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SERVICEMAN’S LOG Another mixed bag of bits and bobs Dave Thompson recently surprised us by visiting Australia. It was such a surprise, we didn’t know he was here! Having recharged his batteries, he’ll be back in July. So, for now, here are some stories from our readers. Mystery amplifier toroidal transformer replacement The mains light-bulb limiter is not a new idea, but not everyone is aware of it. My version is the simplest, comprising a 100W 230V incandescent lamp (these are getting scarce) mounted in a batten holder screwed to the wall above my bench. This is wired with a piece of two-core mains flex to a PDL40A Interrupted Phase Tapon Plug. This means that any appliance plugged into the Tapon has the lamp in series with its live connection. I leave the Tapon plugged into one outlet of a power board, and I can choose to plug the appliance under test either directly to the mains or via the Tapon. I use an inexpensive power meter to measure the mains voltage and the current drawn by the test load. Turning to the repair in question, a PA speaker had blown its mains fuse, so after fitting a replacement, I plugged it in via the lamp limiter. I expected to see the lamp briefly lighting brightly, then fading to a dim glow. This is because the amplifier main capacitors charge when power is first applied, drawing a large initial current, which then subsides. However, the lamp lit up at full brightness and stayed that way. If I had plugged it straight into the mains, it would have likely blown another fuse. There must have been a heavy short circuit somewhere in the amplifier. I dismantled the amplifier module from the cabinet and noted a large toroidal power transformer at one end. The secondary wires were easily identified and fitted with Faston connectors, allowing me to quickly disconnect them. When I plugged it back into the Tapon, the lamp immediately lit at full brightness again. That indicated the transformer was the likely culprit, but to be sure, I disconnected the transformer primary and tried again. This time, the lamp didn’t light at all. So a new transformer was required, but it had no markings on it to tell me what the secondary voltages should be. A label on the back panel told me the total power consumption was 160VA, so I needed a 160VA transformer with a 230V The original transformer (above) and replacement (below). 88 Silicon Chip Australia's electronics magazine siliconchip.com.au primary and a centre-tapped secondary of unknown voltage. I had a schematic of this unit, but there was no mention of the transformer voltage. However, someone had written 35V next to the DC rails. That seemed a little high to me, so I looked at the main capacitors on those rails and found they were only rated at 35V. The actual rail voltage would be less than that. The amplifier uses LM3886 power amplifier ICs, so I consulted the datasheet. This gives different rail voltages for 8W or 4W loads. The woofer was a 4W unit, and the datasheet said the rail voltages should be ±28V. That would suggest the transformer secondaries should be roughly 20V AC. A look at the selection of 160VA toroidal power transformers available from my regular suppliers showed two contenders: 18-0-18V or 22-0-22V AC. From experience, I know that a transformer rated at 18V will deliver closer to 20V with a light load because it is designed to deliver 18V at its full rated load. I selected the 18V unit and, when it arrived, I was surprised to find it was somewhat larger than the original. I fitted it to the chassis with a bit of fettling and wired it up. When I applied power again, all was well and the DC rail voltages measured a touch over 28V DC. I should mention that the light-bulb limiter can give tricky results with appliances with switch-mode power supplies. Most are OK, but some can draw a lot of current at startup, lighting the lamp and lowering the voltage to the power supply, which may subsequently not start. Not long after that repair came another, this time a wedge floor monitor made by the same manufacturer and using similar technology. Only this time, the woofer amplifier was a discrete design with a higher power output. The customer said there was a crack sound, and the HF horn stopped working. I initially powered the box through my light-bulb limiter; again, the lamp came on at almost full brightness Items Covered This Month • Mystery amplifier transformer replacement • A curious remote control problem • HP 8660D signal generator repair • Arlec NL0009 LED Night Light repair 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 and stayed that way. The customer said it was still working, so I held my breath and plugged it directly into the mains. It came on and, as described, the woofer was working but not the horn. A quick resistance check of the horn driver revealed it was open-circuit. Connecting my ‘scope to the output of the horn amplifier explained