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SERVICEMAN’S LOG Sometimes it all just falls into place Dave Thompson Often in the service industry, we get these weird coincidences where a new appliance in need of repair comes in, then a few days later, another similar unit turns up as well. Although I have no experience repairing this type of device, I was fortunate that both had simple faults that became apparent once I dug into them. This sort of weird coincidence happened to me recently when a computer-repair client mentioned they’d just opened the packaging on one of those oil diffusers that seem to be all the rage these days. They’d purchased it a while back, but when they went to plug it in, they discovered it wasn’t working. Of course, I said I could take a look at it (it’s an electronic device, after all), though I made it clear that I’ve never opened one up before, so this would be a new experience for me. They were happy for me to crack it open and have a look, as it was now out of any warranty that might have applied, and they accepted that doing something is better than doing nothing. I understood completely because I know that many of these diffusers are not cheap; some go for hundreds of dollars, a significant outlay in anyone’s money. If I plugged in a brand-new device and it didn’t work, I’d also be more than a little miffed about it! I hadn’t even started on the repair yet when another client called and asked me if I’d ever had an oil diffuser in 96 Silicon Chip for repairs. I replied that, of course I had, before explaining to them that it had only been one day, and I hadn’t even had a chance to look at it yet! They too claimed that it had cost a pretty penny and, while it had been working fine for a while, it had started failing to stream vapour properly. In the meantime, their cat had knocked it off the table it lived on, and now it sounded like something had come adrift inside. Could I look at it? Bemused by the coincidence, I agreed to take the job on – I mean, how hard could it be to repair something as apparently simple as an oil diffuser? Before the second one arrived at the workshop, I decided to crack open the case of the first one and see what was going on. Preparing for surgery The main body of this diffuser is made of injection-­ moulded plastic and consists of three sections. The base contains the power input socket and controls. A water tank section is mounted on top of the base, while a removable funnel-shaped ‘chimney’ caps off the whole caboodle. Vapour streams from the open ‘chimney’ when the device is operating. This diffuser has other features; a digital clock and on/ off timer are included, as is one of those sound synthesisers that can simulate rain, wind, the ocean and, in this case, a forest with birdsong or a running brook or stream. A row of pushbuttons and a rotary volume control (similar to what you’d find on an old transistor radio) are set around the middle of the base part of the body. These control everything to do with the diffuser, the clock and the sound generator. Removing the funnel is simple enough – it is designed to be removable and is simply press-fitted onto the middle section (the water tank). This is how water and oils are added. The cone is then re-fitted, and the diffusing process starts. With the cone off, I could ensure that no water was trapped in the internal components. It was bone dry. The next thing was to separate the two bottom parts – this would reveal all the actual components. The two parts were fastened together with two simple PK-style screws. There were also three clips at 120° positions around the circumference of the body; these required a little careful fettling to remove. This method of clipping things together instead of screws or other fasteners is increasingly used these days to hold plastic cases together. The screws in the base are likely a Australia's electronics magazine siliconchip.com.au Items Covered This Month • Sometimes it all just falls into place • Tips for fixing an LCD TV backlight • An unfortunate series of battery chargers Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Cartoonist – Louis Decrevel Website: loueee.com legal backup requirement, given that there’s power floating around inside. I’m always very wary of breaking clips when disassembling devices – many modern laptops and monitors are renowned for having very flimsy (possibly intended as single-­use) clips holding everything together. There’s nothing wrong with the method itself, but on many devices, it reeks of the “no user-serviceable parts inside – not intended to be repaired” philosophy. Regardless, I got it open. As I suspected, a single PCB took up the majority of the room inside the base. A short wire lead from the tank assembly was plugged into the PCB via a connector, and this had to be removed before the two parts could be separated. I set the tank aside for now. The board was screwed to the base with the external buttons toggling standard small SMD switches positioned around the board’s edge. Power to the device is via a phone charger-style plugpack and a standard 3.5mm barrel connector at the end of a one-metre cable. The power supply connects to a socket mounted into the plastic base. The pick of the plugpack The connections to the socket were easy to get to, so my first task was to plug the barrel jack into the socket, plug in the supply and test the voltage coming from the socket. With my