Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Roped into fixing a friend's dishwasher Dishwashers are ostensibly quite simple mechanical appliances with pumps, solenoids, a timer and a little thing called a “wax motor”. What's a wax motor? I hear you ask? I didn't know the answer either and this was my introduction to fixing a friend's dishwasher. Fortunately, I did not have to fix it before we had dinner. I was invited to the dinner by an engineer acquaintance I hadn’t seen for a while, which was a pleasant surprise. Over dinner, the conversation veered toward work. Then my host asked if I knew anything about dishwashers. Sensing the loaded question, I informed him that I didn’t know much about them, never having had one apart. When I asked the reason, he told me his dishwasher was just out of warranty (of course) and had started misbehaving. The powder dispenser was no longer opening, meaning the wash cycle was completing without any powder or rinse-aid being introduced. Could I perhaps take a look? I wonder if mechanics who visit their friends get asked to replace a leaky head gasket after dinner. Or whether doctors out for a nice evening of food and wine with acquaintances end up getting roped into doing a quick surgical procedure on the dinner table. But I digress... I had a quick look at the dishwasher, checking the patently obvious, ie, that the powder/tablet dispenser’s door could indeed open and close freely. Beyond that, we were into (for me at least) a technical grey area. While I’d have thought nothing of opening the appliance up there and then, some serious shade was being thrown my way by the other half, reminding me that we’d come for dinner, not to work! I arranged a time to go and have a proper look, secretly relieved as I never did like going into anything blind, especially with someone looking over my shoulder. siliconchip.com.au At least I’d now have time to look into dishwasher operations and potential problems and solutions before committing to a repair. Dishwashers for dummies These appliances turn out to be a very simple idea cleverly implemented. Essentially, they are just a watertight box with trays that hold dishes at different levels while heated water is sprayed around by (usually) waterpressure powered rotary nozzles. At a certain time in the wash cycle, the powder drawer is opened and the cleaning agents are introduced. There is then a rinse cycle or two – with or without rinse-aid added – be- Australia’s electronics magazine Dave Thompson* Items Covered This Month • • • • Dishwashers for dummies Ants on the drive Earth leakage fault Heat pump repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz fore the water is pumped away and the dishes gradually dry off in the warmed cupboard. Some models may also automatically pop the door ajar during the final, drying phase to aid moisture evaporation; some also use ultra-violet light as an anti-bacterial "sterilisation" feature. While that all sounds pretty straightforward, as with all appliances there are many potential things to go wrong. Leaks, blockages or electrical-mechanical faults can cause mayhem if the cycle gets out of whack. August 2018  57 In a worst-case scenario, we could end up with a shed-load of water all over the kitchen floor, something I can tell you from experience is what you don’t want happening. The majority of my kitchen furniture is made from melamine-coated particle board and while the sides and edge-banded areas may be watertight, the bare edges of all the cabinets sitting on the floor act like a sponge to any moisture they are exposed to, so a few centimetres of standing water doesn’t do them much good! If exposed to too much moisture, the edges swell up like the half a digestive biscuit I always drop into my mug of tea, causing drawers and cupboard doors to stick and, in more serious cases, entire cabinets to sag and bow. I know this because it happened to us and repairing that kind of damage is difficult and expensive. Modern dishwashers utilise various sensors and onboard computers to control all the various functions, while some older or less-expensive types use mechanically-operated timers and actuators instead. Some early models used bi-metallic strips to regulate water flow or open the dispenser when the right water temperature was encountered, while newer or more sophisticated units use electronic temperature sensors or electrical and thermally-operated solenoids called “wax motors” to achieve the same thing. Other older types used mechanical timers to pop the powder drawer at a certain point in the cycle so there is plenty of scope for variety. The meat of the repair The dishwasher in question, a Haier branded unit, is regarded as a basic but usually reliable unit if internet chatter is to be believed. This model has a very simple control panel: an on/off pushbutton, a half-load button and a single dial on the right-hand side for different cycle settings; no fancy bells and whistles to complicate things (or go wrong!). According to my research, it utilises a wax motor drawer-release mechanism and the problem was most likely to be the heating coil for the wax motor going open-circuit (more on this below). If not that, there is likely some mechanical reason for the drawer not opening. The only way to know for sure was to open the thing up and get 58 Silicon Chip probing with a set of eyeballs and a multimeter. Thankfully, this dishwasher manufacturer