Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Repairs for a ‘key’ client Dave Thompson I’m getting a wider variety of items into the workshop for repair these days, and I’ve noticed that almost all are high-quality electronic devices which were generally manufactured before the 1990s. It could just be that this is the age where equipment tends to fail, or folks who own appliances of that vintage are of a generation that typically loathes to bin their hard-earned possessions at the drop of a hat. But it’s a sad fact that so much hardware these days is not built to the same quality as it once was. Finding a replacement appliance made to the same standard as your old one can be frustrating (if not impossible), which is why many try to extend the useful life of existing devices by refurbishing or repairing them. We also live in an economic climate where vendors and retailers aim for the lowest common denominator buyers, which usually means keeping the price low rather than keeping the quality high. So most modern appliances are built ‘down’ to a price. Nevertheless, many appreciate quality and are prepared to spend more (sometimes, a lot more) on something well-built and made to last. Unfortunately, choices for those people are becoming more limited. For example, I can go to the nearest ‘big box’ store and buy a ridiculously-oversized stereo/radio/DVD-player combo, with a blow-moulded plastic case, too-many gaudy flashing LEDs and an offensive amount of bass boost for a mere couple of hundred dollars. But if I want anything decent, there’s almost nothing between it and a very expensive, name-brand 100W/channel Class-AB reference amplifier, with rubber mountings, oxygen-free-copper transformers, hand-wired circuit boards and heavy-gauge matte-black steel case. I’d much prefer this high-end amplifier, but would be soon destitute after purchasing the matching speakers, solid gold cables, Oracle turntable and Accuphase tuner to go with it. While 68 Silicon Chip the big-shed special will likely blow itself to bits after a few too many rowdy all-nighters, the high-end amp and components would easily see me out (and quite possibly whoever inherits it once I’m gone!) Don’t get me wrong, I’m a ‘gearhead’ at heart and am always looking for an excuse to buy a better mobile phone or upgrade my computer with the latest goodies; the difference is that I know these devices have a finite lifespan. Given time though, even a $6000 amplifier can fail. Whether it’s a scratchy potentiometer, a blown output transistor or a dried-out capacitor, these ‘wear and tear’ issues can usually be resolved quite easily, mainly because quality devices are designed to be disassembled and repaired in the first place. A job arrives through the grapevine Which brings me to my current challenge. A while ago, I repaired a Yamaha electric piano for a neighbour. To be honest, it wasn’t a particularly taxing job, but it was laborious. The sheer size of the thing and the number of fasteners, clips and plugs to undo makes working on large instruments a pain, especially with the limited bench-space in my small shop. Another neighbour heard about that Yamaha repair and called me with her own tale of woe. She’d purchased a then top-of-the-line Roland KR500 keyboard back in the 1980s. She didn’t say what she’d paid for it, but it was likely a small fortune. About ten years ago, it was repaired by a local music store; they’d replaced half-a-dozen keys that were physically damaged by a friend’s kid, who thought playing it involved smashing the keys repeatedly with a timber Australia’s electronics magazine block. Since then, it has been covered when not being played. Recently, the owner noticed that one or two keys would intermittently not sound, and when a couple more started playing up, she sought out the same repair shop. Unfortunately, this business had closed after the quakes and had never re-opened. The owner called around a few other music stores and was told the instrument was “too old” to repair. They all kindly offered to sell her the latest model, though! Out of desperation, she’d shipped the thing up to the main Roland distributor in Auckland, who sent it straight back, stating that it was nonrepairable. Given the size and weight of the keyboard, even without the solid-wood pedestal it is usually mounted on, transporting it to them and back would not have been cheap. When she heard via the bush telegraph of a local who could fix keyboards (ie, me), she couldn’t get on the siliconchip.com.au Items Covered This Month • • A keyboard without conductor 50W CO2 laser tube replacement *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz phone fast enough, asking if I’d take a look at this one. I’ve been down High Expectations Road before, so I told her all I could do was open it up and see what was going on. After that, we’d know the way forward – if any. She was OK with this and happy to pay