multimeter across the contacts, I got a reading of exactly nothing; there was no power getting to the socket. I re-checked that I had actually plugged the supply in properly, and there was voltage at the power board on my test bench (I’ve been caught before by first not ruling out the basics!). Other devices were running from the power board powered on OK, so it was time to check the power supply output. Testing barrel jacks is always a bit of an act, especially when the meter probe is too large to slide down the centre contact. I use part of an old dental pick that broke off years ago; being tapered down to a point, it is a universal fit into almost all of these smaller barrel jacks. Once in place, it is much easier to just hold the meter probes against that and the outer contact. Top tip – ask your dentist next time you visit for any old picks and probes – they are the handiest things for all electronics tinkering, especially SMD placement and other delicate work! This time, I measured 5.2V, close enough to the 5V listed on the product labels. So, power was coming out of the supply but not reaching the output of the jack socket. I flipped the whole thing over so I could eyeball the socket more closely. Looking at both the plug and the socket with my loupe, the plug looked fine, but there was something siliconchip.com.au in the socket. The plug didn’t feel secure and looked to be protruding slightly, even when pushed as far in as I could. With a light beamed into the socket, I could see what looked like a piece of plastic in the way. I used another of my handy dental picks to fish around, and the plastic moved when I touched it. It seemed to be right around the centre pin of the socket. Flipping it all back over, I used the time-honoured method of shaking things loose by holding the base in my right hand and clapping it down into the palm of my left hand, in the hope the sudden stop would dislodge the foreign object and gravity would do all the work for me. I did this several times and could see the plastic was moving. The piece came out with a bit more probing with various picks and tweezers and a few more soft taps. After plugging the power source back in – noting this time it went all the way in – I measured the same 5.2V at the socket connections. The clock display lit up a very nice blue and happily flashed 12:00, so I knew I’d found the problem and that now it was going to work. The debris prevented the power jack from going all the way in, so no contact was made. I’d save trying the diffuser part for when it was all back together. Quality assurance backfires On closer inspection under a magnifying glass, the plastic ring turned out to be the top part of the insulation ring separating the two contacts of a barrel jack plug. The plug on the supply that came with the unit was intact, so I can only assume that a QA tester used a single power source to quickly test all the diffusers coming off the production line. My guess is that they pulled that power plug out, leaving the last bit of the ring behind. They might not have even noticed it for a while, and by then, they wouldn’t know where the piece had gone. My client had drawn the short straw! While it was apart, I looked at the other components. I was most interested in the diffuser itself and had no idea how it worked until I started looking into it. I assumed heat was involved, which vaporises the oil and water mix, creating the stream of ‘steam’. Not so, or at least not in this one. Australia's electronics magazine January 2023  97 Some nebulisers operate that way, but they are usually found only in high-end medical devices. These so-called homeopathic diffusers for domestic use utilise ultrasonics; no heat is involved. An ultrasonic disc transducer is mounted in the centre at the bottom of the water reservoir. When power is applied, ultrasonic waves vaporise the oil and water in the tank and the specially shaped funnel corals it all into a nice stream of scented vapour. Safe and very clever! Editor’s note – they are basically the same design as ultrasonic humidifiers; the ‘steam’ generated is actually a cloud of tiny water droplets that quickly vaporise unless local humidity is very high. Once I had it all back together, I filled it with water to the embossed mark on the side of the reservoir and added some ‘essential oil’ I’d had stored for years. I originally used it with a different type of scented oil diffuser, which used a simple tea-light candle to heat a ceramic bowl containing the oil. With the funnel back in place, I hit the button and a fine stream of mist poured from the top of the outlet. It is surprisingly powerful, totally cool to the touch and very 98 Silicon Chip fragrant – though I think I used a few too many drops of oil. It turns out these are very efficient and only need a few drops for a full tank of water (roughly 200mL on this model), depending on the concentration of the oil and the scent itself – some scents are far stronger than others. I set the clock and messed around with the sounds and the timer function, and it all checked out OK. So, a relatively simple fix then; it would be interesting to see what was happening with this other one, though, because it had apparently been in use for quite a while but now didn’t work ‘properly’. Plus, it had been dropped. Diffuser #2: Electric Boogaloo The client brought that second diffuser in a few days later and I asked him to be more specific about