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SERVICEMAN'S LOG The alarming false alarm system I’ve written before about home alarms and the problems that DIYers like myself run into working on them. The main problem is that I don’t know what I don’t know, and given my proclivity to ‘have a go’, it’s no surprise that I sometimes come unglued. The silver lining is that everything is a learning opportunity; next time I’ll try again, and if necessary, bring in someone who knows what they’re doing. This means – rightly or wrongly – that I sometimes try to do jobs usually best left to professionals. However, getting a professional to do the job doesn’t guarantee that it’s done right either! Admittedly, my expectations might be unrealistic. But if I’m paying (usually handsomely) for a job, I expect a certain level of competence when it comes to the final result. I’ll be the first to admit that I’m an average serviceman; I win on some repairs and fail at others. My Dad, and then the airline I served with, siliconchip.com.au instilled in me the will and skills to do a good job, and this set my standards high. Given the environment at the airbase, and the number of people who worked there, it is inevitable that there would be others far more capable than myself. Those guys studied hard, got licensed to the hilt and more often than not, saw out their careers pushing papers in technical support. Then there were the guys who weren’t as ‘booksmart’, but who were very manually skilled. Australia’s electronics magazine Dave Thompson Items Covered This Month • • • • Alarm systems with false alarms are alarming Two Yamaha amps and a Bose FX unit restoration Behringer RX1202FX 12-channel mixer repair Rangehood repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz I knew engineers who could weld wood to concrete without a visible bead, or strip jet engines or avionic systems, overhauling every component and then reassembling it all blindfolded (not that that was encouraged)! I was somewhere in the mix; everyone had different skills, and with a bit of good fortune, we all ended up where we wanted to be, doing what we wanted to do. Now and then an extrachallenging job would come across the bench to spice up the workday. I sometimes miss being part of that environment. Lately, I’ve encountered several ‘professionals’ where tool skills didn’t seem to be part of their job requirements. It pains me to shell out for a job that, even when viewed with hindsight, we could have done ourselves; and sometimes done better. I’ve had workers come to service appliances, repair water pipes and install fibre broadband and in all these cases, I reckon I could have done at least an equally good job. I know about as much about plumbing as I do about mathematical formalism in quantum mechanics. But with all due respect to plumbers, charging $700 to zip-cut a metal pipe off and replace it with a plastic section over two hours is awfully steep. And the less said, the better, about the gas-fitter who scribed a divot into our brand-new benchtop when his power drill slipped. November 2019  61 I could have fitted that hob, or done the pipes; I just wasn’t “qualified” for the job. These situations remind me of the old yarn about the boilermaker who was brought in to fix the misbehaving boiler on a steamship. He walked in, looked around for a couple of minutes, took out a tiny hammer and gently tapped a valve. The boiler then worked perfectly. The ship owner was irate to receive a $1000 bill for this fix, and asked for an itemised invoice. The invoice he received read: Tapping the valve: 50¢ Knowing where to tap: $999.50 Total: $1000 So I guess my point is, you hire the professionals to do an easy job so that if it turns out not to be so easy, you don’t get into a lot of bother. But it’s still galling when someone charges you a lot to do a sub-par job, especially when you know you could do better. Installing the old alarm system When we moved into this house, we installed a security system. This was for our own peace of mind and also the safety of my tools and my customers’ hardware in my workshop. (April 2018; siliconchip.com.au/Article/11033). It is debatable whether a neighbour 62 Silicon Chip would even bother glancing at our residence if the alarm was blaring away. But it is nice to know that in a dimlylit bunker somewhere across town, an operator is sitting at a console, waiting for our monitored alarm to go off, so they can send a security guard around and charge us a hundred bucks for the privilege! If I hear a nearby alarm sounding, I always wander down the street and have a look at what’s going on, even though I’m not sure what I could do if I do find something amiss. I’ve accidentally set my car alarm off in public several times. On one occasion, I’d locked myself out of it in a supermarket car park and was trying to break in. Shoppers walked past without a raised eyebrow, though admittedly some did look annoyed at the noise! My first stop was the alarm monitoring company we’d used for the last 20 years. They had provided our original alarm system by way of a deal where you got the system free if you signed up for three years monitoring at a buck a day. We’d moved that system a couple of times over the years and weren’t keen on moving it again. The bad news was the alarm companies don’t do those deals anymore. Australia’s electronics magazine I would