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Simple service tips for your • nncrowave oven Most people take their microwave oven for granted and never do anything to ensure that it is always working at top efficiency. In this article, we detail some routine jobs which will help you to get the most from your microwave oven. By LEO SIMPSON Why should you have to do anything to a microwave oven? After all, modern electronic appliances are very reliable and so you may feel well justified in not touching anything on your microwave oven, apart from giving it the occasional wipe over. The problem is that simple cleaning is not good enough. Essentially a microwave oven is just a big unfiltered DC power supply feeding a magnetron. And a magnetron can be regarded as a big thermionic valve which pumps microwave energy into the oven. Whenever the microwave is actually cooking THIS PHOTO CLEARLY SHOWS the difference between Pozidrive (left) and Philips head screwdrivers. The Pozidrive has extra flutes between the blades. 6 SILICON CHIP something, the magnetron is dissipating quite a lot of heat and that is removed by a ventilation fan. You can hear that fan running when you are using the oven. For that fan to work properly, all the vents in the rear and base of the oven cabinet need to be kept clear. If your oven has been in use for several years, you will probably find that these vents are clogged with grease and dust. It's easy to understand how the dust gets there - it is pulled though the oven by the fan - but how does the grease get there? It comes from the food being cooked in the oven. The ventilation fan which cools the magnetron also removes steam and fumes from food cooking in the oven. All these fumes are pulled right through the oven and vented out the back. So if you cook a lot of fatty foods and the food is not covered with some sort of plastic film then not only will grease be deposited inside the oven itself, it will also be deposited inside the enclosure at the rear and sides of the oven. Mix this grease with dust and you will find a pretty daggy mess in the vents of your oven. The problem is likely to be worse with ovens that are permanently installed in kitchen cabinetry, although ovens which sit on bench tops can be pretty messy too. So every few years or so, it is a good idea to clean all this mess out of the vents. This is best done with a vacuum cleaner, using the brush nozzle fitting. You can do most of this basic cleaning job without having to gain access to the cabinet. To do the job properly, though, you need to remove the outside casing of the oven. WHILE YOU HAVE THE OVEN casing off, it is a good idea to remove and clean the interior lights and the glass window. Dry them thoroughly before re-installing them. Now it might seem obvious, but the power plug must be disconnected from the power point before you even think of removing the screws. Microwave ovens contain a large high voltage transformer and capacitor inside. If you accidentally came in contact with live wires to these units, it could very easily be fatal. So disconnent the power plug from the power point. screwdriver which doesn't fit properly. In an extreme case, if the screws are very tight, you might damage the heads so badly that it won't be possible to remove them. We don't want that, do we? How do you tell a Pozidriv screw from a Phillips head screw? Our photographs show the difference. Unfortunately though, they don't tell the whole story. Crosshead screws from some Asian sources may look like Phillips head types but a Pozidriv screwdriver will be a better fit. Most hardware stores these days stock Pozidriv screwdrivers, along with their Phillips head types. Those made by Stanley have blue handles while their Phillips head drivers have green handles. The size to fit the cabinet screws of most ovens.are Phillips point 1 or 2 or Pozidriv point 2 or 3. For a comprehensive article on the many diverse screws and screwdrivers in use today, see "Screws & Screwdrivers: What You Need to Know" in the November 1988 issue of SILICON CHIP. Having removed the casing from the oven, you are likely to find more dust and grease inside. This should all be cleaned off as well as possible. But while you are doing that, make sure you don't disturb any of the wiring. Cleaning the lights To remove the screws from the casing of the oven, you need the correct screwdriver. Most likely, the screws will be a crosshead design but a Phillips head driver will not necessarily be a proper fit. They might need a Pozidriv (pronounced "pozzy drive") screwdriver instead. Because the screws are likely to be very tight, you are likely to damage the heads if you use a Many microwave ovens have interior lights although these are not in the oven itself. They shine through perforations in the oven wall and are further isolated by a layer of glass. As time goes on, you will find that these lights become more and more ineffective, particularly in microwave/convection ovens. Ultimately, you won't be able to see anything inside the oven, even though the lights are on. What happens is that grease circulating around the oven is progressively baked onto the lamps and onto both sides of the glass. This happens even in the most fastidious households so that even if the oven itself is kept spotlessly clean, the globes and glass will still have a coating of baked on gunk. COCKROACHES CAN CAUSE big problems in microwave ovens. The dark patch indicates where a cockroach has urinated on the PC edge connector, leading to erratic keyboard operation. A CLEAN-UP WITH A TOOTHBRUSH and some methylated spirits was all that was needed to cure the erratic