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SERVICEMAN'S LOG When a GPS loses its way GPS satnav systems are widely used in cars, boats and for personal navigation when walking in country but it is safe to say that most of these would be discarded when they stop working. That is probably the most practical approach but what if you were using GPS tracking collars which are fitted to wildlife? These are much more expensive units that are quite costly to replace if they fail. I am certainly getting a variety of work these days and I can no longer complain about doing the same “boring” sorts of repairs. I get all sorts of jobs and I wonder if it is because the servicing game has changed so much here in New Zealand. So many repair business have closed or maybe just given up. . . I’ll bet a lot of service businesses here looked at the silver lining when the quakes struck Christchurch, with many taking the seemingly God-given opportunity to close with dignity. There are few other explanations as to why so many of these businesses never re-opened. Some of us have kept going though... siliconchip.com.au 73  S ilicon Chip A client from “down south” recently visited Christchurch and found me working on my new workshop. He’d heard that I fixed GPS units and asked if I was interested in looking at his. I told him that I’d repaired a few in the past few years as word got around that despite many industry claims, they might actually be fixable. This guy was a typical kiwi “southerner” and I say that with a lot of respect. I mean that he is one of those characters that spends much of his life in the far south of the country, where bush is thick, the terrain harsh and the weather beyond inclement. There are still uncharted areas down there, and this is my client’s backyard. Dave Thompson* Items Covered This Month • • • • Garmin GPS animal trackers Cambridge CD player repair Fixing a useless machine A Pony 3 mobility scooter that just wouldn’t scoot *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Having the GPS working properly could be the difference between coming home safe or spending a night (or longer) out in the boonies, so they are an important piece of kit. What he wanted me to check over was a Garmin hand-held GPS unit and three Garmin Alpha T5 tracking collars, the sort you might fit to a lion or a bear in order to keep tabs on their whereabouts. They are certainly not the dainty “domestic” types sold by the likes of AliExpress for pet owners to monitor Snuggles’ nocturnal antics. My client uses these collars, together with the hand-held GPS, to monitor animals in the wild and gather information about their movements so that SSeptember eptember2017  73 2017  73 Serr v ice Se ceman’s man’s Log – continued programs can be devised to ensure their continued survival. The collars are made using heavyduty synthetics, hard rubber and some metal parts for the clasp arrangement, all of which have to be robust enough to withstand natural hazards and the animal’s efforts to rid itself of the annoyance. Apparently, all of these collars had failed with the same symptoms; they no longer acquired satellites and were thus useless for tracking. Due to the cost of replacement, the guy thought he’d ask around to see if anyone fixed them and for some reason, my name popped up. However, there was a snag (isn’t there always?). Garmin made these collars to withstand the rigours of extreme conditions; to that end, they are built like the proverbial masonry ablutions block. The external connections are wellsealed with some formerly-liquid armour and the GPS module – which is housed a separate small plastic “box” along the collar from the main electronics case and connected by a shielded cable – is completely enclosed in a case with clear potting compound and thus completely isolated from the environment – and potential repairmen. 74  Silicon Chip The main box of electronics goodies is three times the size of the GPS module and is home to the battery, charging ports and a small, doublesided PCB stuffed with surface-mounted components and edged with tiny, multi-colour LEDs that indicate what’s happening with the unit. At least this board is accessible after removing half a dozen long, finethreaded screws and prying the lid away from the seal that (supposedly) keeps the contents safe and dry. The guy mentioned that he, and others with the same issue, thought the problem was the shielded cable from the GPS module and commented that it was often under some strain, so they thought all it needed was re-terminating into the main module. Or at least that’s what YouTubers and posters in online forums reckoned. Just by looking at it, I doubted this was the issue. The cable was embedded in the plastic collar and appeared well-connected, with all the strain relief necessary. And given that it really didn’t flex or move that much when the collar was worn, I found it difficult to accept that this was the problem. We’d see though; I’ve been known to be wrong before. The first thing I did was try the collars out. Two of them had flat batteries since they’d been sitting on the shelf for ages after failing and so they were non-starters. The third one gave a healthy series of beeps on button-push and the middle of three LEDs flashed solemnly every few seconds. This informs the user when enough satellites are acquired for accurate operation; one flash is no satellites; two flashes indicates two satellites and three flashes indicates at least three satellites are acquired and this will provide the most accurate positioning. The problem with this collar was that it wasn’t acquiring any satellites at all; the LED only blinked once every few seconds. My initial thought