be looking at “about a grand” to have a basic system installed and configured by Chap and Bloke, the two overall-clad likely lads who were contracted to do this company’s alarm installations. I’d had dealings with these guys in the past and wasn’t too impressed with their work, so I thought I’d check the internet first. There are literally thousands of alarm systems on AliExpress (one of my favourite websites back then), some cheap and some costly; the biggest problem was which one to choose. Talk about upskilling by proxy; I had to learn and translate a whole raft of new acronyms and technical doublespeak. I also had some strict requirements; many of the newer systems used only SIM cards and the digital GSM (cellular) network to send data to the monitoring company. Not only does this involve an ongoing cost for any calls made from the alarm, but back then, the monitoring company couldn’t accept signals sent this way. So we had to ensure any alarm we installed used the increasingly ‘old-tech’ copper-wire based PSTN (Public Switched Telephone Network) system. siliconchip.com.au This proved to be no real problem as many compatible PTSN systems were available, and the majority of those systems used 433MHz wireless technology to connect sensors to the ‘base’ unit. This was appealing on many levels, not the least of which is that I am getting too old to be crawling around in the roof or under floors to route wires. I ended up purchasing a mid-level digital base unit with both PTSN and GSM capability, along with suitable wireless pet-sensitive PIR sensors, a couple of photo-electric smoke detectors and some magnetic door/window switches. I could have up to 99 zones with this system, and while this was a few more than I needed right away for Casa Thompson, it would allow me to expand. Who knows, I might eventually add 92 more rooms to my house. Any more than that and I would need a new alarm system! Installing this system was as easy as it sounds, with the majority of the work going into deciding where to put the sensors and screwing their mounts to the walls or doorframes. Then it was just a matter of programming the base unit and adding the sensors to it. I wired in a copper phone line using the supplied cable, which I first had to modify by cutting off the original RJ11 plug and crimping on a kiwi Telecom BT-style connector. I had a siliconchip.com.au spare SIM, so for ‘belt-and-braces’ monitoring, I put that into the system as well. I then encountered the first hiccup: the new panel wanted to know my monitoring company’s phone number and my customer number, neither of which I knew. The only way to get this information was to call up the company, and of course, they weren’t about to dish that information to just anybody, and (quite rightly) didn’t want some cowboy messing around with their system. In the end, I had to book Bill and Bob to make a visit, just to watch them press a couple of buttons and make a test call to the monitoring centre. Note to self: make sure to factor another $200 into the cost of any future alarm system. For the most part, this alarm worked well. It came with four keyfobs for wireless arming and disarming, and these are extremely handy; especially because after a few years, I forgot what codes I originally programmed into the panel for manual disarming, since I never used them! A flawed system One of the big downsides is replacing batteries in the sensors; the PIRs use that old standard, the 9V battery, but the window and door sensors chew up those little 12V 23A-style buggers, and neither are cheap to replace. Note Australia’s electronics magazine to self: factor in many more dollars for batteries for any future wireless alarm system. Replacing batteries is to be expected, but as the voltage drops, some sensors get unstable, so we were continually having false alarms. In fact, this was usually how I found out that a sensor battery was going flat. Being rudely woken up at 4am by a shrieking piezo siren is not my idea of fun! Being at home when a false alarm happens is one thing; being out and about when it happens results in a whole other set of problems. The alarm calls the monitoring company, telling them what zone is triggered and what type of emergency it is (fire, intruder, panic etc). It also calls my mobile phone via the SIM with a pre-recorded message with similar information. The problem, as I discovered, is that the monitoring company usually get straight on their phone to call my registered number to report the alarm going off. But they get a busy signal because the alarm system is repeatedly calling me, tying up my phone. If I didn’t answer, they’d send a security guard around and charge us the fee. This can be a real pain, especially as Murphy’s Law dictates this happens only when I’m in an important meeting, driving, or sitting in the dentists’ chair. I soon reconfigured things to make this process smoother. Another lesson learned. Note to self: factor in November 2019  63 many potential visits from security guards in any future wireless alarm system. Long story short, I got sick of constantly replacing batteries and paying security guards to tell me we had another a false alarm. I can’t remember our old wired system, which we’d used for decades, ever going off (unless we tripped it accidentally). So I resolved a few months back to look for something similar to that and replace this