has its act together and everything to do with working on this unit was a breeze. For starters, getting it out was easy; the rear two support rollers slot into corresponding locating/ holding clips that are screwed to the floor, while the front is supported by wheels; an arrangement that prevents the dishwasher moving around in any direction once installed. Grabbing the edges of the closed door and pulling straight out was all it took to roll the unit clear of the bench cavity. The power cable was plugged into a socket located on the wall up behind the unit, while the water and waste hoses fed through a sizeable hole in the adjacent cabinet wall. There was plenty of slack in all the leads and hoses and getting behind it to disconnect or unplug them was no problem. I removed the power plug but left everything else in place; no point tempting fate by needlessly disconnecting hoses and potentially introducing leaks if I didn’t have to. There were no visible screws on the outside of the cabinet but upon opening the door I could see everything I might need to undo was easily accessible from the inside and along the edges of the chassis. Hopefully it wouldn’t come to that, but either way the door front would have to come apart, as that is where the powder-drawer dispenser and actuator lives. Like most white-ware, the thin steel panels are held to the main frame using medium-to-large PK-style countersunk screws. The coarse thread pattern on these fasteners is excellent for this type of sheet-metal joinery. However, due to their size and the screwdrivers typically used to fit the screw heads, they are very easy to over-torque during re-assembly. This often leads to servicemen finding oddball-sized screws utilised, as the original’s mounting holes are stripped and ever-bigger fasteners are used by over-enthusiastic re-assemblers. I’ve even found the odd large wood-screw holding fridges or oven panels together! Thankfully, it appeared I was the first serviceman into this appliance, so no mismatched screws to deal with here. Half a dozen removed screws later, the faux-enamelled front panel fell away from the pressed-steel interior Australia’s electronics magazine door section and sprung, tubular-steel support frame, revealing some very non-technical components. Getting down to nuts & bolts There were several sound and vibration-deadening carpet swatches stuffed into cavities, a couple of purpose-shaped plastic foam packing pieces and multiple wiring looms coming from beneath the bottom door hinge and going off to points (for the moment) unknown. One small loom disappeared into some sound-proofing material about half-way up the door, to the barelyvisible, injection-moulded plastic dispenser assembly. While this was mostly buried in foam packing, I spotted the wax motor mounted on the left-hand side of the dispenser. According to Wikipedia, a wax motor is “a linear actuator device that converts thermal energy into mechanical energy by exploiting the phase-change behaviour of waxes.” It apparently contains a small tube packed with wax and a piston; heating the tube at a certain time into the wash cycle, usually electrically via a thermistor, causes the wax to gradually expand, forcing the piston to extend, which pushes on a plastic bell-crank and, via a lever arrangement, disengages a sear which then allows the powder drawer to spring open. It is a simple yet effective method of opening the dispenser. In some models, a secondary actuator allows rinse-aid to enter the cabinet during the rinse cycle. The powder drawer itself sits opposite the wax motor assembly on the other side (inside) of the door panel and a rubber seal keeps any water from entering the door cavity. As the drawer is opened by a spring and manually shut once the powder or pellet is added, it doesn’t take much to release the retaining sear and pop it open. Once open, the drawer is usually stopped by and rests under the top dish tray, a position that helps guide water into the dispenser to ensure all the powder and/or rinse-aid is washed out. On many dishwashers, when you open the door to remove the dishes, you hear the dispenser door flick open all the way, a somewhat disconcerting sound if you haven’t heard it before. However, in this case, the drawer remained closed. Testing it in-situ siliconchip.com.au would be a problem, as running the appliance through a wash cycle is both time-consuming and inconvenient, especially since I had the covers off and it was sitting in the middle of the kitchen floor. What I needed to do was to see if I could measure the resistance of the thermistor which heated the wax to operate the wax motor; this would at least tell me if the thing was still electrically alive. If it was, the problem could lie with the wiring, the controller board or something mechanical. I removed the leads from the motor terminals and with my trusty analog multimeter set to the 10W range, I placed one probe on each of the terminals. I measured roughly 2kW, which according to Google (such a clever chap!) is about average. I wasn’t too concerned with the actual resistance at this point, as long as it wasn’t at either extreme; I just wanted to know if the motor had continuity, which it did. The next step was to see if it actuated and to do that, I’d need to add power. The problem with this is that applying mains voltages to an in-place component using a pair of flying leads is a bit hairy. Do you have any good