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SERVICEMAN'S LOG Rangehood repair full of red herrings Dave Thompson* No matter how good you are or how much experience you might have, sometimes servicing just comes down to luck. That can mean good luck, where you take apart a monstrously complex device and immediately spot the simple problem which is preventing it from operating. Or bad luck, when you are trying to repair the simplest device and nothing you try seems to help. This is the second kind of story. I've had some brilliant successes but more than once, and especially lately, I’ve completely misread clues and then made assumptions that steered me away from the true cause of a problem. Once, I stripped a lawn mower motor down to the block, looking for all manner of non-existent faults, when all I needed to do was change the spark-plug; something even the least- experienced DIY enthusiast knows is the first thing to check! Another time, I stripped every nut and bolt from a vacuum cleaner in order to disassemble it when just a few screws held it all together ­— I just didn’t twist it the right way to pull it apart. We've pretty much all had this kind of experience and while you just have to chalk it up to experience and learn from your mistakes, it's still incredibly frustrating. So I'd like to play a game with you, dear reader. I'll describe the symptoms of a very real problem we had with our just one-year-old range hood and see whether you can guess what was wrong while you're reading through all the trouble-shooting I did, which only served to demonstrate what wasn't wrong with it. The appliance in question is a Robin Hood Range Hood, model RWV3CL6G. It was installed almost exactly one year ago, having been purchased brand new from a local big-box store. It wasn't even at the age of a toddler yet but was already starting to spit the dummy. This particular model has five illuminated buttons along the front panel (called "sophisticated electronic controls" in the user manual) to control the fan and the two 24V, 1.5W LED downlights in the front corners. The buttons turn a function on with one press, and off with another, each time accompanied by a soft beep. A blue LED backlight indicates the button’s state; illuminated when on and dark when off. The buttons are positive, tactile and worked well, and the three-stage fan and LED lights were both powerful and efficient. We use this appliance frequently. It sits directly over a gas hob in the kitchen and is vented to the outside world through the wall, via a short piece of steel ducting. Items Covered This Month • • A rangehood of repairs MIG welder repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz siliconchip.com.au Celebrating 30 Years November 2017  61 All is not well in the serviceman's abode A few weeks ago, while cooking, and with the fan and lights on, the LEDs flickered in unison before coming back to full brightness. I switched them on and off a few times and they still operated normally, being flickerfree for the next few days. I put that event down to a one-off power issue and forgot all about it. Less than a week later, though, the same thing happened again, with both lamps dimming to a dull glow before coming right. The following day, I switched the lights on and halfway through cooking our meal, they dulled down and then went dark altogether. Of course, this had to happen just a few weeks after the (rather short) warranty ended. Like many others, I suspect companies choose their components very carefully so as to maximise the chance this sort of thing happening, so you have to run out and buy another one... and another... and another. Many of my own customers have often joked that computer manufacturers must install a timer into their machines so they fail just out of warranty. All joking aside, though, it does seem awfully coincidental and convenient for the manufacturer… So, both LEDs were not working. Pressing the light control button still resulted in a beep and the blue LED lit up but there was no light output. Occasionally, when switching on, the LEDs would flash at full brightness but almost instantly go dark again. This always happened to both lamps at the same time but after this, for the most part, there was no output at all. So here's the fun part. What do you think? What would you have done, which parts would you have blamed from such a fault? See if you can outsmart the serviceman and figure it out before the end of the story. Given these symptoms, I suspected the LED driver. One lamp behaving badly might be a failing LED, but both behaving in an identical manner? And anyway, surely LEDs would last longer than this, while drivers can fail quite early due to dodgy electrolytic capacitors, voltage spikes on the power lines or various other woes. access issues. I disabled power to it at the breaker board before removing the lamp