how it operated before it had been dropped. He said it worked fine at first, but the output had reduced significantly of late. As it just wasn’t as good as it used to be, they had stopped using it. This model was quite a bit different than the last one; it didn’t have anything as fancy as a clock, timer and sounds, but it did have RGB lighting, controlled by a single pushbutton switch that toggled between the different colours and modes. Ominously, it rattled when I lightly shook it, so something had come adrift inside. I’d have to open it up to see what was going on. The funnel on this model also pops off easily for filling, and I could see a problem straight away; there was a small plastic coin-sized disc sitting in some sludge at the bottom of the empty water tank. I set that to one side. The whole inside of the tank, funnel and recessed ultrasonic transducer was covered in a thick film of oil residue. It did smell nice, though! I’d need to clean it out properly at some point, but in the meantime, I used a paper towel to wipe as much of it out as possible. There was still something loose in the base somewhere, so that had to come apart. This time, I encountered three ‘security screws’ holding the bottom to the tank stage. Fortunately, I now have a good collection of bits that undo these fasteners, so it only slowed me down a little. With the screws out, the two sections came apart Australia's electronics magazine siliconchip.com.au Refresh your workbench with our GREAT RANGE of essentials at the BEST VALUE. Here's just a small selection of our best selling workbench essentials to suit hobbyists and professionals alike. ALL THE REGULAR OSCILLOSCOPE FUNCTIONS IN A SMALL FORM FACTOR 2 CHANNELS SuperPro Gas Soldering Tool Kit SOLDER ANYTHING, ANYWHERE! 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Prices correct at time of publication but are subject to change. Jaycar reserves the right to change prices if and when required. ONLY jaycar.com.au/workbench 1800 022 888 easily. I also had to unplug the transducer lead from the small circuit board inside to separate the parts fully. I could see a fan had come loose in the base and was floating around on its wiring. It had been screwed to two plastic posts, which had come adrift; they were still screwed to the fan. I left it all as it was and placed the fan back where it should be. The broken-off parts matched up very well, so I decided to glue it back into position. Everything else in there looked OK – there really wasn’t much to see. A strip of SMD LEDs was mounted in a ringshaped moulding around the inside of the base and hardwired into the PCB. All seemed OK, so I plugged the diffuser in and tried it. The fan spooled up and the lights came on and changed colours when I repeatedly pressed the button. I’d have to plug the ultrasonic transducer back in and fill the tank with water to test it properly, but I felt sure it would work after I gave it a thorough cleaning. So I went ahead and glued the fan back in with 24-hour epoxy; I didn’t want it moving again. While that set, I took the tank and funnel assembly inside and used a good detergent to clean the inside and outside of both parts. The transducer appeared to be glued to the bottom of the tank, so I wasn’t about to disturb that, but I used an old toothbrush to give the recessed, visible part of it a very good clean. I reassembled the diffuser, filled it with water and added a few drops of oil. I put the funnel back in place and hit the button. Instead of a nice stream of vapour, I got water spitting all over the place! The sharper knives in the block among you will recall I found a small, coin-shaped plastic piece lying in the tank. On closer inspection, I could see where this baffle had broken away from the inside edges at the top of the funnel. After fiddling it back into position, I tacked it with instant glue. On testing, it worked perfectly, so I glued it properly with epoxy. Job done then, and two happy clients. An example is a Linden L55UTV17a TV I looked at. It has six strips each of 15 LEDs connected in series, as shown in the photo below. I tested these with an LED backlight strip tester, and only three of the strips were OK. I also looked at an LG 49LB650 TV. LG uses a higher current in the backlight strips, which causes them to go blue after a while. This gives a blue cast to the picture. In this case, one of the LEDs had burnt and actually damaged the strip. With LG TVs, it is best to replace all the strips when they fail; AliExpress has a good range. Newer TVs divide the backlight into sectors. The Hisense 50P7 has eight sectors, and the power board that drives the backlight is now on the serial bus, so the board is more complex. Similarly, the LG 75 NANO86 power board is also on the serial bus and drives 12 LED strips. Due to the more complex power boards, it is becoming more challenging to determine if the main board or power board is faulty. Most TV repair places deem it uneconomic to replace backlight strips due to the time involved and the risk of breaking the LCD panel. Still, it is worth a try if you are doing it to fix your own TV. Up to about a 55in (140cm) TV, you can, with care, be successful. First, from the circuit board side, very carefully disconnect the long board that connects to the LCD panel ribbon connectors. Then turn over the TV and remove the screws holding the retaining edge around the panel. Do not do this from the circuit board