servicing stories that you would like to share in The Serviceman column? If so, why not send those stories in to us? We pay for all contributions published but please note that your material must be original. Send your contribution by email to: editor<at>siliconchip.com.au Please be sure to include your full name and address details. which is usually supplied at the right time in the cycle by the controller board. For testing, I dialled in 240VAC (indicated) on my non-Variac branded variac and waited. After a short time, the motor’s plunger slowly deployed, moving the plastic lever system. However, the drawer remained closed. From what I could see, the lever system didn’t move far enough to trip the door mechanism. From this, I could deduce we were likely looking at a mechanical fault. I removed mains power and while the motor was still warm enough, manually moved the solenoid’s plunger as far as it would go; while some of the links go off to the rinse-aid dispenser, I could see the bell-crank and cam arrangement that opens the dispenser door. The plastic arm that trips the drawer release didn’t move far enough to open it, though it did if I gave it some extra help. I took a closer look and could see the pivot pin on one of the levers was out of whack. It was either misaligned from new or slogged out, I couldn’t tell which. As no individual plastic parts are available for this dispenser, I would have to fix it if I was to avoid buying a costly replacement. The lever popped off easily enough but it wasn’t repairable using the original pin, so I removed it altogether and bored the linkage’s moulded pivot point out. I found a suitable cap screw and self-tapped it well into the base, tightening it down enough to hold everything yet allow it to move freely. Now, when I manipulated the solenoid, the drawer popped open every time. I didn’t bother running another heated simulation, as I was sure this would work now. I’d spent a lot of time on this, and as the owner assured me he could reassemble everything, I left him to it. However, I received a text message later that night saying that while the dispenser now worked fine, the dishes were still not as clean as before. I dropped by the next day to check it out and discovered that while he’d had everything apart, he removed the upper and lower rotating spraying arms in order to better clean them, but had re-assembled the bottom sprayer with the holes pointing downwards! After reversing them, a test wash cycle proved everything was working properly. Whew! Australia’s electronics magazine siliconchip.com.au Once bitten, twice shy Many years ago, I vividly recall my brother and me, who were just old enough to get into trouble, messing about in Dad’s workshop. My brother had found and wired up a mains plug, or perhaps just cut and stripped the leads of an existing cable and plug, while I watched over his shoulder. We’d found a bulb after rummaging amongst Dad’s bits boxes and were keen to fire it up, the way we’d seen him do it many times before – or so we thought. What we had done is inadvertently created what olde-worlde servicemen – in their typically dry way – call a “suicide lead” or “death cord”. In other words, a cord with a mains plug at one end and bare wires (or alligator clips) at the other. I must admit to possessing such a lead, which is typically used for testing valve-based equipment. But for obvious reasons, it makes me extremely nervous. My modern version has a crocodile clip to connect Active and Neutral to the equipment I’m testing but as kids back in the workshop, we just used bare wires and I held the bulb while we each applied one wire to one of the bulb’s terminals. It lit up, albeit extremely briefly and with the shock of the brilliance of it, I reacted and touched my lead to my brother’s. Of course, this splattered and crackled and arced, scaring the bejeebers out of us and temporarily blinding me as well; a situation made worse by the fact the workshop was now plunged into darkness as all mains power went out. Yikes! We were in for it now! I envisaged all manner of trouble was about to befall us and to this day remember the unusually stern talking to we both received from Dad. Not only were we told not to mess about in his workshop with anything we didn’t understand, we were especially not to go anywhere near anything to do with mains-level voltages. He then showed us how he powered up bulbs with a battery and encouraged us to experiment with that but not to mess about with anything that plugged in. He also demonstrated how easily we could have killed ourselves by using his multimeter to demonstrate the voltage from the mains socket. That made me paranoid enough to still be extremely wary of it today. The moment of truth I was going to use my suicide lead to test the wax motor but not with direct mains voltage. I’d brought my autotransformer along to use with it, and while the output could still easily kill me, I made darn sure the connections to the wax motor were attached properly and well-insulated from anything else before cranking up the juice. In order for this wax motor to fully actuate, it needs around 230-240VAC Servicing Stories Wanted 60 Silicon Chip Empire of the ants J. R., of Woy Woy, NSW, recently fought a pitched battle with a small but numerous army of invasive critters which threatened to overheat and gum up his electronic equipment. He did not welcome these new insect overlords and eventually