new-fangled-but-flawed wireless thing. I should also add that recently, the monitoring company let us know they had upgraded their system and could now do GSM signal monitoring. While this meant we might be able to ditch our old copper phone lines, they also said it would incur higher per-call costs than what we currently pay. I briefly pondered how that worked, since we paid for any alarm-generated cell-phone calls anyway, but gave up and decided to stick with the monitoring system we already had. Fed up with wireless hassles I found a new alarm system on AliExpress almost identical to our old 8-zone wired system, but with 16 zones, a lockable metal box and PSTN dialling. It was quite reasonably priced and included a couple of fob remotes for wireless arm/disarm. I could also pick whatever sensors I wanted to go with it, and opted for three pet-sensitive and three ‘normal’ PIR/microwave combination sensors. These are Canadian-made, and apparently they have the lowest false-trigger figures in the business. I also got two smoke detectors and an extra keypad. All I’d have to add is a 12V SLA backup battery and some cable; I already had a 100m roll. One obvious downside to this decision is the requirement to run those cables. This is usually not too much of a hassle in any normal house (at least here in New Zealand) with reasonable roof or underfloor space. However, our house was converted from a single to a double-storey home 30 years ago. So much of the groundfloor ceiling space needed for routing cables has a whole other house sitting right above it, leaving almost no usable gaps. While there is a very narrow crawlspace around the perimeter of the roof, I (and a builder friend) pondered 64 Silicon Chip this sensor location and cable-routing problem for weeks. We eventually decided that the only way was for one of us (that means me) to suit up, get into the roof space and to probe aptly-named fish tape (or fishing rods) through any gaps we could find between the floors. Hopefully, we could route the wires as close to the ideal sensor positions as possible. If the worst came to the worst, I’d run the cables out through the roof tiles and around the eaves. Note to self: before buying wired alarms, check out potential cable access problems! I’m not claustrophobic enough to have a problem crawling around inside the wing tanks of aeroplanes, but that was 35 years ago, and I had proper gear then. Wiggling through dark, spider-web and mouse dropping-infested gaps I can barely fit my shoulders through while dragging a long fibreglass pole is not how I pictured spending my increasingly autumnal years. Someone had to do it, though. I am happy to report that with some surprisingly agile gymnastics and inspiredbut-educated guesswork from both of us, we were able to run all the cables we needed to the positions we wanted. We only had to drill one hole in a less-than-ideal position, around 50mm away from where we wanted it. While I was up there, I ran Cat6 network cabling out to my workshop and a couple of other rooms I wanted connected, so we got a lot done in one day. Note to self: allow several days for physical recovery after cabling work. I mounted the alarm box by the access door in the roof space, which is a natural junction of all the cables coming from the sensors. I mounted a couple of cheap LED lights up there too, which made connecting up the sensor wires a lot easier. This was all relatively straightforward work. But I did need to ensure the sensors’ operating mode (normally-open or normally-closed, set by jumpers on their PCBs) matched the panel configuration. Normally-closed operation requires a so-called “end-of-line” resistor (2.7kW) wired in series with the sensor’s ‘hot’ lead, while a normally-open sensor requires the resistor to be wired in parallel with the hot and ground leads. The alarm’s user manual had these two diagrams transposed, but I eventually worked it out. I still had to deal with those unknown monitoring numbers. Luckily, I found them in my old wireless panel, so retrieving them and programming them into the new system was a cinch. It’s been working now for months and not one false alarm, so I’d call that job done, and not a ‘professional’ in sight (thank goodness?). Yamaha amplifier and Bose guitar effects unit restoration D. D., of Petrie, Qld is a serial repairer and recently managed to easily fix two different Yamaha amps and a guitar effects unit, two of which had already been relegated to the tip! That’s a pretty good effort and here is how he did it… The local tip has a recycling section where you can drop off your unwanted gear to sell to people who can use it, but their policy recently changed, and they no longer allow mains-powered equipment to be sold in this manner. But as I was recycling some bits and pieces, I happened across a Yamaha RX-V457 7.1-channel surround receiver. A tip worker saw me looking at it, so I asked him if I could have it. He said no (with a wink), but if he doesn’t see me take it, then he can’t do anything about it. He then walked away. So I became the proud new owner of an amplifier. I got the unit home, plugged it in, and nothing happened. So, Google to the rescue. There is a common fault with this amplifier, a capacitor on the inlet power circuit board goes bad. I tested capacitor C4 and found it much lower than its rating of 22nF, 630V. I replaced it with a new one from