Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN’S LOG A bang in the night! Dave Thompson Dave Thompson is currently busy trying to break the world record for the most Kiwi birds juggled while singing Aotearoa atop a ride-on lawnmower. So instead of his usual column we have a collection of stories from readers. In the 1970s, I worked as an Electronics Technician at Whenever an HF transmitter was re-tuned to a differthe National Broadcasting Service transmitter site at Wan- ent operating frequency, a spot distortion and noise check neroo Road in Hamersley, WA. During one evening shift, would also be conducted. a fault developed in the 50kW HF transmitter that eluded One night, towards the end of our evening shift, we were several maintenance staff. getting ready for the shift change at midnight. All was quiet The site was operated on a 24/7 basis by three eight- except for the transmission monitoring speakers operathour shifts. The day shift was from 8am to 4pm, followed ing in the transmission hall. Just after 11pm, we heard an by an evening shift from 4pm to midnight, and the night almighty bang from the transmitter hall, followed by the shift was from midnight until 8am. sound of HF3 cycling back up. The ABC Radio services transmitted from the site at A check of the transmission output using the monitorthat time were the local AM transmitters for 6WF (55kW ing speaker did not indicate a significant problem with the <at> 720kHz) and 6WN (10kW <at> 810kHz), along with the transmission (with hindsight and tuning a critical ear to three high-frequency shortwave transmitters for the VLW the monitor output, one may have been able to glean an Service, which transmitted to north-western WA. idea of what the problem was). 6WF would commence transmission at 0500 hours and We then proceeded to check the meter readings against would continue until midnight. 6WN would commence those that were recorded earlier in the night at 10pm but transmission at 0600 hours and would continue until 0100 could not find any significant differences. At that point, it hours. The three VLW services were transmitted at 6140kHz was decided to leave it as-is and wait until service close (VLW6), 9610kHz (VLW9) and 15,425kHz (VLW15). The at midnight. The night shift could follow it up, rather than time of day for these transmissions varied due to iono- losing the service altogether for the short duration before spheric propagation. closing time. VLW6 operated from 0500 hours until 0900 hours and The next day, when we reported for duty at 4pm, we then from 1725 hours until midnight. VLW9 operated all found that the HF3 transmitter was out of service (and had day, from 0500 hours until midnight. VLW15 operated from been all day), with various transmitter parts being placed 0700 hours to 1730 hours. on the transmitter hall floor adjacent to HF3. Two of the HF transmitters (HF1 & HF2) were 10kW STC The night shift had measured unusually high distortion 4SU48B units. These could operate on any of the three fre- and noise figures when they checked the transmitter after quencies used by the VLW service. Normally, HF1 would re-tuning it to the daytime operating frequency (15,425kHz). operate at 6140kHz and HF2 would operate at 9610kHz. They had commenced fault-finding unsuccessfully and HF3 was capable of 50kW, with an STC 4SU48B front end had passed it onto the following day shift. Day shift confollowed by a 50kW power amplifier section and an asso- tinued the fault-finding unsuccessfully until we arrived ciated 25kW audio modulator. for our shift at 4pm. HF3 was used to transmit the VLW15 service from The first thing I did was look around the transmitter to 0700 hours until 1730 hours. It would then be re-tuned to see if I could find the source of the loud bang we had heard 9610kHz and took over transthe previous evening. Those mitting the VLW9 service at high-power transmitters tend 1741 hours until midnight. to leave tell-tale marks (burnt • A transmitter at Radio National (Radio 2) All three of the HF transor otherwise) when a large mitters were built in Australia amount of energy is dissi• Another MIG welder for the pile by Standard Telephones and pated quickly, as indicated • Cleaning up cabling Cables (STC). by the loud noise. Opening it is half the battle • When preparing the transWithin five minutes, I had • Repairing a toy skating pond mitters for service, the usual located the cause. Inside • A fault in a car antenna procedure was to conduct the left-hand access door, Dave Thompson runs PC Anytime in Christchurch, NZ. spot (400Hz) distortion and there were six high-voltage noise checks on all transcapacitors used to filter the Website: www.pcanytime.co.nz mitters (both MF and HF) to high-voltage supply for the Email: dave<at>pcanytime.co.nz ensure that they were ready large vacuum tubes used in Cartoonist – Louis Decrevel Website: loueee.com to go. the 50kW power amplifier Items Covered This Month 98 Silicon Chip Australia's electronics magazine siliconchip.com.au and audio modulator sections. These capacitors were protected by inline fuses. The fuses are mechanically designed so that when the fuse blows, a metal lever (held up by a length of fuse wire) drops down to short out the capacitor; no bleeder resistors were used. This prevents the capacitor retaining a high amount of stored energy when the fuse fails, which would pose an electrical shock hazard to the servicing staff. I can only assume that one fuse had failed first, and that caused the next one to fail due to the extra loading, and so