banished them from his domain. Here is how it went down... My "man-cave" is in our garage which I share with my car. The walls are bare single brick and although it gets a bit nippy in the colder months, the insulated ceiling helps a bit. I have a desk, workbench and adjustable steel shelving and I keep it all pretty neat. We get a few mozzies and the odd fly in the warmer months but they are mostly dispatched with a very distinct zap by my high-voltage bug killer with UV attracting light. Ants are a different problem though. Ants need food and shelter and it seems they like to be warm. Wifey prefers a minimal amount of technological stuff in the house so I keep my computer gear in the cave, including a couple of hard drives, the NBN modem and a router to network the house. Some of my gear stays on 24/7 and has a nice, stable temperature which is most attractive to insects, especially ants. These industrious little beasts never seem to tire and they are mostly very good at staying out of sight as they move about. One day I was happily working away when a single black ant sauntered past. I brushed it to the floor and thought no more of it until a while later when I spotted another one that I imagined might be looking for its mate. I thought there might be more about so I decided to look for their source. I don't use the inbuilt keyboard or screen on my laptop so it hardly ever gets moved. I spotted a few more ants walking along a cable and then disappearing down a gap in the screen's hinge. Then I saw others leaving by the same path. Alarm bells were now ringing in my head and I immediately powered down the laptop and disconnected it. There were probably a few alarm bells ringing in the ants' heads too because more appeared from the screen siliconchip.com.au hinge and the keyboard. Without closing the screen, I picked up the laptop and took it outside to an old kitchen table we use for potting. A quick brush down of the tabletop and a strategically placed towel made a good clean work surface to open up the laptop. Fortunately, I had a set of tiny Torx drivers that I bought when I replaced the hard drive with an SSD. More ants were now starting to leave their hightech home and found themselves in a strange, very exposed place. I undid all the bottom cover screws with the laptop sitting on the towel like an A-frame. I did this because I didn't want to squash any ants – the acid inside them would affect the components and PCBs. I removed the bottom panel and saw what looked like millions of ants, their eggs and larvae all packed into every nook and cranny. With the air now freely moving around them and the internals cooling down, it was every ant for itself! They were swarming all over the table, with many of them carrying an egg or larva. I did not use any insect spray. I quickly set up an old vacuum cleaner I use in the garage and sucked most of them up. It took over an hour before they stopped appearing. I wish I had taken pictures whilst all this was happening and still can't believe how tightly packed they were. What is more amazing is that there was zero impact on the computer – it had continued to function perfectly, with no overheating or errors at all! The ants must have been occupying my laptop for days or maybe weeks before I spotted a couple of their scouts. Australia’s electronics magazine I left the laptop out in the open for another hour or so to give the last stragglers time to leave. Once it was apparently emptied of ants, I removed everything that could be removed and found a few dozen more here and there. Some compressed air blew out a few more ants and eggs and eventually, I was satisfied that they were all gone. I cleaned every surface I could with a cloth very lightly soaked in CRC 2-26. It is my weapon of choice against corrosion, moisture and electrical leakage and I feel sure the odour and the oily film would discourage future ant incursions. It doesn't go gooey either. The only visible clue I could see left behind by the ants were white marks in tight places where the eggs had been and some CRC 2-26 on a cotton bud removed them almost totally. I left the laptop on my workbench overnight and reassembled it the next day. It has been working perfectly ever since. But this story does not end here. A few weeks later I again noticed an ant on my desk. My response was swift. I whipped the laptop out of action and checked it thoroughly, but no ants, not even one – CRC 2-26 in action! Nervously, I unplugged each item one by one and moved them outside. I saw no ants until I unplugged one of my hard drives and lifted it up to reveal a rectangular carpet of ants underneath. Yikes, not again! Outside went the hard drive, onto the table, to wait for the ants to depart of their own accord. Back inside I quickly fired up the vacuum cleaner to suck up all the ants now running every which way on my desk. How had they been getting to the drive? August 2018  61 Well, the desk the drives are on is screwed to the wall with a dress strip making a nice dark tunnel out of the mortar line and delivering the ants to the hard drive. That strip is now gone. I went back out to inspect the hard drive and only a few ants were exploring the tabletop. I peered into the drive's cooling vents and could see ants, ants, and more ants! This time they were not leaving, so using a few impromptu spudging tools made from scraps of fibreglass PCB and a screwdriver or two, I was able to carefully