Jaycar and the amplifier now sounds fantastic, although I did have to buy 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. Australia’s electronics magazine siliconchip.com.au a remote control, which was the most expensive part of the repair! Here’s another story of a tip rescue. My brother-in-law’s brother works at his local tip and picks up bits and pieces all the time. Lately, he happened across a Boss ME-50 guitar multiple effects unit. These are pretty cool and have pretty much everything a guitar player could want, with some 22 effects. The unit had no (or very low) output signals. Disassembling it took a while due to the 20 odd potentiometers holding it together. I found a very nice looking PCB populated with SMDs. A quick internet search revealed the service manual and a circuit diagram. I applied a sinewave signal to the aux input using my smartphone. I could see the signal going into the circuit but nothing coming out. While looking at the diagram, I noticed there are muting transistors on the outputs of all the channels, so I lifted one of the legs of all of these SMD transistors, but there was still no output. Next, I tried removing each op amp one by one to see if one was causing the problem. As luck would have it, the very first op amp I lifted (IC6, NJM4556) solved the problem. It appeared to have a short circuit across its inputs, which was shunting the input signal to ground for all the op amps. As soon as I replaced that IC, the whole thing worked. My third repair was of a Yamaha RX-V2067 7.2-channel surround sound amplifier that was given to me. It would turn on but then switch itself off after a second or so. I initially thought great another easy fix with a faulty capacitor on the power board, but it was not to be. So I downloaded the service manual and put the unit into service/no protection mode. The unit prompted me with an error code, “PS2_PRT 168H”. A perusal of the manual showed that this error code is related to the voltage rails labelled, ±12V, ±12RY, +5A, +44V and +5DK. The schematic showed the regulators for three of these rails were on the PCB labelled “video 2”, which is right at the bottom of the unit, so after removing four PCBs and many screws and unplugging many connectors, I got to those board. I set it up on the bench with my bench supply and measured all the voltages. They were all in spec. I then re-assembled the unit and powsiliconchip.com.au ered it on, testing all the other rails; they were all in spec too. I scratched my head and had another look at the schematics. The PS2_PRT line is a sum of all the above voltages via a resistor voltage divider network, resulting in a voltage going into the A/D converter which should be around 1.6V but I measured 2.2V. I removed the PCB labelled “video 2” again and started checking the resistors related to this voltage divider network. I found one which measured 70kW, but it should have been 47kW. These are all small 0603-sized SMD resistors. On removal of the suspect resistor, I tried to measure it again and found it open circuit briefly, before it flew off somewhere, yet to be found. I didn’t have any 0603-sized 47kW resistors in my home stockpile, but I had a couple of 0.25W axial versions which, with a bit of lead manipulation, I soldered to the pads. After reassembly, the PS2_PRT line now reads 1.5V and the unit no longer goes into protection mode. Unfortunately, it only worked for a couple of days before all sound disappeared. I put the unit into service mode again and found that I could get sound out of the speakers using the service modes A2: analog direct test and the A7: manual test. So I knew the amplifiers were still working. I then used my phone as a signal generator and fed signals into all the channels one by one. They all worked on pure direct and A2 test mode. So all inputs and outputs were working. But there was a fault when the DSP function was switched on. I started to follow the signal and found that nothing was coming out of IC811, a PCM1803 analog-to-digital converter (on the Function 3 PCB). Replacing the chip permanently fixed it. Behringer RX1202FX 12-channel mixer repair A. M., of Port Macquarie, NSW had to go into full sleuth mode to fix the power supply of a fancy mixer. Several parts had failed, and not just the usual culprits... The RX1202FX is a rack-mountable 12-channel mixer with an integrated effects unit, designed by Behringer in Germany but made in China. The mixer arrived with no signs of life at all. My initial thought was that it was likely due to a failed power supply or fuse. Australia’s electronics magazine Of course, a failed fuse is usually a symptom of another problem and replacing the fuse won’t necessarily fix it. The mains fuse is located in an integrated IEC socket. Prising this open and testing the fuse showed the fuse to be intact, so a more a time-consuming repair would be required. Opening the unit up revealed a front panel PCB, rear panel PCB and a small switch-mode power supply. The front and rear PCBs are connected via three ribbon cables, glued in place with hot glue. The power supply is mounted vertically between the front and rear panels with an output connector linking to the rear PCB. Removing the power supply board involved undoing two screws, disconnecting the output connector on the rear PCB and desoldering the mains input cable from the rear panel switch and Neutral connection. As