assemblies. These were held in using clip springs and simply needed to be prised out with something flat and thin. Once out of the housing, the LED units hung on their respective power cables. The lens and reflector assemblies were held together in a bayonetstyle fitting; a quick twist anti-clockwise separated the lens, reflector and the brushed-aluminium trim ring, revealing a G4-type LED. The heavy-duty lamps were virtually all aluminium heatsink, with the LED die itself set into the very end. Rated as being equivalent to a 15W incandescent bulb but drawing just 1.5W, I was reasonably sure that they were bulletproof, and with no signs of overheating, I set them both aside and dug further into the hood’s internals. Removing the aluminium filter from underneath gave both better access and more light, so I could at least see what was going on. This model range hood has a thick, curved glass canopy, which is held onto the body of the appliance with four large set screws. To get to these screws, I had to remove the stainless-steel chimney, which is a three-sided cover that conceals the fan, output ducting and electronics. It is simple enough to remove the two small PK screws holding it in place but awfully fiddly to get free due to it being right at the ceiling and with me not wanting to stand on the benchtop or gas hob. A bit of wiggling around (and cursing) got it free, but I wasn’t looking forward to having to get it back up there; we’d had issues lining it all up when we first installed it, and there were two of us on the job back then. Still, I’d cross that bridge when I got to it; all I could do now was carry on and find out why these lamps weren’t working. With the panels and canopy off, I had a clearer view of the internals. The front switch panel was connected with a ribbon cable to a controller circuit board further back in the guts of the range hood somewhere. To get to the board, I’d have to dig deeper. However, right in front of me, mounted on the shoulder of the main frame, was the LED power supply/driver. This unit was smaller than I expected, being only about 30 x 30mm and around 20mm thick. It had a twopin terminal block for mains power input and a similar terminal block that the dangling LED holders were wired into. It only had to deliver 3W in total, so I guess it didn’t need to be very large. Much wasted effort later Pulling the thing apart was relatively simple but the fact it is stuck to the wall above the hob introduced some 62 Silicon Chip Celebrating 30 Years siliconchip.com.au Make the Switch to WAGO Ensures Scalability Of Your Network Infrastructure The Power to Provide Solutions! Complete Flexibility with Greater Security! The Individually Configurable Industrial Management Switches reliably network all ETHERNET nodes and ensure continuous access to machines and systems. The network ranges of redundancy protocols enable the creation of redundant netwok structures with a short recovery time of less than 20 ms. This guarantees secure communication, even when connections are faulty. Every WAGO switch also features a redundant power supply for uninterruptible data communication (transmission rate up to 1 GBit/s). This value-add feature contributes to secure operation of machines and systems. Industrial Management Switches also support up-to-date security functions, such as “Mac Limitation,” “Port Security,” and authentication per IEEE 802.1x. Furthermore, “IGMP Snooping broadcast and bandwidth limitation enables additional data flow control. For further information visit WAGO Australia & New Zealand www.wago.com.au or call us on (03) 8791 6300 siliconchip.com.au Celebrating 30 Years November 2017  63 With the driver nicely accessible, I broke out my multimeter and, ensuring all the wiring was in a relatively safe state, restored power. I pushed the light switch, saw that the blue LED lit up and with my meter sitting on the kitchen benchtop, I carefully measured across the input terminals of the driver. The meter read around 230VAC on the 250V scale, which was close enough for me. Add cornflour to plot, stir until it thickens I use an analog meter for this type of work, as it is easier for me to read a meter needle position than having to read and understand a numerical value. So after adjusting the meter range, I measured across the driver’s output. Hmm, I got 24V; I was expecting nothing, or thereabouts. Still, it might be intermittent so I switched it on and off a few times, checking each time. getting a stable 24 volt output. Curious. After powering things down, I reinstalled the bulbs into their respective holders. This time, when I pushed the light switch, for a brief moment, both bulbs lit up to full brightness before going dark again. What was going on here? With the light switch on, I set about wiggling and measuring in equal amounts, but aside from the briefest of dull glows, they remained stubbornly