keyboard operation. A squirt with silicone water repellant completed the repair. Use the right screwdriver A PRIL 1990 7 SUSPECT THE THERMISTOR IF the cooling fan runs continuously. It's easily removed for replacement by unplugging the leads and undoing two self-tapping screws. THE VENTILATION SLOTS in the back of the cabinet should be cleaned regularly so that the oven's cooling fan can work effectively. This is best done using a vacuum cleaner with a brush fitting. While it is usually possible to change the lamps from a compartment on the side of the oven, it is not possible to remove and clean the glass, unless the oven casing is removed as we have just described. Once you have the casing off it usually is an easy matter to remove the whole bracket which supports the lamps and glass. Be careful when removing the screws that you don't let the glass fall out and break. Give the glass and bulbs a thorough cleaning and dry them off before reinstalling them. fortunately, the place which is often the cosiest for a cockroach is just behind the vacuum fluorescent digital display or behind the keyboard. Sometimes they die there and that can spell a problem for the electronics. If the clock display on your microwave oven starts to dimly light segments which are not supposed to be lit or the keyboard starts behaving erratically, the most likely suspect is a dead c9ckroach. Inevitably, the cockroach will have urinated on the printed circuit board and the resultant leakage will cause the erratic behaviour of the keyboard or clock display. If left too long, this problem can also lead to severe corrosion on the printed circuit board tracks. The oven in our photos had this problem and the cure was to thoroughly clean the affected area of the board with a toothbrush and methylated spirits. If you have a can of CRC 226 or similar silicone water repellant, it is a good idea to give the affected area a squirt or two to seal it. This will help avoid a re-occurence of any "ghosting" of the clock display. One point to be watched in disassembling the oven's control electronics is to make sure that you note the position of every screw and cable connector. You will find that a variety of different screw threads and screw lengths will be used and it is almost impossible to keep track of where each screw goes. So make notes as you go. Better still, get a piece of white styrofoam and stick each screw into it as you go, and note where it came from. Removing the cockies Another prob!P.m which even the most fastidious households can suffer is cockroaches. And these little beasties love microwave ovens. Why? Because they have nice litte niches which are warm and cosy. Un- Thermistor replacement BE SURE TO NOTE the position of every screw and cable connector when disassembling your oven. Check that the power plug is disconnected from the wall before you begin work. 8 SILICON CHIP The oven in our photos, a combination convection/microwave type, has also had a recurring fault with the temperature thermistor which controls the fan. It monitors the temperature of the cooling air and causes the fan to run until the oven cools down. When the thermistor fails, the fan runs continuously. The cure is simply to replace the thermistor, as shown in one of the photos. Lightning protection Before we conclude, we should say something about lightning protection. Because microwave ovens are usually permanently powered and they contain sensitive electronic components, they are . particularly prone to damage during thunderstorms. This is due to high transient voltages caused by nearby lightning strikes. Now most microwave ovens these days incorporate surge protection components and these will be sacrificed if a voltage surge occurs. The problem is, if such damage occurs the microwave will not work afterwards. It will have to be repaired at a cost of somewhere between $50 and $100 and perhaps a lot more if the damage has gone further than the surge protection components. Many areas of Australia have had very severe thunderstorms in the last few months and if the present trend continues, we are likely to have many more. Therefore, if a thunderstorm is coming your way, it is a good idea to disconnect your microwave oven (and any other electronic equipment such as VCRs, computers and TV sets). Sure, the surge protection components will work but why put them at risk? For the same reason, if you are going away for the weekend or a longer period, disconnect your microwave oven and any other electronic equipment. ONCE THE CASING HAS been removed, it is a good idea to vacuum out the interior of the oven to remove dust and foreign bodies. Take care that you don't damage any parts or wiring during this process. ANTRIM That way, you don't have to worry about thunderstorms while you are away. For more reading on the subject of lightning damage to electronic equipment, see this month's article in the Serviceman and an article entitled "Lightning! How To Protect Your Electronic Equipment" in the February 1989 issue of SILICON CHIP. ~ TOROIDAL TRANSFORMERS General Construction OUTER INSU LA TIO N - OUTER WI NDI NG - WI NDING QUALITY TOROIDAL POWER TR ANSFOR ME RS, MANUFACTURED IN U.K. NOW AVA ILABLE EX-STOCK AT REALISTIC PRICES. INSUL AT ION - INNER W INDING - CO RE: INSULA TIO N TAX PAID PRICES 15VA 30VA 50VA SOVA 120VA 160VA 225VA 300VA 500VA 625VA 1- 9 32.80 36.00 38.50 41 .75 44.95 55.70 62.00 72 .80 100.00 112.00 Enquiries from resellers and OEMs welcome. Quantity prices and data sheets available on request. Distributed in Australia by Harbuch Electronics Pty Ltd , 90 George St., HORNSBY, NSW, 2077 Phone (02)476-5854 Fa x (02)476-3231 APRIL 1990 9