why. There was a solid -30V across the horn, meaning the IC amplifier driving it had failed and had taken the horn with it. If I had just replaced the horn driver, the new one would have burnt out at switch on. After replacing the IC (LM3886), I tried again with the light-bulb limiter, and this time the bulb came on bright and then faded to a dim glow as expected. This suggests that the faulty IC was also drawing a lot of current; I think if it was powered up for any length of time, there would have been smoke. Paul Mallon, Christchurch, New Zealand. A curious remote control problem I had noticed that our air conditioner remote control was working poorly. It would control the AC, but it seemed less sensitive, and the AC unit did not display the set temperature. I have a separate control for each room, so tried another one with no better result. That evening, I also found that the LG TV was responding strangely. I changed the batteries in the remote and even tried a second one with no change. The next morning, I tried using the sound system with a new (replacement) remote control. This was also acting strangely; since the batteries were low, I replaced them. I then had the bright idea of checking the sound system remote itself and found that it was continuously transmitting. Opening it and carefully reseating all the buttons stopped that, and now everything else worked properly. So, although the controller was transmitting codes not recognised by the AC and TV, it was enough to interfere with both systems. Graham P. Jackman, Melbourne, Vic. HP 8660D signal generator repair I went out to the radio shack intending to check some VHF receivers using my Hewlett-Packard 8660D signal generator. However, my effort was short-lived – the sig gen didn’t want to produce any useful output. Of course, the HP that built this sig gen is very different from the HP we know today as an IT company. The test and measurement arm of HP that created the 8660D siliconchip.com.au Australia's electronics magazine June 2025  89 was spun off from the IT company many years ago and became Agilent, later renamed to Keysight. When it was released in 1971, the 8660 was truly ‘bleeding edge’; it was the first fully synthesised sig gen built by HP. Although HP built frequency synthesisers before the 8660, they lacked the modulation capability and a wide range of calibrated output levels. The 8660’s specifications were impressive. In its early form, it offered 0.01MHz to 110MHz in 1Hz steps, AM or FM modulation, and an output from +10dBm to below -140dBm. Its output was also clean, with all in-band unwanted (spurious) outputs at least 80dB below the level of the wanted frequency; a real achievement. Truly impressive for the time! Add two front-panel plugin bays for a modulation section (with various options available) and the output section (again, different modules available) and it was a very flexible design. It also had an internal plug-in bay for a “Frequency Extension Module”, which was required for the later 1300MHz and 2600MHz versions. The one specification that was a bit below par was phase noise; the analog HP 8640B signal generator stayed king of the phase noise heap for many years after the 8660 was released. Phase noise is the wideband noise created by all oscillators, with some designs much better than others. Still, the 8660 was still very usable for most purposes. All of this did not come cheap or small. Despite being designed in the early 1970s, the 8660D was still on sale in 1990. The list price in 1990, for the 2.6GHz version with plugins, would not leave you with much change from US$40,000. But you got a lot of hardware for your money – a 4U (about 175mm) high 19 inch rack-mount box over 500mm deep and weighing about 30 kg. So, when I switched it on and no signal appeared, what to do next? My first check was with a spectrum analyser and frequency counter, which confirmed that with the 8660 set to a nominal frequency in the VHF range, it had an unstable (frequency varying) output at a few MHz. Fortunately, HP instruments came with excellent documentation, usually including an operations and service manual with full schematics, part layouts, fault-finding guides and parts lists down to individual components. Scanned versions are often available online. One useful source of info on the higher-end HP equipment is the Hewlett Packard Journal. Although essentially a sales strategy, the HPJ often had articles written by the project development teams about the high-end new equipment they had developed. For the 8660, the March 1971 and December 1971 HPJ issues both had very useful information, one about the 8660 mainframe, the other about the plugins. There are two basic fault-finding options in a complex system such as this, where no functional block stands out as the most likely to create the problem. One is to start near the signal source and work through the instrument to locate where the correct signal