dark. Somewhat puzzled, a notion was beginning to poke its way through to 64 Silicon Chip the front of my mind; after all, despite the appearances of the LEDs, there could only be one explanation, given the results of these tests. To confirm my diagnosis, I took out the LEDs again and departed to my workshop, where I broke out my benchtop power supply. After carefully setting the output to 24V and limiting the current to about 100mA, I connected each LED up to the supply using small, alligator-style clips. As had slowly dawned on me, neither bulb showed any sign of life. Actually, that’s not quite true; once or twice, as power was applied, I saw the briefest of glow from the LEDs. If I interrupted the supply by touching and releasing one leg of the LED, I could very infrequently get a fullbrightness flash, almost like a flashgun, from one of the bulbs. In laymans’ terms, they were both poked. How could I be so thick? A simple bulb failure, and here I am with the range hood lying all over the kitchen bench. Both bulbs gone, failed at exactly the same time and exhibiting (for all intents and purposes) exactly the same symptoms. Apparently, failed LEDs can show full brilliance for short periods of time. I know this seems unbelievable, even unlikely, but I assure you I’ve described things exactly as they happened. If only one bulb had failed and had flickered from full brightness to a dull glow, I would have assumed that bulb Celebrating 30 Years had failed and simply replaced it; a five-minute job (if that) which doesn’t involve disassembling the range hood into component parts. To my mind, both LEDs exhibiting exactly the same behaviour as they failed beggars belief. Surely, the driver had something to do with this? Perhaps it surged and took the LEDs out, fooling me into thinking it had failed instead. To determine if this was the case, I removed the two screws holding the driver to the frame and took it out to the workshop, where I proceeded to connect some strip LEDs I had on the bench and powered them up using mains power input, just as it would be in the range hood. I ran the strips for about an hour at full power, monitoring the voltage output and current, and everything was as stable as could be expected. Annoying to say the least but at least I know the driver likely had nothing to do with anything, and it is just a LED failure, pure and simple. An easy fix. Or is it? Well, this one really got me good. Hopefully, I’ll be a bit more aware in the future but the way these LEDs failed is a new one on me. Now all I had to do was find some replacement LEDs and get the range hood reassembled. But finding replacement LEDs turned out to be tougher than putting the range hood back together. siliconchip.com.au The only G4 LEDs I could find locally (and when I say locally I mean within New Zealand) were listed on a campervan manufacturer’s website at a whopping $43 each! Needless to say, I wasn’t paying those daylight robbers anything like that for a LED. After a few more wild goose chases, with similar results price-wise, I decided to try my luck over at AliExpress. Sure enough, there were hundreds of listings of G4-type LEDs, the vast majority being replacement lamps for chandeliers and cove lighting. But they would likely be suitable for our needs as well. The LEDs I ended up buying have a 12-24V AC/DC input range and are rated at 3W each. The cost was just US$7.80 for a pack of six. While they might be a little over the top for a range hood, there was nothing with a lower power rating that I could find with a G4-style connector. We put up with not having lights in the range hood until the new LEDs arrived. They are actually rather impressive in the flesh. For a start, they are quite large; at 15 x 45mm they are about 10% larger than the originals, and therefore barely fit into the receptacles in the range hood. However, they are also flexible, being made out of some kind of amazingly-clear and pliable silicone material, so a bit of fettling got them fitting OK. However, it appears I didn’t choose well, as the driver won’t power up the new LEDs. All I get with one LED plugged in is the LED flashing at full brightness at about two hertz. I didn't want to try plugging both LEDs into the driver since that would be a 6W load and as far as I know, it's only designed to handle the original 3W worth of lamps. I tried powering the new LEDs with my benchtop supply, thinking perhaps I’d purchased blinking LEDs by mistake but I found that even at 10V and 20mA, they easily lit up. So perhaps there is something awry with that driver after all. That said, it did test out OK, powering up the strip lighting with no apparent problems, so it is more likely just a mismatch between it and the new LEDs that leaves it unable to power them properly. Regardless, I’ll have to either replace the LEDs or find another driver. After another quick look