on until all six fuses failed in quick succession. That large amount of energy being discharged very quickly would have been the cause of the loud bang we had heard. After replacing the fuses and restoring the transmitter parts that had been removed, distortion and noise checks produced normal figures. The transmitter was returned to service. Finding the fault quickly was a feather in my cap and a memory that has stayed with me over the years. I received a considerable number of jibes from the other shift members who were involved in the fault finding, along the lines of, “That young upstart showing us up!” Looking back with hindsight, I think that the night shift had not investigated the cause for the loud bang we told them about, and just continued with the normal routine of re-tuning the transmitter for the day’s operation. With ABC radio and television services being distributed via AUSSAT, the VLW services were no longer required and were shut down in the late 1980s. I believe the HF transmitters were broken down for scrap. L. H., Geraldton, WA. A Workzone MIG welder repair Mk2 I was welding a frame to make a small table with my Workzone flux-core MIG (metal inert gas) welder when it suddenly went haywire, shooting the wire out at high speed, making it impossible to make any welds. I had this same thing happen to my smaller SIP MIG welder that I used before getting this welder from Aldi Special Buys a few years ago. In the case of the SIP MIG welder, it was dry joints on the circuit board that were easily fixed by re-soldering. A few months back, I had to repair this Workzone welder when the wire for the trigger broke and I had to run a new wire through the sheath. That repair was described in the October 2024 issue, starting on page 92. Now it had another problem. I wondered if it might be the same thing that happened with the SIP MIG; there was one way to find out. I removed the front panel from the welder, and the problem was obvious. One of the wires going to the circuit board plug had broken. I don’t know how that could have happened. It is a very unusual fault in my experience. I didn’t have much room to move with the front panel still attached to the welder, so I removed all the plugs from the circuit board. That let me get the front panel out of the way, so I would have room to repair the broken wire. Some of the plugs were a bit difficult to remove, but I got them all out and put the front panel aside for now. The next problem was how to affect a repair so that I could get the welder back in working order and finish the job at hand. My quick solution was to cut a nick in the plug to provide access to the pin that the wire used to be connected to and solder it back on. icomretail.com.au       siliconchip.com.au Australia's electronics magazine May 2025  99 Making a small cut in the plug plastic gave me access to the pin to reconnect the wire. Two unrelated cable faults within a week, in a system that had been running reliably for nearly 20 years? Unlikely! Had someone or something disturbed the cabling? If so, we were likely to see more faults in the near future. Out came our trusty PMG cable tracer. It was made by Melbourne company Aegis back in the 1970s, back when we still had a strong electronics manufacturing sector in Australia. We traced the cable from the meeting room RJ-45 port up into the false ceiling, across the building and back to the computer rack. It had a patch bay across to one of the audio racks, where the clock signals originated. The signal I got my 20W soldering iron and plugged it in to heat up ended at the computer rack. while I prepared the plug by trimming out a small piece On closer examination, we noticed a patch cable going with a utility knife. This was the easiest and quickest way to a port that we knew was unterminated, and another port of repairing it, rather than going to the trouble of extract- that should have been patched that wasn’t. That explained ing the pin. the faults, but who was the culprit? I plugged the plugs back into the circuit board and reasIt turned out that our contracted IT support guy had sembled the welder. I then made a couple of test welds on been in and ‘tidied up’ the IT cabling. Yes, you guessed it, some scrap steel, and the welder was back in working order human intervention had caused the problem. We re-patched again. I could now finish making the table frame. Some- it, stripped out the temporary Cat 5 cable and restored the times, a very simple fault can put a device out of action, original one. but a simple fix gets it back in working order again. At one level we were annoyed that the IT guy had disB. P., Dundathu, Qld. turbed the audio patching, which was none of his business (then again, maybe he didn’t realise it was even there). At Studio signals suspiciously stopped another, we were pleased that we weren’t facing a bigger At my local community radio station, the GPS-­ problem. synchronised clocks and open-mic indicator went offline R.P., Melbourne, Vic. in one of the three on-air studios. We quickly substituted Sometimes opening it is half the battle a spare clock to make sure that wasn’t the problem. Breaking out the RJ-45 connector in the studio verified I was asked by a friend to look at a radio, as the volume that no signals were present. As both signals are supplied control did nothing, but they liked the sound quality when via a single Cat 5 cable from a rack in the control room, it was working. and similar services in the other studios were unaffected, When I accepted the radio, I got