for my time. I also suggested that I come and look at it first; if it was going to be an obvious non-starter, this would save some effort. It turned out that she lived literally around the corner. This KR-500 is pristine, a vision in vintage dark-brown, looking for all intents and purposes as if she’d just had it delivered from the shop. It didn’t have a mark on it, and even though it had been well-played, it showed none of the usual wear-andtear I usually see on older instruments. I powered it up, and doing my best Rachmaninov impression, tested all 88 keys one by one; I found at least a dozen not working at all and as many intermittent. The period-fashionable linear pots, LCD screen and all of the dozens of micro-switches and tiny red LEDs on the control panel appeared to toggle and work properly. So did the retro, analog (but still cool-sounding) ‘rhythm’ section. So the fault lay only with the keys. I told her it was likely the keyboard contacts were causing problems and that a good internal clean might fix things. But if that didn’t work, we’d have limited options. She was happy for me to assess it further, so I arranged for her and her husband to remove it from the stand and bring it to my workshop in her van; my MG isn’t the best vehicle for transporting full-sized piano keyboards! Prepping the beast for surgery In my small workshop, the keyboard siliconchip.com.au looked even bigger. Like the majority of Japanese-made instruments of the era, it is incredibly well-built using long-lasting, high-quality materials. Splitting the case was easy; just a matter of removing the dozen or so standard screws holding it all together. It was obviously made to be serviced, with the top section hinged at the rear corners to the internal metal chassis at the bottom. So after releasing the various power and ribbon cables linking the two halves, it simply opened up like a sandwich press. The inside was as clean as the outside. These older keyboards have a lot of PCBs stacked with rows of large, VLSI integrated circuits. There are a ton of components compared to modern instruments, where one or two (likely proprietary) ICs would do the same job. Here, all the parts were clearly marked and the circuit boards had screen-printed values and part numbers, so if I later found that I needed a circuit diagram, I would have no trouble figuring out how it corresponded with the actual hardware. I made an educated guess that since most of the keys worked, the electronics were probably OK and the fault lay with the keyboard itself. After all, it’s the component that’s given the hardAustralia’s electronics magazine est time by the user. Even though the rest of the keyboard may be pristine, over time all manner of rubbish, sweat and other unmentionables migrate down through the gaps between the keys to foul things up. Most keyboards have this problem and there isn’t a lot that can be done about it, other than avoiding smoking, drinking and perspiring while playing. Giving the keys a good wipedown and a thorough hoover now and then helps too. This keyboard assembly was a solid piece of kit. Built like the proverbial masonry ablutions domicile, the backbone frame is a z-shaped piece of folded, heavy-gauge steel securely bolted to the substantial timber bottom of the case. Sections stamped out of the frame accommodate and hold the springsteel key springs and other peripherals while a couple of 90cm-long flexible connectors span the length of the keyboard at the bottom, folding and splitting off at right-angles at the mid-point to connect to the main PCB. After removing the whole assembly from the base and flipping it over, I could see a green, ‘thin-film’ circuit board peeking out from under the keys. The problems likely lay somewhere July 2019  69 within. I couldn’t get any access at all to the circuit board or the contacts under the keys, so the only option was to remove everything from the frame. This was a mission in itself. Along the top of the keyboard, flush with the ‘heads’ of the keys, were a series of plastic locking strips. These had been attached to the frame with very strong double-sided tape, and I had to carefully pry them off one by one. Once off, each key could be pushed slightly forward to release the back ‘hinge’ and then maneuvered out of the frame. With the first key, the steel spring pinged off into the distance; fortunately, I found it after much foraging under the bench. I daren’t lose any because re-creating them would be extremely difficult. Getting to the heart of the matter The PCB was dusty underneath, so my hopes rose that a good clean would sort it out. I could also now see the strips of grey rubber contact pads that make up the top half of the keyboard switches. These were still looking very good and felt nice and supple; hopefully, the carbon-composite contacts moulded into the strips and their corresponding printed contacts on the circuit board below would be in a similar condition. 