disappears, or work backwards from the output and find where the fault stops. Starting at the source was not attractive. It is a 100MHz master oscillator which is phase-locked to a reference oscillator at 5MHz or 10MHz, which can be either internal or external. Many different signals are derived from the master 90 Silicon Chip oscillator by frequency division or multiplication, to provide the reference signals for the phase-lock loops – 7 PLLs in the mainframe, plus several more in the Frequency Extension Module and in the output plugin. That means a lot of signal paths to check, including several complex programmable frequency dividers. Starting at the output and working back looked a lot easier. The output plugin module receives only two signals from the Frequency Extension Module: one tuning from 2.750GHz to 3.950GHz in 100MHz steps, the other tuning from 3.950GHz to 4.050GHz in 1Hz steps. The desired output frequency is the difference between these. A fault found here on one signal would immediately provide a path to further investigation ‘upstream’. Although a service manual is available for the output plugin, there is none online for the Frequency Extension Module, and in any case, the Frequency Extension Module is basically impossible to test without special (and unavailable – of course!) ‘extender’ cables. If the fault was in one of these modules, the instrument was probably a write off. So, trace backwards from the output it was. I just needed to locate the relevant connections and check with a spectrum analyser to see if the expected signals were there. The instrument is a maze of cable looms carrying various signals; most of the cables are terminated with special-purpose plugin connectors that have both coaxial and standard connections – which are inaccessible with modules installed in the chassis. Sigh! As it happened, the two required signals were actually on SMB connectors, so it was only a few minutes to make a suitable adaptor cable and have a look with the spectrum analyser. Lo and behold – one signal was absent. So, the fault was probably not in the output plugin module – so that was one critical module cleared. The Frequency Extension Module has four RF signals feeding into it, all via a multi-way connector. After a bit of fiddling, I was able to make a cable that kind-of mated with the coax connectors in the multi-way plug, with the Frequency Extension Module unplugged (and hence inoperable). Another test with the speccy showed the reference signals to the Frequency Extension Module were not present. Australia's electronics magazine siliconchip.com.au So the Frequency Extension Module was probably OK, too. Big sigh of relief! Now I had a new place to look – the circuits that generate the reference frequency signals used by all the phase-locked loops. The 100MHz master oscillator is used to generate 500MHz, 100MHz, two 20MHz, two 10MHz, 2MHz, 400kHz, and 100kHz reference signals. These are easy to check in what’s called the A4 assembly, which has all the circuitry to generate the reference signals. Most, but not all, reference frequencies were MIA, so at least the master oscillator was operating, but the circuits to generate most of the reference frequencies weren’t. Then I spotted some greenish corrosion on a small area of the “A4A4 reference loop and dividers” circuit board. Closer inspection showed that an electrolytic capacitor used as a bypass on the -10V power rail had leaked onto the board and the electrolyte had eaten a couple of power supply tracks. Bingo! After that, it was easy. Clean up the board, replace the missing tracks with copper wire bridges, a new capacitor, and we were in business. Or so it seemed. As a final check, I hooked up the sig gen output to my frequency counter, which is locked to a GPS reference. That showed the sig gen was putting out a signal significantly low in frequency, which drifted in frequency as I watched. Bother! Was there another fault with one of the phase-lock loops? Then the penny dropped – the sig gen was using its internal reference, an ovenised crystal oscillator, very likely the venerable HP 10544A or something similar. This was drifting low in frequency as it came up to temperature during its warm-up phase before settling to something very close to the correct frequency. Previously, I had always used the sig gen with an external reference from a GPS-derived 10MHz frequency source, so I never saw this behaviour. So, with relief, I decided it was all good! John Morrissey, Traralgon South, Vic. Arlec NL0009 LED Night Light repair We have two separate car garages and, a few years ago, we decided that a motion sensing night light in each garage would be ideal to help find our way to the light switch or door when we come home at night. So we purchased plug-in Arlec NL0009 PIR motion sensing LED night lights from the local hardware store and installed one in each garage. These are low-cost plug-in devices and worked very well until a year or so ago, when the first one became Servicing Stories Wanted Do you have any good servicing stories that you would like to share in The Serviceman column in SILICON CHIP? If so, why not send those stories in to us? It doesn’t matter what the story is about as long as it’s in some way related to the electronics or electrical industries, to computers or even to cars and similar. 