is usual with a switch-mode supply, your mind immediately jumps to the capacitors as the source of the fault. I thought it would be an easy repair; just replace the dried-up electrolytics and it will work again. Visual inspection of the supply, once it was removed from its aluminium heatsink/mounting frame, did not reveal any catastrophic damage, but did show it provided multiple supply rails and was designed by Behringer (many equipment switch-mode power supplies are generic devices made by third parties). An internet search indicated that this supply was used in a few different Behringer mixers, but a complete replacement supply did not seem to be available. The search did yield a schematic, though; the commentary with the schematic was not in English so it may or may not have been an official diagram. But it did match the part numbers and values and general configuration of the supply, and indicated the output voltages. The PCB silkscreen gave the component values as well as part numbers but not the output voltages. Powering the supply up outside the mixer showed that all the supply rails were absent and the big input filter capacitor stayed charged once the mains had been removed. Being bitten by the 340V DC on these capacitors is something you always remember and good quality capacitors with no load can hold a charge for a long time. November 2019  65 The capacitor keeping its charge was a clue that the primary side of the supply was not switching. The two small electros on the primary side are were wedged between the large filter capacitor and the transformer. Both tested OK with the ESR meter and measured a reasonable capacitance in circuit. All the surface-mount resistors on the primary side of the supply seemed to match both the schematic and their values, measured close to the markings on the PCB, except for R5. This is a 10kW surface-mount resistor between the X pin on the switching regulator IC (a seven-pin TOP245YN) and ground. I desoldered R5 to check it further. It was apparent that the regulator had an internal short between the control input pins, damaging R5; strangely, the switching device had not failed. My previous experience with these devices is that the output device usually fails short-circuit. While awaiting a replacement regulator, I decided to check the rest of the PCB and found both the 100nF X2 capacitors on the input filter to be under 10nF. While you would not expect that to stop the supply working, replacing them is easy and the designer of the supply put them there for a reason, so I did so. The replacement regulator arrived and was duly fitted. The excitement of powering the supply up again was short lived when the 15V rails were sitting at 21V and the 12V rail was sitting at 19V. The 5V rail was correct; it was regulated off the transformer second- ary with its own 7805 linear regulator, so this was to be expected. At this point, at least the primary side of the supply appeared to be working correctly; no smoke, no explosions, just a little too much voltage on the secondary. After sleeping on the problem, I thought that maybe the excessive output was due to a lack of load; after all, once in the mixer, the supply would always be loaded by the rest of the circuitry and the voltage would drop. A dummy load was hastily knocked up from some resistors and the supply fired up again. This slightly reduced the loaded rails but they were still nowhere near 15V and 12V. Clearly, the feedback path between the low voltage section of the supply and the control TOP245Y was not acting to regulate the output voltage. As is common on this type of supply, the feedback path consists of a voltage divider off the 15V rail, a TL431 shunt regulator and a 4N35 optocoupler to isolate the signal between the primary and secondary. The voltage divider and filter capacitors all tested within acceptable tolerances and the reference pin on the shunt regulator voltage was close to the expected value given in the data sheet. Static tests with a multimeter suggested that the optocoupler had not catastrophically failed. At this point, it was tempting to load the parts “shotgun” and replace everything on the board that had not already been replaced, as this would be quicker than further analysis and fault finding. But having heard stories of failed optocouplers and there being a general distrust of them amongst some parts of the design community, plus having plenty of spares, I rolled the dice and decided to replace the 4N35. This was the magic that finally reduced the rail voltages to within normal limits. The unanswered questions now are about the chain of events which caused the supply to fail; was there a long-term over-voltage condition before it finally failed? Could this have damaged other parts of the mixer? Due to the length of the interconnecting cables within the mixer, the only way to test the other boards was to fully reassemble the unit. Once assembled, the mixer powered up correctly and the expected LEDs on the front panel lit up. The next test involved injecting a sinewave into each channel and looking at the output on the oscilloscope and checking the response of the controls. This was a time-consuming job on a 12-channel mixer but it revealed no apparent damage with a clear sinewave on each output matching the input, allowing for the effect of the controls. A final test with some music and an amplifier