70 Silicon Chip By now I could see the whole PCB, or at least most of it, given that some of it was still obscured by the rubber key switches. The circuit board was in two halves; one for the lower four octaves and one for the upper four. These are stuck (probably with similar ‘gorilla’ tape) to the metal frame, and I really didn’t want to have to remove them. The fact that multiple keys up and down the keyboard were failing meant that the problem wasn’t localised to one or the other circuit board, so it must be something in common with both causing these problems. It didn’t take long to find a potential smoking gun. At the end of each PCB is a multi-pin, thin-film joiner that curls under the frame to link up to the long flexible main-board connector strips beneath. While there is a socket mounted on a bracket for the joiner’s silver-coated contacts to push into, on the top the graphite or carbon pads are just pressed onto and stuck to the corresponding thin-film key PCB connections with conductive tape. This is all held down by a clear-plastic link pinned to the metal frame at each end with plastic push clips. On closer inspection, I could see that the conductive tape had let go in places, making contact sporadic at Australia’s electronics magazine best. I carefully plugged the piano in and positioned everything while the case was open so I could power it up. With it switched on, I could press the rubber key contacts directly and with the right pressure on the flexible joiner connections at each end, could get the previously non-working keys to sound. I then used a couple of small-but-strong spring-clamps (like clothes pegs on steroids) to firmly hold these contacts in place while trying every key. While they all worked, just tapping on the clamps resulted in dead keys again, so merely clamping something stronger than the existing plastic bar onto the connections wasn’t going to work. Figuring out how to make a lasting repair There are 16 contacts each less than 1mm wide, separated by a similar-sized gap, on each joiner piece. I could see most of the original contact material had been stripped away by the lifting tape. I’d need to rebuild these contacts, and that could be a challenge. I had to take a break and ponder the problem. My initial thought was to replace the strap with a computer IDE ribbon cable or similar; I could solder one end of it directly to the socket’s PCB pads underneath, but I’d have to work out a way to connect the other end to the missing contacts at the thinfilm PCB end. Then I remembered conductive paint; I’d used this a long while ago to good effect. Perhaps it would work here. Maybe I could simply paint in the missing contacts and voila! Feeling hopeful, I ordered some from an auction site, mainly because it was considerably cheaper than the paints listed on local electronics suppliers’ sites. However, when it arrived, I discovered it had gone off and was useless. While I arranged for a refund, I bit the bullet and shelled out $60 for a pen-style applicator with conductive silver paint from a more reputable source. Annoyingly, this wouldn’t work properly either; the contents had partially hardened. I’m still waiting to hear back from them, but in the meantime, I scooped a bit of the material from inside the pen and mixed it manually. siliconchip.com.au It looked OK, so I painted it onto two of the contacts on the joiner. It looked the business, and after letting it dry overnight, I tried some continuity tests. It didn’t conduct at all, and when I tried to remove it, I only succeeded in stripping more of the meagre amount of contact material remaining on the joiner! This effectively ruined it and ruled out using that option again. What to do now? I went back to my ribbon cable idea and found an old floppy-disk cable. The wire spacing was identical to that on the socket PCB, so I peeled off 16 strands, cutting off about twice the length I’d need. I stripped 3mm of insulation and carefully twisted and tinned each wire. I then soldered the wires to the socket’s PCB pads. This was the easy part; it was the other that caused me difficulty. While long-time servicemen out there are probably eye-rolling and shouting into their magazines that I could have used product X or Y, I settled on using self-adhesive conductive copper tape to replace the halfmissing contacts on the end of the thin-film PCB. I cut the tape to precisely the right size, then stuck it down with enough left over to solder (quickly!) to the stripped and tinned ends of the ribbon cable. I then clamped the plastic link back with screws instead of pins and tested the keyboard. This part of the keyboard now worked a treat, so after wrapping the joins carefully in polymer tape, I repeated the process on the other side. I cleaned everything, painstakingly reassembled the keyboard and got the owner over to test it out properly. The result was