a shock. I am in my the basic problem was clear: the cable was broken some- eighties and from the analog age, so I had anticipated a where between the Krone block in the control room rack loose wire broken off the potentiometer. But this radio was and the RJ-45 connectors in the studio. a modern digital one. The volume control was not a pot, it Since the studio was booked for use within the next few was a rotary encoder. It was used to not only set the volume hours, we simply patched in a new cable. Problem solved? but also select the input, set the clock and everything else. Not quite. How to dismantle A few days later, the open-mic indicator in the same it? There were studio came on permanently and the synchronised clock no screws in the meeting room stopped working. The meeting room v i s i b l e ; fault also revealed that no signals were arriving at the clock, the first despite being present at the control room end of the cable. place to 100 Silicon Chip Australia's electronics magazine siliconchip.com.au look was under the felt feet in their recessed hole, but still no screws. I asked a friend, who looked online for that model but didn’t find anything helpful. Finally, in desperation, we prized off one end plate. That broke some plastic mounts, but revealed that the screws were under the metallic face plate that was glued to the front. We were able to remove the faceplate, take out the screws and open the radio. That exposed the top panel that the PCB was attached to. There was a rotary encoder with a push switch, plus a further eight tactile switches to set up and operate the radio. In my days, this would have needed up to twelve wires to operate, but to my surprise, only three wires were needed! This PCB contained only two capacitors and twelve resistors, eight tactile switches, and the encoder, all surface-­ mounted. All was revealed once the board was in a suitable lighted area: liquid had been spilt over part of the board. The owner confessed later that this would have been wine. I wiped off as much as possible, but found that one switch had sticky residue under it and did not work, so I removed it and left the board soaking overnight in isopropyl alcohol. Our local Jaycar did not have a suitable switch, but a colleague had one, which was gratefully accepted. Still, it would be three weeks before I received it. Unfortunately, I found it to be unsuitable; the protruding stem was too long, the one required would have been about 1mm long. Since the delays were increasing, I spoke to the owner to explain the problem of finding a suitable replacement. It transpired that the radio was never used as a clock or even as an alarm clock. The ‘snooze’ button (a long bar) had a switch at both ends, wired in parallel, so I borrowed one and replaced the faulty one. I packed the end of the bar with a small piece of foam to make it horizontal and look good. The snooze function would still work, but only if pressed at one end! Reassembling the radio was a bit of a challenge due to the broken lugs, but I finally got it together, and had a happy friend with a now-working radio. The second item I repaired was a small battery-­operated audio amplifier. The complaint was that there was no sound and it had a rattle from the enclosure; something had obviously come loose. After removing five screws, the cabinet would not come apart, so based on the earlier repair, I decided to remove the metal grille in front of the speaker. This revealed eight more screws (the manufacturer did not want the speaker to get out!). The photo shows that the rattle was the magnet. It had detached from the speaker housing; all four rivets had given away. I wonder what sort of handling could cause this? As the complete unit was only worth about $120.00, a new speaker and my time made the unit an uneconomical repair. R. R., Morrinsville, New Zealand. Skating Pond toy repair My daughter presented me with a Lemax Village Skating Pond from one of her friends. It was not working. I have been fixing faulty electronic equipment for many years and this is just another one of those challenges for me. These Chinese-manufactured devices usually come out at Christmas time to amaze the children. It is designed to have miniature figures skating around the table-top pond siliconchip.com.au Australia's electronics magazine May 2025  101 to the sound of a crowd and carousel type music. There is a magnet connection through the pond surface from the mechanism to the skating figures. This one had working sound but no movement on the pond. I took it apart and was presented with the motor and belt driven mechanism. After powering it up again, I spun the motor shaft with my fingers and the mechanism ran, but the beautiful sound deteriorated to a horrible noise, which I worked out later was commutator hash. I then measured the voltage going to the motor and found they were feeding the red and the black wires on the PCB motor socket with reversed polarity. The motor is a model RF-300CA 11440, which provides clockwise rotation looking at the motor drive end. It appears they worked out the mechanism design and then added a motor and found that the motor style that was chosen rotated the wrong way – so they just reversed the connection to it! The mechanism design would not work if the motor rotated in its intended direction. [The initial design probably used a different motor, but they subsequently changed it due to availability or price – Editor.] I switched the unit off to work on it and removed the belt. I then reversed the motor wiring