was that perhaps the guys were right in thinking that the GPS module’s lead had come adrift. It would certainly explain the lack of satellite acquisition. This would be well worth checking out anyway, if not to confirm the diagnosis, then at least to rule it out. I decided to start with the one that powered up; I could use that battery to check the others as the client neglected to bring the specialised charging dock for the collars. Once I had the battery out I could use my bench supply to top it up if necessary. I started by removing the six screws holding the main module together. Two of those screws hold a smaller, separate cover and another, smaller machine screw and two tiny PK-type screws beneath that held the GPS module’s connection harness to the main module. With those smaller screws removed, the end of the collar and the embedded GPS module’s shielded cable could be pulled away from the main module. But not very far; the portal where the shielded cable enters the main module is heavily potted and the material is somewhat elastic, but very tough. The VHF antenna, which is about 350mm long and follows the contour of the collar due to it feeding through various holders, is basically a chunk of heavy gauge, multi-strand steel cable with a basic crimp terminal at the module end and a red, plastic antenna tip at the other. This connects to the main module via a relatively large machine screw but this isn’t potted in and is easily removed. With all the screws and bits removed, I used a small flathead screwdriver to gently pry the metal frame out of the main module’s thick plastic body. It fits very tightly and aside from a few animal hairs and some dried mud, it came out cleanly, revealing two plugs from the board; one to the battery and one to the charge port, which were screwed and moulded into the main plastic housing respectively. Once unplugged, the PCB came away with the metal base, and I could see the PCB was attached to the base with a few more of those tiny PK screws and stuck with potting compound in several places. siliconchip.com.au The first thing I noticed was a lot of grub between the VHF antenna terminal and its connector into the module. As I said, that end of the antenna is not potted in and only has an unsealed, thin plastic cover over it in the wild, allowing moisture and other debris to work its way in. I cleaned the terminal with some isopropyl alcohol on a rag and used my 30-year-old contact-cleaning diamond file to clean the face that contacted with the one in the module. The module side of things was a little dirty but looks to be nicely polished or even chromed steel, so I didn’t file that. Instead, I used my fibre-glass-bristled PCB cleaning brush to spruce it up. Looking further onto the PCB, I could see that moisture had gotten into this one. There is a rubber O-ring type seal between the metal base and the plastic body of the main module and it looked to be intact, so I’m not sure how the moisture got in, but it had started to corrode some of the solder joints on the board. Once again, I used my PCB brush to clean the board and with a very fine tip in my soldering iron, I went through and tidied up every dodgy-looking connection on the board before setting that aside and checking out the GPS module. The GPS module had a plastic bottom, which was held on with four small screws. Once removed, the base came away easily, revealing a completely potted PCB board taking up the whole interior space. The connecting cable exited via a purpose-made channel in the collar and entered the potting material, which was clear, so I could see the cable gently curl around and end up soldered to the PCB. This cable was also heavily potted in at the main-module end, so it wasn’t easily accessible for ringing out. It needed to be tested for continuity though, if only to prove or disprove the client’s theory that it was the problem. The easiest way to do this was to drill a small hole through the potting material down to the joints on the PCB. I used a standard 1.5mm “jobber” drill to start with, drilling slowly down by hand with a pin chuck until I was nearly to the joint, a distance of about 5 or 6mm. I finished off with the same-sized drill, but with the bevels ground off, making it flat-bottomed. This I twisted in until it just touched the soldered siliconchip.com.au joint. Luckily, the refracted light didn’t throw me off the mark, as it certainly looked odd from certain angles as the drill went in. I then used my dentists’ pick to clear the way for one of my multimeter leads and after touching one lead on that, went to the main module’s board and used the other lead to “ring” out the shielded cable. Although the main board end was also potted over, I could touch various parts of the board and get readings, and on the grounded side, could make a one-to-one contact with earth points on the main board, even when twisting and manipulating the cable at either end, so that confirmed to me that this cable was not the problem with this collar. I refilled the holes I’d drilled in the potting compound with 5-minute epoxy and though probably not as tough or hard as the original, for filling a 1.5mm x 6mm hole it was sufficient for air and moisture protection. I assembled the VHF antenna and plugged in the battery – which by this time I’d removed from the housing – pushed the ON button and took the whole caboodle outside and sat it on the rag top of my car. Within about 30 seconds, it was double-flashing and by one minute, was flashing three times, indicating that at least three satellites had been acquired. When I fired up the handset and selected one of the dogs listed, two didn’t show any data, though the third indicated a