remove the case without stirring up the ants. This time I took a couple of pictures too (as shown above). It was the same story as the laptop when it was opened – legions of ants with their eggs and larvae were now swarming all over the table and the vacuum cleaner did good once again. It was then a simple matter to unclip and remove a metal shield to inspect around the hard drive and unplug the tiny controller PCB. Hard drives have dust-proof construction so no ants could get into the mechanism. As before, compressed air removed the remaining eggs and CRC 2-26 was used to clean up all contaminated surfaces and generally protect everything. Reassembly the next day was easy. On testing the hard drive, I was relieved that the ants had caused no damage and the drive has been running 24/7 ever since. You may think that was the end of it – no way! 62 Silicon Chip Days later, more ants were observed in the man-cave, just wandering about looking for their next warm abode. An extensive search eventually found millions more living in the extruded aluminium frame of my one and only window, right behind my desk, with a few hundred extra ants and eggs crammed into the little homemade headphone switch box which is screwed to the side of my desk. I tried various surface sprays in and on the window frame and many days and dead ants later, they were still in evidence. Good grief, what does it take to get rid of them? I don't like the idea of long-lasting toxic chemicals being sprayed copiously where I spend a fair bit of my time. After some trial and error, I eventually found that a Permethrin-based ant and wasp powder in a puffer pack was very effective and the safest way to go (Permethrin is a synthetic version of natural Pyrethrin). Result – no more ants – yay! An earth leakage fault in the house One morning, B. P.’s wife turned on the light switch but nothing happened. He thought it was probably just the globe that had failed but then another light could not be switched on... The household appliances were still operating, displaying clocks and so on, so it obviously wasn’t a blackout. I headed over to the laundry where our sub-board is located to see what was going on. The safety switch for the light circuits had tripped. I tried to reset it but it just tripped again, indicating a serious earth leakage fault. We have a modern house and our electrical installation complies with the latest Queensland electrical regulations. We have two safety switches, one for the power circuits and one for lights. Three power circuits and two light circuits are protected by the safety switches. Further investigations indicated that there was a fault on light circuit #1, which tripped the safety switch when on but light circuit #2 did not. So we could have light in part of the house for the moment. The wiring in our entire house had been checked on three separate occasions by licensed electricians. The first time was when we had the underground power connected to the house, the second time was when we Australia’s electronics magazine had the solar PVR system installed and the third time was a safety check following several electrocutions associated with the installation of foil insulation in house ceilings. So this was a new fault. There was nothing for it but to head up into the roof space and have a look around and see if I could see anything. There's been a lot of activity in our roof over recent years, with the running of cables, first for the ABG satellite internet, then the interim NBN satellite internet, then again for telephone and network cabling when we got ADSL. Also, as we'd been unhappy with the foil insulation that we had installed (it didn't seem to be effective during winter) we'd been putting “Earthwool” insulation under the foil. It had to be removed and re-installed on all of the above occasions too. So, could any of this activity have caused the problem to occur? It seemed unlikely as it had been several years since anyone had done anything in the roof space. The next thing I thought of was rats in the roof. It's pretty common to find rodents in your roof space, so maybe a rat had eaten through a wire. As I entered the roof space, I could see evidence of rodents and nothing stood out as problematic. However, I decided to lift the insulation and inspect all the wiring anyway. I could still find no problems. The wiring was still in excellent order with no sign of any damage whatsoever. At this stage, I decided to give my mate Ray a call. Ray is a licensed electrician and he lives in town, but he often works in the Bay. Our place is between, and as it happened, he was just about to leave to do some work in the Bay, so he said he'd call in on his way past for a quick look. When he arrived, he brought in his “megger” and tested the circuit, which gave a reading of 33kW. That’s way too low and explained why the safety switch was tripping. I checked with my multimeter and I got a similar, but siliconchip.com.au higher reading, so we knew that there was an Active-to-Earth fault somewhere in the #1 light circuit. Now we just had to find it. The quest begins Ray headed off and said he would call in again on his way home when he would have a bit of spare time, to look into the matter further. When he got back, I suggested that we could start by disconnecting parts of the circuit until we found where the fault was. He agreed so we got started. The #1 light circuit starts at the subboard