on AliExpress, I found a new driver that should do the siliconchip.com.au job. While a little bit larger physically, it will still easily fit in the space for it within the range hood, and with an output of 12V and 2A, it has more than enough grunt to drive these two LEDs at full noise all day long. At just US$4.50 with free shipping, it is also affordable and, I think, a reasonable upgrade to the range hood. It is yet to arrive, but I have no doubt it will be the answer to this whole problem. So once again it seems I overthought what turned out to be a stupidly-simple fault. In my defence, given the symptoms and the fact that both bulbs failed at exactly the same time, I didn’t even consider the LEDs could be the issue. I expect LEDs to last a lot longer than a comparable halogen or incandescent bulb of similar output, as per all the marketing hype, and to have them fail at just over a year with very little actual use is a big disappointment. I would guesstimate that, on average, they haven’t been used for more than a couple of hours a week over that year, resulting in a lifespan of only around 100 hours. And I consider a replacement price after such a paltry life of 40 plus bucks each a huge rip off. The pair of halogen bulbs in our previous kitchen’s range hood only needed to be replaced twice each in 10 years and likely had more Celebrating 30 Years use than the LEDs in the range hood at this house, so either we got unlucky, or we got burned. Time will likely tell which it is. Well, I hope you enjoyed playing "outsmart the serviceman" and I'm sure many of you made better guesses than I did initially. At least I'm on the way to having this one sorted but really, you'd think that replacing a lamp would be an easy job for an old hand like me. Metal inert gas (MIG) welder repair The next story is from M. H., of Albury, NSW who had to first reassemble and clean out what was left of a MIG welder before he could even have a go at fixing the fault which left it in this sorry state. His story is as follows: My friend’s mate gave him a MIG welder that was not working and he asked me to fix it. When I first looked at the beast, it appeared that lots of different people had previously had a crack at getting it going again. The handover ceremony involved the presentation of an ice-cream container of assorted parts along with the statement "It's beyond us, Mike. If you can get it going, it is yours!" I own a handbag-sized IGBT welder with rod and TIG options and at the time, had developed just enough skill for backyard, weekend-warrior type November 2017  65 The control knob is connected to a stepped rotary encoder on the front display PCB. The mounting pins were repaired and copper wires added to provide anchoring. welding. I had observed professional welders using a good MIG unit, but when I investigated the cost, I decided I really did not need one. Well, this was my opportunity to gain a working MIG unit as well as further my skills at saving stuff from the landfill. The broken unit I was given is a BOC Smootharc 180 MIG Welder. MIG stands for metal inert gas and it works by feeding a wire out of the handset end to maintain the arc and also provide the metal for welding. A gas is simultaneously discharged under the hood around the arc to exclude oxygen, which would prevent a proper weld from forming. You can also use "gasless" wire that contains a material which vaporises, providing the same oxygen displacement protection as the inert gas would. This has the advantage that you don’t need to rent a gas bottle, lug it around or have it periodically re-filled. Anyway, having decided to fix it, I placed the unit on the bench in my workshop and attacked the obvious faults first. No need to remove the cover; it was already in the ice-cream container. I cleaned out all the bent and melted tips from the wire feed compartment. It seemed that this side of the unit At a glance everything in the power section of the welder looked like it would work OK, until it was switched on. The fault lay on the main PCB. has been used as dumping ground for busted parts. I then checked the action of the motor drive for the wire feed by manually turning the motor parts and all seemed okay. The other half of the casing contains the main circuit board and it was lined with dirt and dust and metal shavings from years of grinding near the unit. Evidence of attempted repair action was everywhere. The main heat sinks were floating free off the board. The IGBT drivers were floating free and there were broken wires on the gas solenoid. Transistor clamps had also been removed and a heat-sensing thermistor had been torn from the mountings. What a mess! It was going take a lot of work to put everything back in place before this baby got mains power connected again. The front of the unit housed a control knob to vary settings on the display, which was broken and wobbling free. It was connected to a stepped rotary encoder. The shaft retaining pins were broken and it required some restoration work and wire strips added over the base unit to give it strength. With all the obvious faults restored, Servicing Stories Wanted 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. 