. Indeed, with a little practice and by always using the same values, one hardly needs to apply the above formula; a glance at tlte thermometer is all that is needed to tell the story. By the way, it is desirable to monitor the mains voltage during this test and to make due allowance if it is more than a few volts off normal. In this case, one magnetron could barely raise the temperature by 9°C, which meant that it was delivering less than half its rated power. And the other magnetron? - well it didn't do TV TEST EQUIPMENT TO EARN YOU A HIGHER RETURN (Australian Made) SHORTED TURNS TESTER Built-in Meter to check EHT transformers including split diode type, .yokes and drive transformers. $78.00 + $3.00 p&p DEGAUSSING WAND Strong magnetic field, larger than usual coil with multicore centre. Double insulated for your safety, also fitted with momentary on/off switch. 240V A/C 2.2 amps. Just about as important as having a soldering iron in your toolbox! $75.00 + $10.00 p&p HI-VOLTAGE PROBE Built-in meter reads positive or negative 0-50kV. For checking EHT and focus as well as any other Hi-tension voltages. This view shows the magnetron & its associated high voltage 1.14µF capacitor. Care should be exercised when dealing with these components inside a microwave oven as they can deliver a fatal electric shock. But not this model Sharp. In any case, there is always enough emission left in the magnetron filament after switch-off to bleed the capacitor, even with a sick magnetron. Not that I had ever relied Oil' this before. As I mentioned earlier, I have a well insulated earthing probe with which I normally discharge any capacitor in an oven, even if there is a bleed resistor. So why not this time? That's what I mean by being "a little bit careless". Working under pressure, I'd forgotten that the Sharp had no bleed resistor. And I'd chosen the one time when there was no magnetron operating· to discharge the capacitor. Could such a shock have been fatal? I seriously doubt it, considering the short path involved. But had I had my other hand on the chassis, instead of in my pocket, it could have been a different matter. Make no mistake, a lµF capacitor at 4000V is a very dangerous device. All of which adds up to an obvious lesson. No degree of urgency can justify taking a short cut which bypasses safety. And the irony ofit all was that, when everything was sorted out, I had one sick magnetron and one totally dead one. So , with no replacement magnetrons in stock, I had to order them and that meant a couple of days delay before I could even tackle the job again. All the sense of urgency, which had undoubtedly contributed to my mental lapse, had been for nothing. Makes you think, doesn't it? Previous story That brings me to Jim Lawler's story, the coroner's report, and the editorial comment in the January issue. In my opinion, the coroner's report is totally inadequate. One can hardly blame the coroner, who must rely on his technical advisers, but these advisers have let him down badly. It's not what the report said but rather what it didn't say that's the problem. It also highlights the ignorance of so many people, particularly the do-it-yourself types but also some who are in the commercial field. Jim Lawler clearly identified the crux of the matter: the difference between the high voltage in a TV set (anything from 15-25kV) and the much lower voltage (around 4000V) in a microwave oven. The high voltage in a TV set is relatively harmless; the lower voltage in the microwave oven is lethal. The reason is very hard to get across to some people. Everyone thinks in terms of voltage and voltage alone. We have been taught that high voltages are dangerous; low voltages are not. $98.00 + $5.00 p&p REMOTE CONTROL TESTER (INFARED OR ULTRASONIC) Designed to test lnfared or • Ultrasonic control units. With the , extension lead you can also test infared units which cannot be placed in front of the testing unit. Requires a 9V battery. Output is via the LED diode and piezo speaker. $85.00 + $4.00 p&p LASER DETECTOR PROBE A new addition to the remote control tester. Comparable with units costing $500 or more. You can test the laser pickup in compact disc players. $27.00 + $2.00 p&p LOW VOLTAGE PROBE Ideal for checking microwave ovens and TVs. The ranges are from O to 5kV Negative and from O to 10kV Positive. Double insulated for your safety. $84.00 + $5.00 p&p TUNER REPAIRS from $17.00 exchange + p&p Cheque, Money Order, Bankcard or Mastercard ~v. TUNERS) ············ ···••.