and goes to the family room, kitchen, dining room, lounge room, entry, en-suite and master bedroom, with branches going to the end of the back verandah, the side verandah and the front verandah. So by disconnecting the wiring along the way, we would be able to isolate where the fault was. We started in the family room, where Ray disconnected the circuit at one of the lights. This cleared the fault, so the fault was further on. After reconnecting the wiring there, we moved to the lounge room and did the same thing. This again cleared the fault, so we moved to the en-suite. This time, the fault was not cleared, so it was between the lounge room and the en-suite. We'd skipped over the entry light, so we headed there and Ray found that the branch line from the light socket, which took power to the light switch near the front door, was where the fault was. Ray then removed this power wire at the light switch and tested the wire, which showed the fault. So for some reason, the Active wire from the light socket in the entry to the light switch near the front door had an Earth fault. We would need to replace this cable and I had a cable drum with about 10m left in my shed. This was the older-style grey cable with thicker insulation, compared to the newer white slimline cable, so my left-over cable would match the original wiring. We both knew that running this cable would be difficult, so Ray said that he would leave it with me and he'd call in again tomorrow afternoon to connect it up, once I'd run it between the two locations. In the meantime, he left that branch disconnected, so we would then have light in all the house except for the entry and the front verandah. The next morning, I tied the end of the new cable to the old cable at the light switch and went up in the roof siliconchip.com.au to pull the cable through, while my wife fed the cable up from the drum. Our roof is 22.5° pitch, so while there's enough room to get to the walls, it's a tight squeeze. I managed to get over to the front wall with the aid of a plank set between the roof trusses which I could lie on. The first thing I noticed was a large snakeskin in the area where the cable ran down the wall but there was no damage to the cable at this point. We often find snake skins around the place. There are times when we also find the owner of the skin, which is typically a carpet python, luckily a harmless type of reptile. Occasionally, I have had to remove snakes from the house or the back verandah as my wife doesn't like them very much for some reason; I don't have any problems with them, particularly as they help to keep the rodent population under control. I usually just re-locate them clear of the house. Anyway, I pulled the new cable up and ran it over to the light socket and poked the end through the ceiling, ready to be connected. I took the old cable down with me so that I could inspect it but initially, I couldn't see anything obvious. I'd been expecting to find some severe damage to the cable, but on a quick glance, it looked like it was still OK. This indicated that there must be an internal fault in the cable itself. I decided to rip the cable open to look for an internal fault and while I was doing this, I found the cause of the problem. Around 10 years ago, we'd been away for a few weeks and when we got home, I found that termites had tracked across the front verandah in one of the joints in the concrete and eaten the pine panel under the front door. They were despatched quickly, but in any case, they can't do any structural damage to our house as it has a steel frame and is on a concrete slab. It would appear that when these pests had been present, they had actually chewed a hole in the cable in the front wall of the house. As to why it had taken so long for this fault to materialise, I suspect it was because the weather has been exceptionally dry in this area for the last decade and we'd experienced several severe droughts during that time. It's only last year that we've had unseasonal wet weather in spring, with Australia’s electronics magazine August 2018  63 very high rainfall and high humidity levels. In fact, it was mentioned on the weather report just recently that this was the wettest spring in decades. This must have caused any left-over termite material in the small hole to absorb moisture and become conductive. When Ray came back to re-connect the wiring, I showed him the damaged cable. He was amazed and he said that he'd never seen anything like it in all the time he'd been an electrician. I told Ray that I'd previously seen wiring chewed by rodents, but I'd never heard of termites attacking electrical wires before. Ray finished connecting the wiring and we once again had all our lights working. It was fortunate that we know Ray and we often do favours for each other, so that saved us quite a bit, compared to if we'd had to pay an electrician to locate and rectify the fault. If we hadn't had the safety switch in the circuit, it is possible that the current flowing between the conductors could have caused it to overheat start a fire. Fortunately, the safety switch stopped it before it could escalate. I had previously been wondering why there was a need for a safety switch on the light circuit, as it would be difficult for anyone to come in contact with a live wire, other than through carelessness when changing a light globe with the power still on. I can now see a very good reason for