66 Silicon Chip Celebrating 30 Years it was time to review what this thing was actually capable of doing. 15A mains power passes through a toughlooking mains filter to keep the noise inside the box from getting out on the mains and then immediately through two large bridge rectifier blocks and then off to the main board via oversized red and black wires. A thick cable runs from a very large heatsink to a lug terminal secured by a 10mm bolt, to connect the welding cable to the front of the unit. The other work cable passes though a large toroidal coil and then through what looked like a final filter. I assumed this section basically operated as a “buck” step-down converter, but that’s unimportant at this point. Since all was looking good and clean, and everything appeared to be in place, it was time to "light this thing up". So with mains on, the fans were running and display on. Good. The knob varied the display numbers. Pressing the wire feed button caused the wire to spool out. All was looking great. I pulled the trigger on the handset and tapped the work clamp with no result; no sparks, no action. I now had the original fault to chase. What should have happened when I pulled the trigger was that the wire feed motor should have started. When the wire hits the workpiece, this should cause the current to flow, creating the welding arc. But tapping the wire to the workpiece produced nothing. It was time to visit the main board and inspect it further. I am always pleased to see designsiliconchip.com.au This interior shot of the MIG welder shows the main control The wire feed motor is mounted at the opposite end of the PCB and the power supply filter board above it. The failed case. One of the symptoms of the fault was that it did not component is at lower left; see the close-up image below. operate when expected. ers of equipment using LEDs on the boards to indicate conditions. A LED blinked when I tapped the wire to the work, so clearly it was detecting this action. But nothing happened when the trigger was pulled. I then investigated if there was a fault in the trigger or handset. Using my old faithful Dick Smith analog multimeter it was easy to prove the trigger signal arrived at the main board. Near the connector for the trigger was a cluster of resistors, a transistor and a few diodes to form what looked like a trigger detection circuit. Its output was fed to an opto-isolator. I powered the unit up again and measured the voltage across the input side of the opto-isolator, which revealed 14V when closer to 1V should have been present, ie, the forward voltage of its internal infrared LED. So this LED must be open circuit. A quick Google search showed that the opto-isolator was nothing special and just had the typical transistor output. As I am a “throw nothing out” sort of guy, a quick search in the junk pile revealed a replacement. I used a heat gun to de-solder the SMD opto and the replacement was soon in place. At last, I was ready to apply mains again. The fans ran, the LED displays came on and the unit seemed ready. So to test it I tapped the wire to the work and pulled the trigger. Sparks flew everywhere and red dots dashed about on the floor. The carpet mat at my feet was smoking with burn holes. Wire was feeding out of the handset. It was like Christmas siliconchip.com.au on Black Saturday and my workshop filled with smoke. I decided that perhaps I should control the situation a little better before the next strike on the work clamp. To control the wire feed, I removed the drive clamp. This will allow the motor to drive without feeding wire out of the handset. I then pulled the trigger and viewed the main controller board. Lots of LEDs blinked, indicating the IGBT driver section was operating and output was present. When the trigger was pulled, the output floated about 20V. It was now time to move outdoors to make more smoke and try melting stuff. It all seemed to do what it should, so I was convinced it was time to visit a friend that has a proper MIG to compare with my now-working unit. It compared well, so the conclusion is that this wonderful beast had been brought crashing down by the failure of a 20¢ opto-isolator and was almost relegated to the dump because my mates tried to fix it by "just undoing stuff" with no real idea of what the fault may be. Luckily I managed to rescue it! SC The faulty opto-isolator is near the top of the photo (U16), while the hand trigger switch connectors are at lower left (CON3). Both solder joints had blown for some reason or another. Celebrating 30 Years November 2017  67