music to both of our ears! 50W laser tube replacement This time, we had the opportunity to be our own serviceman. The laser tube in our laser cutter stopped lasing (it had one job!). This is how we got it going again... We use our laser cutter to make custom case parts out of acrylic sheets for some of our projects. It is one of the “K40” types that can be purchased from any number of online stores. It’s a CNC machine; stepper motors move the head over the top of the workpiece. A 50W CO2 laser provides the ability to cut and etch (by running the laser at reduced power) as the head moves around. siliconchip.com.au TM Creative Making Technology www.picokit.com.au email sales<at>picokit.com Flowchart Coding • Vinyl Cutters • Soldering Tools • Curriculum & Resources • Filtration Systems Laser Cutters • 3D Printers • CNC Plasma Cutters • CNC Routers • Coding Kits • CAD Software The cutter has a water cooling circuit to keep the laser cool, as well as a ventilation fan to remove the plastic vapours that are generated. There is also an “air assist” system which pushes fresh air past the lens, to keep it from being contaminated by dust and fumes, and to help burn away the plastic. The laser cutter (laser and XY table mechanism) works quite well, although we did initially have some trouble with the ventilation and cooling components. We documented our fix for these in an article in the June 2016 issue (siliconchip.com.au/Article/9960). Since then, the laser cutter has performed well, until one day we went Australia’s electronics magazine to check on the progress of a cutting job, and found that it had not only stopped cutting, but was emitting a high-pitched whine. The laser is a fixed glass tube around a metre long which is fed from a high voltage supply; it’s kind of like a neon tube on steroids. The beam is reflected by three mirrors and focused by a lens onto a point directly below the moving head. The laser tubes do not have a long life span, and the four years that this one had been working appears to be par for the course. When it was powered up, there was a corona discharge visible near the anode, but not the usual, healthy glow along the full length of the tube. July 2019  71 Left: the old laser tube with the water cooling tubes and supply wires detached. Middle: the glass tube is mounted in a saddle lined with rubber pads, which had to be carefully removed. Right: the anode wire join for the new laser tube. We suspected breakdown of the high voltage insulation around the anode connection, and attempted repair by adding some silicone sealant. Unfortunately, while that stopped the discharge, the laser still wasn’t working, so we suspect that the critical CO2 gas may have escaped through a small hole. Since the tube is blown glass, it’s almost impossible to service. So we bit the bullet and ordered another tube. After a few anxious weeks, the new tube arrived in one piece. This one was slightly different to that originally fitted to the K40. For example, it has a slightly smaller diameter and is also a bit shorter. It generally looks a bit better made, and the high voltage anode lead features a locking insulating sleeve that protects and insulates the wire join we would have to make. Thus began the delicate process of removing the old tube and replacing it with the new tube. We started by draining the cooling water circuit, using the cooling pump to empty it into a bucket. We then opened up the hoses near the laser tube and forced air in, to get the rest of the water out. The tube is held in place by clamps at either end, with the glass tube protected by rubber pads which fully encircle it. We cut the two supply wires (a red anode wire and a black cathode wire) near where they are terminated to the old tube, giving some extra length for 72 Silicon Chip connecting to the new tube. The new tube already had wires fitted and insulated to the anode and cathode, much more nicely than the old tube, so we wanted to keep as much of the wiring as possible. After this, having already detached the cooling water tubes, we carefully removed the tube by lifting it out. The new tube has a warning to refer to the user manual, but there was no manual included. So we were going to have to figure out the installation process by ourselves. We fixed the new tube using the existing clamps. Its slightly smaller diameter meant that the hex machine screws had to be screwed in further than previously, but we were able to clamp it securely. We then attached the water pipes. These merely push onto the barbs on the tube. All seemed in order, so we turned on the pump and refilled the small outflow tank with fresh water. The cooling circuit quickly filled, and the bubbles made their way to the outflow pipe. It’s important to get rid of air bubbles so that there aren’t any hot spots inside the laser tube. Our cooling system monitor reported no faults, so the flow appeared to be adequate. We