stationary distance of two metres, and when I moved the collar to the end of the driveway, twenty metres. That was good enough for me, so I reassembled everything bar joining the main housing and metal base together; I’d need the battery for testing the others. The second collar was pretty much a replay of the first; cleaning up all the connections resulted in another working collar. The client was well pleased, and at this stage mentioned there was a YouTube video of a guy fixing one of these collars with the same symptoms as ours. I had a look, and that guy simply replaced the GPS module with a new part, which was overkill in my opinion. The third collar defeated my attempts at basic repair and I think the GPS module has really gone in that one. I’m currently stripping the potting compound out of it. After all, I’ve nothing to lose by doing that and I think I can pick up a suitable module for a lot less than the YouTube guy paid. We’ll have to see. Repair to Cambridge Audio 640C CD player D. R., is a tinkerer living in a small country town, who sometimes gets asked to look at various non-operational devices... A friend recently asked me to look at her CD player. I have had a few CD players requiring a lens clean, but as the front panel showed that it was reading the info off the disc, that wasn’t the case here. There was a signal at the digital output socket, but nothing at the analog audio output sockets. I found a circuit diagrams on the web which showed that there was a relay which could mute the output. There was no “mute” button on the unit or the remote control so it wasn’t going to be that easy. The relay was a 5V DC coil unit and checking around, I found a mute connection (CN4) on the board near the relay. This had either five or zero volts on it depending on whether play or pause/stop was pressed. I (stupidly) jumped to the conclusion that the relay coil must be open. I ordered a suitable replacement, but of course replacing the relay made no difference. Searching around on the board, I noticed that four capacitors appeared to have leaked brown gunge onto the board. I could only get higher voltage rated versions so one of them had to be fitted horizontally on long leads. I half-hoped this might make a difference to the voltages, but the relay was still not operating. 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. September 2017  75 Partial circuit diagram for the Cambridge Audio 640C CD player showing the output mute control, as described in the text. The diagram showed a circuit with four transistors associated with the mute relay. I tested these and they all appeared OK. To try and work out what was going on, I soldered a few flying leads around these transistors so that I could monitor voltages while the unit was operating. I realised (a bit late) that the mute 5V signal was present when the relay should be off and zero when it should be on. This meant that the circuit must invert the mute voltage. I finally traced the fault to R11 which was difficult to find as it was covered in brown gunge from one of the capacitors. It was difficult to test in circuit as it effectively had a large capacitance in parallel, but it was open. I did not have a 2.2kW resistor handy but a 1kW and 1.2kW in series worked as a replacement. This fixed the problem and it was reassuring to hear the relay click on and off and get audio via the sockets on the back. After removing my flying leads and reassembling, I checked that all was still operating. My friend was very happy to have her music back, but since I had deprived her of it for so long (waiting for parts to arrive and putting it aside out of frustration), I felt I couldn’t charge her anything. I might have saved time and frustration if I had done some better testing at the start and applied (correct) logic. Pony 3 mobility scooter J. W., of Aspendale, WA, was recently asked if he could repair a connector on his friend’s mobility scooter so naturally he agreed to have a look at the machine. . . My friend said that the scooter was not going as fast as it used to. He had 76  Silicon Chip been fault-finding the problem over a period of time and had isolated the fault to a 2-pin Molex connector. So he delivered the scooter and we set it up in the workshop. I removed the cover from the controller and checked the “faulty” connector. It seemed to be OK but I gave it a clean anyway. With the scooter out of gear, we were able to hear that the motor was still not revving fast enough, although at one stage it did rev up for a short period. I traced the wiring from the 2-pin connector and found that all it did was connect the ignition switch to the controller PCB. So it seemed highly unlikely that this would have any effect on the speed of the scooter. I suggested that he leave the scooter with me and I would investigate further. I could not find any service information on the ‘net so decided to check the obvious and hope to find a cure. The speed was controlled by two potentiometers: a throttle control with levers for forward and reverse and a speed control potentiometer which set the maximum speed. I disconnected and removed the throttle controller which looks like a rectangular potentiometer. I found on the ‘net that this was called a wig-wag controller with a self-centring position that was supposed to give a resist- ance of half the total. The wig-wag controller was marked as 5kW and it measured 5kW between the two outside terminals. I then checked between the outside terminals and the centre one. The reading showed a variation of approximately 2.5kW when the controller shaft was moved in each direction. I assumed that this was OK so put it back in circuit. I then unsoldered the