it. Heat pump repair Have you ever called in a repair technician, only to get the feeling that you know more about troubleshooting than they do? That must be how M. D., of Canberra, ACT, felt while dealing with multiple parties, none of which were able to find the fault in the household heating unit. He eventually managed to sort it out himself... Our house is equipped with a reverse-cycle air-to-water heat pump. It is a three-phase device with about 12kW heating/cooling capacity. It's used for hydronic heating and cooling of the concrete slab in our house and we have several hydronic fan-coil units for conditioning the indoor air. The heat pump is an imported Chinese model, rebranded and sold by a local distributor. It appears to be well built and has operated reliably for eight years. But towards the end of last summer, the unit began to intermittently trip. The system controller 64 Silicon Chip reported “Power phase error”. Throughout winter, this problem arose frequently, but unpredictably. At times, the unit would trip within minutes of starting, while at other times it would run for many hours without any problem. When the machine tripped, it could only be restarted by cycling mains power at the circuit breaker and this was becoming tedious. The electrical design is straightforward. The heat pump compressor is switched by a contactor and this is driven by a control PCB. The unit has several safeguards such as a phase fail relay, overcurrent protection (in the contactor) and a system protection mechanism that monitors the heat pump for faults such as low/high refrigerant pressure and excessive compressor discharge temperature. Having tired of this fault, I called the supplier. They suggested that the heat pump might have lost its refrigerant and to get a heat pump specialist to take a look at it. However, the lowpressure gauge was reading the correct pressure during operation. I called a heat pump technician. By connecting pressure gauges to the high and lowpressure service ports, he declared the system to be fine. I then called an electrician. He checked phase voltages and currents and declared all to be in order. Without any real diagnosis, he thought that the phase failure relay would need to be replaced. The phase failure relay trips the main compressor contactor if the phase-to-phase voltage is out by 15% or if the phases are not in sequence. This was an expensive guess and did not fix the fault. The electrician then sought to start replacing every electrical component upstream of the heat pump without further diagnosis. I felt it was time to take a more considered approach. I called the supplier again. They suggested that I bypass the compressor safety switches to see whether that was the problem. Although these switches are low voltage inputs to the controller, I was not comfortable in operating the heat pump without these safeguards. In any case, the heat pump reported a power system fault. The user manual is brief and offered little guidance. As is often the case with imported units, the English translation is ambiguous. The manual suggests that the heat pump is equipped with a system protection PCB in adAustralia’s electronics magazine dition to the main control PCB. The circuit diagram in the door of the unit also shows this PCB but it was not present in the hardware! The error we were getting was supposedly coming from this protection PCB, so I thought that the error message itself could have been in error. I subsequently noticed that the compressor heating band was in poor condition with exposed wires where rats had chewed away the insulation. I disconnected the band, thinking that it may be the cause of the fault, but the unit still tripped intermittently. It seemed to me that the control PCB was deciding to switch the compressor contactor off in response to an unexpected reading from some sensor. So I decided to monitor the low-voltage signals coming into the controller using a Maximite as a high-speed data logger. The contactor provided an auxiliary contact that I used to detect when the unit tripped. I monitored the low and high-pressure switches, the over-temperature switch, the pump flow switch and the phase failure relay at 20ms intervals. Having observed several trip events, I could see nothing that would cause the unit to trip. I even powered the control PCB from an external battery to eliminate any power supply issues. While checking voltages around the control PCB, I noticed that the signal from the over-temperature protection switch was sitting at around 1V and was unsteady. This was odd since all the protection sensors were supposed to be normally-closed switches. I had configured the Maximite to read the switches using digital inputs and so this had remained undetected. I reconfigured the Maximite to read analog voltages and confirmed that the “switch” was producing a varying voltage as if it had gone high-resistance. Replacing the over-temperature cutout switch solved the problem. The replacement cost about $5. It was unsettling to witness lack of proper diagnosis by the electrical technician, the poor support offered by the supplier and the poor level of supporting documentation and the misleading error messages reported by the machine. In the end, it turned out that I was the only person able to properly troubleshoot this system! But it certainly is a great relief to have heating back in place for the Canberra winter. SC siliconchip.com.au