then soldered the two new wires to the existing power supply connections, added heatshrink tubing and fitting the protective anode wire housing over the join. These wires are very fine, but have Australia’s electronics magazine very thick insulation due to the high voltage (tens of kilovolts!). To avoid strain on the new splices, we taped the wires to the outside of the tube (as the original wires had been). This completed the installation, but we still needed to check the alignment of the laser with respect to the mirrors and other optics. Calibration Since the new tube has a smaller diameter than the old one, we expected that the line of the laser beam would be shifted somewhat. Removing and re-fitting the rubber pads may have also caused some variation. So we took no chances and checked the entire beam path. This is done by placing a piece of paper (such as a self-adhesive label) over the mirror in the optical path, briefly firing the laser using the PULSE button, then checking that the laser strikes near the centre of each mirror along the way. The first mirror is accessible from the rear of the machine, the second through a panel on its left side, and the third is on the moving carriage, directly above the lens which focuses the beam onto the workpiece. We found a good guide at siliconchip. com.au/link/aao9 The button press triggering the laser burst needs to be very brief, or the sticky label may smoulder or catch fire. After trying with a second sticky note, we found that the laser was aimed close to the siliconchip.com.au centre of the first mirror, which was no surprise, since it is close to the end of the laser tube. Proceeding to the second mirror, we found that the beam was striking a little low. We adjusted this by turning fine-pitched screws on the back of the first mirror, changing its angle to aim it towards the centre of the second mirror. A fraction of a turn was all that was needed to correct the aim. Repeating with the paper on the third mirror, we found that this was a little low too, so a similar adjustment was performed on the second mirror. The alignment test is repeated with the carriage in all four corners of the laser cutter bed, to ensure that the results are uniform. We found only a tiny amount of variation, so the beam alignment was complete. Air bubbles appeared in the outflow pipe of the laser when the cooling system was refilled. These went away after running the water pump for some time. Testing While checking the mirrors, we took the opportunity to clean them using acetone and a lint-free cloth. The cloth was dirty afterwards, so a clean was undoubtedly due. The laser beam focusing is dictated by the distance between the beam and the bed; it should not have changed, but we decided to check it anyway. We performed a ‘ramp test’ by placing a piece of acrylic on the bed, propped up at one end so that the cutting depth changes along the piece. We ran a cut, and the results showed that the focus was fine, as the cut was cleanest close to the bed. We followed by running a job, and all seemed to be in order. By this time, we had quite a backlog of orders for case pieces, so we had to run the laser cutter continuously for several hours. During calibration, a small piece of paper was placed over the mirror in the optical path, to help centre the beam. Disaster strikes While setting up for one of these jobs, one of the hinges that holds the lid on snapped, leaving the lid hanging by the remaining hinge and the gas struts. The lid had been getting quite hard to close, and appeared to have shifted, but now it was impossible to close as the gas struts were pushing the lid against the remaining hinge, threatening to break it too. Due to the safety interlock, the laser will not operate unless the lid is closed, so this had to be fixed before we could continue. Fortunately, Bunnings has an exsiliconchip.com.au tensive range of hinges. Thus we did not have to wait weeks to get the laser cutter going again. We took the broken hinge to the closest store and compared it to the hinges there, and found one that was a similar size and had a similar hole spacing at one end. We removed the gas struts, as they pushed the lid around awkwardly, and in any case, it made it easier to work on the lid by detaching it entirely from the laser cutter. Although the holes on the new hinges were in slightly different locations, Australia’s electronics magazine by enlarging two holes on each and drilling six new holes in the lid, we got them to fit. These hinges are a different style than the old ones and don’t sit flush when fully open. So we inserted some spacers under them, to ensure that the laser protection switch would engage with the lid closed (the laser is disabled when you open the lid). The new hinges work even better than the old hinges, with the lid not jamming so much and we were back up and running again less than a day later. SC July 2019  73