•:•:•:•:•:•:•:•:•:•:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:::::;:;:::_;::::::-:,• 216 canterbury Road, Revesby, NSW, AUSTRALIA, 2212 ~ (02) 774 1154 [;~ (02) 774 1154 MAY 1991 73 SERVICEMAN'S LOG - CTD Yet fatalities have occurred on 32V home lighting systems while we have all had a belt off a Z0kV plus ignition system, or a TV set EHT system (uncomfortable, but nothing more). The answer is current or, more precisely, the amount of current that can flow through the victim's body. And values of l00mA - or even lower, depending on the body path - can be fatal. Unfortunately, there are so many variables - the resistance in the total path (including contact with earth), the nature of the contact with the voltage source, the body's own resistance, etc - that it is impossible to predict how much current will flow in any shock situation. The closest we can get is to consider the worst case situation; how much current can flow if all these resistances are at a minimum. And that brings us to the one factor that we can assess; the internal resistance of the voltage source or, in simpler terms, how much current the source can supply if asked. And this is where so many people become confused. Because some high voltage systems - such as TV EHT supplies and auto ignition systems have very high internal resistances, they are incapable of delivering more than a few hundred microamps or, at most, a few milliamps. And so they have been lulled into a false sense of security. High voltage warnings go unheeded; they've had a belt from an ignition or TV system and suffered no ill effect. So it's all a lot of baloney. Until you encounter a microwave oven, that is. The voltage is low by comparison, but so is the internal resistance. They can deliver half an amp 74 SILICON CHIP or more if provoked - and that's more than enough to kill. But the coroner's report makes no such distinction. It lumps TV sets and microwave ovens together, in terms of danger, thus serving to perpetuate the confusion over supply impedance. When someone finds that high voltages in TV sets are apparently not dangerous after all, they dismiss the whole warning. Nor can I agree with the safety suggestions in the report - in particular, the reference to rubber gloves. Granted, they can provide a degree of protection - if they are in good condition - but the discomfort and inconvenience they cause is such that, in practice, no one ever uses them. Recommending them may salve someone's conscience but it does little else. The editorial sums it up best. Acquire instruments and develop techniques which avoid the need to work directly on live systems. And don't forget to discharge the capacitor(s). Shark attack! Well, after all that profundity, something a little lighter would seem to be called for. So here is a complete change of scene from J. L. in Northern Antarctica. Here's how he tells it. This is not really a servicing story but it does involve several servicemen colleagues, so I suppose its presence here is justified. One Monday morning, after a particularly fine and sunny weekend, I called into a colleague's shop for a brief chat. I found him and two other technicians engrossed in watching a video tape that was running on a bench monitor. They told me that the tape had been shot the day before during a break in their water-skiing activities and I was cautioned to "be quiet and listen"! On the screen was the image of a large ocean going yacht lying a hundred metres or so offshore. On the beach there was a row of people, some in wetsuits, staring out to sea. Between the yacht and the beach was the unmistakable black triangle of the dorsal fin of a large shark. We could hear someone on the yacht calling instructions to the helmsman as they tried to manoeuvre the yacht closer to the shark. One man on deck held what appeared to be a shotgun. They were obviously trying to get close enough to at least worry the shark, if not to kill it. Unfortunately, they were on a deep keel boat and could not get too close inshore without running aground. In the meantime, the shark cruised backwards and forwards along the beach, just out of range of the seaborne shotgun. This continued for some 10 minutes, with people on the yacht and some of those on shore beginning to become very agitated. There were calls for the police, the navy, even the Prime Minister. But nobody had any idea of how to drive the shark away. Then one of the water skiers, braver or more foolhardy than the rest, waded out into the water. The shark immediately turned towards him. There were screams and gasps from the watchers on the yacht and on shore. Just as the shark reached the skier, he bent down and lifted it from the water. It was a 1-metre long radio controlled model submarine with a large triangular fin fitted to its conning tower! Most of the watchers groaned with embarrassment but the final word came from one of the people on the yacht. "Bloody brainless idiots", he shouted. Readers in other states might think this escapade was in bad taste, and you are probably right. But then, here in Tasmania, sharks are more of a psychological than practical threat. Shark attacks are almost unknown in our colder waters and a prank like this worries many but endangers none. OK J.L., apology accepted. But ya can't help larfin'. SC