side, as you will probably break the panel. Remove the panel very carefully, taking care not to flex it much. Put it aside. Then take off the retaining edge around the sheets of plastic that diffuse the light. Put the sheets aside, keeping them in order. You are now at the backlights. Test them with a backlight tester (available on eBay) to determine the faulty strip(s). See if you can buy replacement strips on eBay or AliExpress. Re-test before reassembly. Tips on fixing LED TV backlighting An unfortunate series of battery chargers R. S., of Figtree Pocket, Qld has found that LCD TV LED backlighting can be troublesome. Still, if it fails, it generally can be fixed, and he has some good tips on how to do that... The change from cold cathode backlight tubes to LED strips for LCD screens was supposed to be an improvement, but they seem to be less reliable. Many newer TVs will not turn on if a backlight fault is detected. J. B., of Burpengary, Qld sent in a saga involving two battery chargers and a seemingly never-ending series of faults, trials and tribulations... The chargers in question are Truecharge 20i (TC20i) models made by Statpower (now Xantrax). They are rated at 20A 12V with three stages and can simultaneously charge two batteries of the same chemistry semi-independently. Two small slide switches select between flooded and gel, and three temperature ranges on the front face: cold, warm and hot. Charging and float voltages are listed for each range for both flooded and gel cell batteries. The charging voltage has a range of 13.8-14.8V and float 13.1-14.2V, both in 0.2V increments. Not ideal for some chemistry types. An eventual upgrade to lithium-ion will need a revisit of what to do, but I only have flooded and AGM at this present time. Two extras are available: a battery temperature sensor and a remote panel. With a temperature sensor connected, the front panel temperature switch is ignored. They connect via two RJ12 6P6C sockets. The three charging stages are the usual bulk, absorb and float. A hidden fourth stage (equalise) is accessed by holding down a small recessed button on the front face with a narrow pointed object, eg, a straightened paperclip, for two seconds. This Linden L55UTV17a TV has six strips of 15 LEDs arranged horizontally and connected in series. 100 Silicon Chip Australia's electronics magazine siliconchip.com.au There are also charging and charged LEDs and an overall current readout in 4A steps on the front face. Short-circuit and reverse polarity protection are built in, the latter via a pair of 30A blade fuses, one for each battery. I will shamefully admit I have blown more than one fuse while using these chargers. The first charger saw service in an ambulance. It was used while parked at the station to keep both the start and house batteries topped up. The house battery runs a small custom-made fridge with strict temperature control to keep drugs in. While going out to a job under lights and sirens, this particular ambulance caught fire in the electrical enclosure due to a flasher unit being under-rated for the task required. The boss was told about the flasher unit but chose to do nothing about it till this fire happened. A recall was issued so the flasher units could be replaced. Firefighters seem to completely disregard the fact that electronics don’t like being doused with water. This particular charger copped an absolute drenching. This ambulance eventually found its way back to our workshop for repair. No doubt there was a big argument over who would pay for the repairs. The whole electrical enclosure and other fire-damaged parts were removed and stored. Before it was all dumped, I managed to retrieve the charger and a 12V to 240V inverter. There was also a remote panel for the charger, but all that was left was the bare board; everything else was gone: the solder mask, the copper tracks, even the vias. The remote panel has a line of LEDs for each battery to show the battery voltage in 0.5V increments and five LEDs to show the overall current. Two LEDs also indicate the charging and charged states. The inverter was a modified square-wave type. I used it only a handful of times over 14 years until it gave out. Once home, a cursory inspection of the charger showed that the IEC inlet socket had the melted remains of the plug in it, and there was a small amount of fire damage to the aluminium cover next to the socket. A screw was also missing that held the socket to the cover. It appears that a lot of heat was next to that screw, and it melted the plastic housing so that the screw fell out. The charger is mostly made from a long U-shaped finned aluminium section that is also the heatsink. A long, thin hat shape made of sheet steel fits over the top and ends and has flanges to mount the charger vertically to a wall. It has a small circuit board to hold seven LEDs, two slide switches and a recessed push button. A ribbon cable connects this panel to the main board near the processor. Removing the top cover revealed that the inside was surprisingly clean. The only thing to do was to remove what remained of the IEC plug and give it a go. It worked straight away, much to my delight. Some six months later, I was charging a battery in the carport when a heavy downpour came through. I didn’t know at the time that the carport leaked water during heavy rain. Naturally, the charger was right under the leak, and it protested the impromptu shower by