was anticlimactic, with all controls working as expected. The reason for the failure of the switchmode regulator and optocoupler will have to remain a mystery. I speculate that the reduced capacitance of the input filter capacitors could have made the supply susceptible to mains-born transients, even though it appears well protected with The Behringer RX1202FX power supply circuit with the faulty parts highlighted in red. 66 Silicon Chip Australia’s electronics magazine siliconchip.com.au a metal oxide varistor (MOV) across the input. Rangehood repair R. G., of Cooloola Cove, Qld got sick of failing switchmode power supplies and decided to take an old school approach to repair a dead rangehood instead. This is what he did... I have been retired a long time and do very little electronic work these days, but an old friend called to tell me that her rangehood no longer worked. As I had helped install it a couple of years ago, and as they could not get anyone to fix it, I said that I would have a look at it and see what I could do. This is one of those rangehoods that has a front glass panel that curves up from the wall at the back of the stove to the top of the rangehood some 700800mm above. The lower glass panel is fixed while the upper one swings out on hinges, and is opened by a 24V electric actuator. It also has the display and three touch buttons on it. Since the rangehood would not operate at all, the first problem was to get it open to get at the control board. To do this, I removed the screws at the top of the rangehood to release the hinges holding the upper glass panel. Unfortunately, I still could not get my hand in far enough to remove the pin that releases the panel from the actuator. I then noticed four screws in line with the actuator, further back on the top of the rangehood. Releasing these enabled me to get the panel far enough open to remove the pin. I then disconnected the wiring from the display and put the glass panel somewhere safe. The actuator then fell to the bottom of the metal box with a loud bang, but fortunately, it did not do any harm. With hindsight, I should have removed the screws on the actuator first. That way I could have opened the glass panel on the hinges, which would have given me enough room to hold the actuator with one hand while I removed the pin with the other, enabling me to put the actuator down more gently on to the bottom of the box. It was then only a matter of undoing three more screws and removing the lower glass panel to gain access to the workings. I removed two screws so I could lift off the black plastic cover over the controller board. I could see a switchmode power supply at one end with the fan control relays along the top. The remainder of the box contained the control electronics. A test showed that there was 230V AC on the top of the board, but zero volts on the rectifier diode cathode connected to the transformer. I de- A new linear power supply (right) was made using a transformer, rectifier, regulator and some capacitors on a piece of veroboard. This was then mounted on a piece of aluminium and connected to the failed board for testing. siliconchip.com.au Australia’s electronics magazine cided to take the control box off the wall and bring it back to my workshop for repair. In my experience, these switchmode power supplies never last long when operating continuously, especially in a hot environment like in a rangehood. With it on the workbench, I unplugged all the cables and undid two more screws, allowing me to remove the control board. I found that a low-value resistor in the 230V supply line was open-circuit, having acted as a fuse. I replaced this as well as the two high voltage electrolytic capacitors. One capacitor was open-circuit and had leaked out some of its contents. But even after those replacements, I still could not get any power out of it. I then connected my bench power supply to the cathode of the rectifier diode. The supply uses 25V DC rated electrolytic filter capacitors, and contains 12V relays, so I thought that I would play safe and set it to 12V with a maximum current of 300mA. It then powered up and everything worked fine. The 24V actuator seemed to work even with the 12V supply. The fan was not running because I had unplugged the unit from the mains. However, I could hear the relays click when I changed fan speeds. A search of my junk box revealed a suitable power transformer, four 1A rectifier diodes, a 2500µF 25V electrolytic capacitor, a 7812 regulator, plus a few smaller capacitors and a piece of veroboard. I used these components to build a new linear power supply for the rangehood and mounted it into a piece of scrap aluminium that I had bent up. I then removed most of the parts from the switchmode power supply, including its transformer and inline filter. I left the transient suppression components in place. I also removed its rectifier diode. I wired up my new supply, tested it to make sure that all was safe, plugged it into the GPO, and away it went, good as new. I just had to fabricate a proper metal enclosure for the supply, which I pop riveted to the back of the rangehood box. The new power supply simply slides into this case and is held in by two screws so that it can be easily serviced. After reinstalling the rangehood, and putting it back together, my friends were pleased that it was all working again. SC November 2019  67