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. siliconchip.com.au Australia's electronics magazine June 2025  91 faulty. It was still functioning, but the light output had become very dim, so I accepted the challenge to see if it was easily repairable at minimal cost. These units come apart easily with the removal of four small Phillips-head self-tapping screws to reveal the PCB. My first thought was it could be a problem on the supply side from the incoming 230V AC to the electronics. I could not find a circuit diagram for these units, but inspection of the PCB showed the power supply to be fairly conventional. This consisted of a 330nF 275V AC rated X2 capacitor paralleled by a 390kW resistor and connected in series with a 47W resistor (on the underside of the PCB) between the incoming 230V AC supply and a bridge rectifier, BD1. Across the DC output side of this rectifier is SMD capacitor C14, plus C15, a 220uF 35V electrolytic (on the underside) and a SOD-80 type zener diode, ZD1. This provides a voltage-limited, filtered and regulated DC supply to power the night light electronics and the white high brightness LED lights. I first checked the 330nF X2 capacitor as I recalled one failure mode of these metallised film ‘safety’ capacitors is to lose capacitance over time due to internal partial discharges, which progressively degrade the metallised film. I measured the capacitance of the X2 capacitor with my DMM and found this to be about 230nF. This was significantly below its labelled value, and would certainly explain the diminished light output from the unit. I replaced this faulty component with a new 330nF X2 275V AC capacitor and its full light output was restored. About six months later, the second night light failed, but this time with no light output at all. I opened up the unit and firstly checked the 330nF X2 capacitor, finding its capacitance to be about 320nF, which was acceptable. I next used my current-limited DC bench supply to apply voltage on the AC (input) side of BD1, checking both polarities. Voltage measurements indicated about 1.6V across the bridge input, for either polarity, before a significant current draw started. This seemed to be indicating a short circuit somewhere on the output side of the bridge rectifier; 1.6V is approximately equivalent to the sum of two forward-biased diode voltage drops in the bridge. Zener diode ZD1 seemed to be the most likely culprit, followed by the two DC filter capacitors. I removed ZD1 from the PCB and, on testing, I found it to be pretty much a dead short circuit. Assuming the zener diode to be the only faulty component, the next challenge was determining what voltage it should be. The zener had what appeared to be one green band with no other markings. A quick online search was of little help, so with no zener in the circuit and the light sensing photodiode (photo 1) shaded with a small piece of black tape, I decided to apply a current-limited DC voltage to the output side of the bridge rectifier. I slowly increased the voltage while moving my hand over the PIR sensor, and at about 22V, the LEDs started to glow. Further increasing the voltage to about 28-30V resulted in a LED brightness level of about what I thought it should be. As a further check, I decided to test, in a similar way, the previously repaired Arlec unit from the other garage. This unit still had the original zener diode installed and showed the voltage developed across the zener to be about 30V, so that was good confirmation. As I didn’t have a SOD-80 type 30V zener diode on hand, I decided to try two series-connected DO-41 zeners of 13-15V, hoping this would be good enough. These diodes were easily installed sitting just above rectifier BD1, and the repair proved to be very successful. While I was at it, I decided to also replace the original X2 capacitor just in case it was heading the same way as the original X2 capacitor in the other unit. So, with a little effort and replacement of a few lowcost components, both night lights are continuing to provide their helping glow when we come home at night. SC Stephen Denholm, Tranmere, Tas. One of the repaired night lights. 92 Silicon Chip Australia's electronics magazine siliconchip.com.au The replacement grey 330nF X2 capacitor is much larger than the original but it still fits. 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See the series of articles in the February & March 2023 issues for more details (siliconchip.com.au/Series/396). siliconchip.com.au Australia's electronics magazine June 2025  93