to correct the polarity. Leaving the belt off, I powered up the unit and found that the motor started by itself and the sound coming out was normal – very pleasant. I then searched the internet for a motor of this style, approximately 25mm diameter with 16.5mm screw mounting, running in an anti-clockwise direction (looking at the motor drive end). However, I came up with nothing. It appears that most cheap DC motors have commutators designed to go best in one direction. Some of the better brands, such as Mabuchi, say that the direction can be reversed by reversing the wiring polarity. They are probably built to work that way. Unfortunately, Mabuchi did not have an equivalent motor that would fit. So I had to work with this motor and somehow change something to make the mechanism work with the motor rotating in the opposite direction. I thought of changing the drive to a figure-8 belt drive, which worked OK, but there was rubber wear where the belt crossed over. I then thought of placing a slippery sheet of something between the belt crossover. What I used was part of an antistatic bag (that was on my desk at the time), with a couple of subtle bends. This was held under one of the motor board mounting screws. That fixed it – the unit was put back together and is still working to this day. Looking at reviews for this unit, many people had problems with it only working for so many hours and then stopping, or it working for one Christmas and not working at the next. I suspect this all comes down to the use of an unsuitable motor. E. R., Marion, SA. Car antenna fault Around a year ago, I started to experience poor FM reception on my 2009 Ford Ranger car radio. It has a telescoping type antenna on the right-hand roof support pillar. The antenna was looking worse for wear, with a fair amount of dirt and corrosion. I found a suitable replacement on eBay and set about fitting it. Disconnecting the cable from the back of the car stereo and pulling the cable over to the driver’s side of the vehicle was fairly easy, but threading the new antenna cable down the support pillar would not be an easy task. I pulled the old antenna cable with a draw wire and fed Australia's electronics magazine siliconchip.com.au the cable up while my wife slowly removed the antenna from the top of the car. I then taped the new antenna cable to the draw wire and pulled it back into the vehicle and to the back of the stereo as my wife fed the cable in from the top. Once the antenna was screwed down, I tried it, and everything seemed to be working. That was until I got about 2km out of town and the reception dropped out. Naughty words were said! I removed the stereo (it is the stock Ford system) at the next opportunity and resoldered the antenna socket in case there was a bad solder joint. I also looked over the rest of the circuit board but could not see any problems. The circuit board is full of SMD components, and I have no circuit diagram anyway. I reinstalled the stereo and it made no difference. I sourced a replacement stereo from the local wrecker and installed it. Guess what? More naughty words were uttered. It was time to get scientific. I hadn’t noticed before, but I tried scanning the AM band and was surprised to find nothing. Usually you will pick up something up due to the long distances AM will travel. I set my signal generator to 639kHz (2HC Coffs Harbour) amplitude modulated at 1kHz. I draped the insulated signal generator lead around the car antenna and tuned to 639kHz. Total silence. I had the signal level full bore (5V peak-to-peak) on the generator. At least I had a clue now. If I touched the antenna with the bare end of the signal generator lead, a loud and clear 1kHz tone came through the radio, but still nothing if I draped the insulated wire over it. I unscrewed the antenna and slowly pulled it out of the pillar while listening, and bingo. A 1kHz tone was heard loud and clear. I slowly lowered the antenna back down the pillar and the signal dropped out again. OK, there must be some insulation damage. I tested with a multimeter set to measure resistance and there were no shorts between the screen and centre pin of the antenna lead. There was also no short to the frame of the car. The base of the antenna was grounded to the body of the car. You can’t test continuity of the antenna to the centre pin, as the car service manual states that the antenna is AC-coupled via a capacitor. Of course, no value for this capacitor is given. I tested with a capacitance meter and found it to be about 6.5nF, which would have a reactance of about 46W at the lower end of the AM band. Does this seem right? I’m not sure. Maybe a radio guru could comment. This was frustrating, as I had to reattach a draw wire and again pull the antenna out to inspect the coax and the telescoping part of the antenna itself. A visual inspection of the coax and antenna body revealed nothing. This was crazy! Maybe there was some sort of capacitance to the frame of the car. In desperation, I wrapped the whole antenna telescope body and about a metre of the coax with two layers of insulation tape, and slowly lowered it back down the pillar as before while listening to the 1kHz tone. It all went back together, with no break in the signal this time. When I switched the signal generator off, I was greeted with 2HC Coffs Harbour. I don’t know the true cause, but after much frustration, the antenna and radio work on both AM and FM as they should. G. C., Toormina, NSW. SC siliconchip.com.au Australia's electronics magazine May 2025  103