speed controller pot and checked it with a multimeter. The pot was marked 20kW and started at a reading of 20kW at the low speed end of its travel. The resistance reduced as I turned it to a higher speed position but as it reached about ¾ of the travel, the reading reverted to 20kW and stayed there. So with the pot turned up to maximum speed it was giving a resistance associated with low speed and not the zero ohms I was expecting. I only had a 50kW pot on hand so I connected it up and found that the motor now started at low revs and increased to quite a high speed with the pot turned to zero ohms, the maximum speed position. So it was off to my local parts supplier to get the correct replacement for just $2. Once it was installed and everything put back together, I did a few laps of the garden to prove it was siliconchip.com.au all OK. My friend was delighted as he had been quoted over $200 to have it looked at by the supplier. Fixing a useless machine J. G., of Princes Hill, Victoria is having fun in his retirement, reliving those halcyon days when he made model planes and played around with electronics. He takes up the story. . . My most recent project has been to make a “useless machine”, invented by Marvin Minsky at MIT in Boston. The first prototype seems to have been built in the 1950s by Claude Shannon, the pioneer of information theory. A useless machine consists of a box with an on/off toggle switch on top. When it is turned on, a hand emerges and turns it off. That’s all it does. You can buy useless machines from Jaycar, but I wanted to make one that is even more useless! It would be more creepy if the hand emerged very slowly but snapped back into its box the moment it hits the switch. Servo motors used to control model planes are ideal for this purpose. They consist of a small brush motor and a set of reduction gears which actuate a “control horn” linked to the rudder or ailerons. The servo is controlled by a stream of pulses, the width of which sets the position of the control horn. Typically, a pulse width of 1.5ms sets the horn at a midway position; a pulse of 1.0ms moves it to one extreme and 2.0ms to the other extreme. It was relatively simple to devise a circuit using a CMOS version of the ubiquitous 555 timer IC, where the pulse width is smoothly increased by a slowly rising voltage on the control pin, causing the hand to emerge slowly, followed by a sudden return to a short pulse, putting the hand back into the box. Preliminary testing without the motor connected showed that the circuit worked well, but the best laid schemes o’ mice an’ men gang aft agley. With the servo connected, the hand oscillated wildly and randomly back and forth. This problem is well known in the radio-controlled plane fraternity, and is known as “servo chatter”. It didn’t take long to confirm that it was caused by noise from sparking motor brushes. Somehow the motor noise was getting into the control circuit but a variety of measures including ferrite beads in the motor wires and a 2000µF capacitor siliconchip.com.au across the battery made no difference. Old-timers will remember a common problem that used to affect valve radios, aptly known as “motor-boating”; characterised by a loud put-putput-put in the speaker. These days it is sometimes seen in valve guitar amplifiers. Motor boating is caused by feedback between the power output stage and earlier voltage amplifier stages via the high voltage supply line. Badly designed circuits can be prone to motor boating but it is typically caused by a faulty electro. Motor boating is commonly prevented in the design stage by decoupling the early stages from the power stages, by using a simple RC filter in the high voltage line to prevent fluctuations in the supply line feeding back into the high gain voltage amplifier stages. Could decoupling solve my problem with servo chatter? The motor and the control circuit were fed from a 6V battery. Measurements showed that the servo motor drew a wildly fluctuating current with peaks of well over an amp and the scope confirmed that the supply voltage jumped up and down randomly when the motor moved. The control circuit only consumed 2mA. How about decoupling? All it took was a 220W resistor followed by a 1µF MKT capacitor to earth. The control circuit still worked perfectly with less than half a volt drop in supply voltage, but the servo chatter disappeared completely. Now when the hand moves out slowly and creepily, and snaps back instantly, it always provokes fits of laughter in young and old. Incidentally, while the labelling on the switch in the accompanying picture may look incorrect, it is not. The switch is pictured in the ON position. The hand pushes the switch to the OFF position. In the “resting” situation, the servo arm presses against an invisible microswitch, keeping it in the OFF state. The microswitch is in parallel with the visible switch but is not seen in the photo, such that no power is delivered to the electronics or the motor. The hand is activated by moving the switch to the ON position. This supplies power to the electronics and the motor. The hand slowly moves forward, such that the microswitch is now turned on. The hand moves out of the box, pushing up the lid, and pushes the visible switch to the OFF position. The hand then moves quickly back to the inside of the box, where a hidden protrusion presses on the microswitch and turns the power off. There’s more to it than meets the SC eye! A useless machine is a functional device that serves no useful purpose. This example was designed such that when switched on, a hand will come out and turn the switch off; using a servo to provide the hand with a variable speed. September 2017  77