ceasing operation. It was time to remove the top again and see what damage had been done. Removal of the circuit board requires the disconnection of five clamps that hold large heat-­generating components, four screws that hold two tabs at either end, and one Earth wire to be undone from the heatsink. The board then slides out. siliconchip.com.au Australia's electronics magazine January 2023  101 Two small-signal transistors had their sides blown out, removing most of the type numbers. There were also some black marks around one of the two IRF840 Mosfets, a blown 4A fuse and a slightly blackened and cracked resistor. The first order of business was to try to get a circuit diagram. The internet revealed nothing, so I sent an email to Xantrax. Their response was to send money plus charger plus return postage. At the time, the exchange rate was not in our favour; it would have cost as much as a new one to do that particular activity. The only remaining option was to figure out what the blown parts were and hope for the best. Looking closely, I discovered that only two small-signal transistor types were used throughout the whole charger: 2N2222A and 2N2907A. There was one of each type next to the two blown ones, and the circuits appeared to be the same as both pairs drove the gate of their associated IRF840. So I felt sure I knew what to replace those transistors with. There is also a UC3845A controller chip (U1) that I felt should be replaced. There are two opto-couplers as well, but figuring out their types was an arduous process as the markings were very hard to see, even with my strongest magnifying glass. After an hour of researching possible type numbers, looking yet again using different light sources, and getting just the right angle of reflected light, I finally found both to be 4N25s. After replacing all the above and the 47W resistor plus the fuse, I fired it up only to reveal that the charger would go through its boot-up sequence but not put any current into the battery. Something wasn’t right. I spent a lot of frustrating time trying to locate the problem. Measuring everything in-circuit didn’t show anything out of order. Eventually, I gave up, put the charger away and waited for inspiration to hit. About six months later, while looking for something else, I came across the charger and pulled it out again to have another look. This time, I measured the three 0.1W 3W resistors out of circuit. One of a paralleled pair was open-circuit, which I very much later discovered is part of the current sense circuit. I found a suitable resistor in an old CRT monitor. Replacing it finally fixed the charger (again!). Buoyed by that success, I noticed a second identical charger gathering dust in the storeroom of my then-employer. I asked if I could have it as it wasn’t working. After fixing one, how hard could another be? Opening it revealed the same two burnt transistors. I replaced all the same parts except for the 47W resistor, but I did have to replace one of the 0.1W 3W resistors. Switching it on without a battery connected, it went through its usual power-up sequence, and no smoke escaped. After connecting a battery, however, it was a different story. Much fire and brimstone issued forth as soon as the startup sequence completed and current was applied to the battery. “Oh, dear!” I shouted, or perhaps a slightly less polite word to the same effect. I was now trying to do things on the cheap by leaving parts out and powering on or not replacing parts that I should have. It resulted in a growing pile of blown-up silicon, much smoke venting into the atmosphere, many sparks and damage to heavy tracks. Smarter people would know that switchmode supplies require all parts present and working, but Muggins here is a slow learner. After the fifth time, I decided to replace all the silicon parts listed above and, while I was at it, fit a socket for the controller chip. I also replaced three 15V zener diodes. After that, finally, the charger fired up properly. I then reassembled it and tested it for three months by running a 12V fridge connected to a small lead-acid battery before declaring it fixed. Unfortunately, on the first camping outing to the “outlaws” (wife’s parents) with this charger, Murphy found us overnight and hit the charger with some strange ‘stop working’ spell. “Bother!” I said quite loudly (and perhaps not so politely). Back home, investigations revealed that the startup resistor (220kW 1W) had gone high in value. I didn’t have one on hand and couldn’t find one in my pile of disassembled bits, but I made a close facsimile from two 470kW 0.5W resistors in parallel. Once again, it worked as it should. This charger subsequently travelled across some of the worst roads in Australia on various camping holidays for several years till Christmas 2011, when I went camping on the largest sand island in the world. For this trip, we bought a small 720W two-stroke generator from a large hardware chain. Its voltage is regulated by adjusting engine RPM. It was backup if the sun decided to hide during the day. It also has a dedicated 12V output for battery charging. Two 35L 12V fridges (actually one fridge, one freezer) take a heavy toll on batteries. So for this trip, I set up two batteries dedicated for both fridges and brought both chargers, figuring I could run the generator half the time. Murphy must have followed me or disguised himself as a dingo as there was very little sun to keep my solar panels busy. I was forced to use the generator. Well, things didn’t go to plan as some 20 minutes after starting the charging process, the generator suddenly started to labour. I quickly determined that the second charger had stopped working. A close-up one of the Trucharge 20i battery chargers. The main PCB suffered some water damage. 102 Silicon Chip Australia's electronics magazine siliconchip.com.au Make building or servicing easier with our Magnifiers & Inspection Aids 4.3" OLED GREAT FOR TECHNICIANS OR ADVANCED HOBBYISTS 600X ZOOM ONLY 9995 $ Digital Microscope • LED illumination • Rechargeable QC3193 FULLY ADJUSTABLE POWERFUL 127MM DIA. 3-DIOPTRE LENS Clamp Mount Desktop Magnifier with LEDs • 1.75x, 2.25x & 3x magnification • 60 LEDs with high/low brightness • Mains powered FULLY ADJUSTABLE ARM ONLY 119 $ QM3554 ONLY 2995 $ RECORD & SNAPSHOT FEATURE FOR A BETTER VIEW LED Headband Magnifier • 1.5x, 3x, 8.5x 10x magnification • Can be worn over eye glasses LARGE 4.3" COLOUR LCD QM3511 720P WITH ILLUMINATION LED ILLUMINATION ONLY 12 $ 95 Handheld Magnifier • 3x magnification • Lightweight, just 200g Inspection Camera • 3x magnification • 3 x probe attachments included • Add an SD card to record vision or snapshots QC8718 QM3535 ONLY 199 $ Shop at Jaycar for: • Eye Magnifier • Handheld Magnifier • Headband Magnifier • Desktop Magnifiers • Inspection Cameras • Digital Microscope Explore our wide range of magnifiers & inspection aids, in stock on our website, or at over 110 stores or 130 resellers nationwide. jaycar.com.au/magnify 1800 022 888 Prices correct at time of publication but are subject to change. Jaycar reserves the right to change prices if and when required. I later discovered, to my disappointment, that if the separate battery charger option on this cheap generator was used, the AC voltage the generator puts out goes up to 265V AC. Oh well, at least one charger was still working. I kept a close eye on the output voltage and managed the RPM for the rest of the time on this island. Once back home, I discovered that everything inside the charger looked pristine. The generator didn’t have the wattage to blow the fuse or let the smoke out. Investigations revealed that the IRF840s were both shorted, as were the gate driver transistors and the UC3845 switchmode controller. I also decided to replace the opto-couplers and the zener diodes. I checked the resistors and the capacitors in-circuit with my multimeter and they all appeared to be OK. However, upon firing it up after replacing the semis, nothing happened whatsoever. I then spent several weeks trying to find out why and replacing many components all over the high voltage section, none of which helped. I finally made voltage comparisons with the other charger on each pin on the UC3845. All voltages were very close to each other except pin 7, which measured 10.5V. This pin is fed directly from the startup resistor and a winding on T2 via a simple regulator. It should have been above 12V. Was it a load or supply problem? Around this time, I drew up a circuit diagram to work out what was going on (reproduced below). I discovered that the two high-­ frequency transformers are identical, but only one has its feedback winding connected. I swapped U1 over, but again, it made no difference. Replacing C27, C28 and C29 made no difference. Replacing R5 and R26 again drew a blank. In desperation, I fed 12V from a small battery directly to pin 7 of U1. To my surprise, the charger fired up and proceeded to work as it should. I could remove the small 12V battery once current was supplied to the battery, and the charger would keep going. Every 15 minutes or so, the charger would stop for about five seconds to, I assume, read the battery terminal voltage before continuing to charge it. It was at this point that the charger would stop dead. Feeding 12V to pin 7 would once again bring the charger back to life. This proved that the feedback from T2 was working, but the startup resistor wasn’t supplying enough current. Or was it? Once again, I replaced R5, but it made no difference. In desperation, I started to replace the small capacitors around U1. C21 broke apart while removing it. Replacing it was the answer to all the troubles. But why? The UC3845 (IC1) has a 5V reference available at pin 8. It appears that C21 was drawing more current that the 5V reference could supply, and at startup that was keeping the supply voltage below the threshold required to start the chip. During charging, extra current from the feedback winding provided the current required. We gave away that generator and now have an inverter generator to run the chargers. Both ran flawlessly for over 10 years. The first charger recently developed a problem where it would go through its startup sequence, then reset and repeat in a continuous loop. Even activating the equalisation mode didn’t stop this behaviour. I just hoped it wasn’t the processor, so I swapped it from the second one. The problem stayed with the first charger. Looking closely, I could see a white film all around the processor but couldn’t get in there to clean it. Removing both RJ12 sockets revealed a white film under them. A good clean and reassembly was the fix. It appears that the drowning SC close to 20 years ago finally showed itself. A reproduction of the selfmade circuit diagram for the battery charger 104 Silicon Chip Australia's electronics magazine siliconchip.com.au