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SERVICEMAN'S LOG Stomping on the pedal killed it We all know that musos need to stomp on their effects pedals as they weave their magic. But ultimately that stomping can kill the pedal and the good thing is that it then needs a repair. Hopefully, they then know “who they’re gonna call”! The term serviceman means different things to different people. 60 years ago, a serviceman was typically someone in uniform, most likely heading off to, or returning from, a war. These days, a serviceman can be the person who turns up to fix your washing machine or TV, or the person who checks the oil and air filters on your car. Those who call themselves servicemen (or women) cover a huge range of careers and callings and that is what makes being a serviceman so interesting; we cannot usually be easily pigeon-holed into any one job or service. My current job title would be computer serviceman but that description doesn’t cover my skill-set and other servicemen probably feel the same way. Most of us bring a range of skills to our trade and this is what sets the serviceman apart from some other tradespeople. There are plenty of servicemen out there who have no experience in fields other than the one they specialise in, and while there’s absolutely nothing wrong with that, those who do bring outside skills to their trade will raise the bar for others and that can only be a good thing. Having skills in the model aircraftbuilding and hobby electronics fields, for example, can be a real boon for my computer repair work, as every now and then I’m faced with a task that requires soldering, custom fabrication or other outside-the-box aptitudes that others in this trade might have to outsource in order to provide the service. I’d like to think this gives me an edge in business, though given there are plenty of very skilled technicians 58  Silicon Chip out there, that might be just wishful thinking! I mention this because I had a job recently that required a range of skills to resolve. Since I’ve been diversifying into musical instrument and amplifier repairs in order to keep my accountant happy, I’ve had some interesting jobs through the workshop. However, this field also has some challenges to the serviceman and having been a working musician certainly helps me with insights into that world. One rather large flaw in choosing this line to diversify into is that 99% of working musicians operate on very tight budgets, from which they have to set themselves up in what can be a shockingly expensive business. This means there is a lot of compromise and innovation as musos try to get by with whatever gear they can afford. It doesn’t help that guitarists in particular are usually searching for an elusive ‘sound’ to call their own. And those ‘sounds’ are big business; all the top guitar players have their signature tone and playing style that others try to emulate, so much so that manufacturers of so-called ‘modelling amplifiers’ try to ‘bottle’ those sounds so other players can easily replicate them (if they buy that amplifier of course!). That’s a pretty big ask, given that those sounds are much more than just what comes out of the speakers. A player’s tone is a combination of many different factors, from the mass of the player’s hands and fingers to the way they strum, pick, hammer-on and tap the strings through to the construction of the hardware itself, such as the timber the guitar is made from, Dave Thompson* Items Covered This Month • • • • Wah-wah pedal repair Dishwasher cockroach removal Gas igniter repair Technics SZ-4000U amplifier *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz whether it is solid or hollow, the size and type of strings, the way the instrument is tuned, the pick-ups and onboard controls and type of amplifier and speakers, the sum of which have a profound effect on the overall sound. All this variation means a lot of work for the instrument and amplifier serviceman, who sometimes must tread very carefully when repairing or servicing some of this gear. It also means that costs have to be sensible and keeping costs down means not spending too many non-chargeable hours working away on problematic gear, which comes back to having the skills to do these jobs quickly and efficiently. It is sometimes a fine line to walk. While this job turned out to be more mechanical than electronic, it did have a troubleshooting sting in the tail. The customer brought in what is generically known as an effects pedal, which is typically a foot-operated device that sits on-stage and is positioned in-line between the guitar and the amplifier. These boxes modify the guitar’s signal in some way before piping it onwards. Check out the floor near any guitarist playing live and you’ll often see a gaggle of these pedals within easy reach. Some guitar players don’t use any floor effects, while others might have a large pedal-board chock full of them; it is very much a personal preference. This job involved a ‘Wah-Wah’ pedal, one of the earliest and most audibly recognisable of all the guitar effects. The wah-wah pedal as we know it today hails from the mid to late 1960s and was introduced to the masses by siliconchip.com.au musical experimenters such as Jimmy Hendrix and Frank Zappa, though it has been used in every genre of music from jazz, country, funk and disco to the heaviest high-gain rock. Essentially just a variable notch filter, the effect is controlled by the up and down movement of a foot operated pedal, giving the guitar a ‘crying’ sound. While sometimes regarded as a bit of a ‘joke’ effect because the wide range of sounds produced makes it prone to over-use by inexperienced players, it was hugely popular throughout the 1970s before falling out of fashion in the 1980s. Lately, the wah-wah pedal has seen a resurgence due to its adoption by hipster bands and lovers of retro, ‘analog’ effects. This means that some of the original pedals, which you once couldn’t give away, are now worth a small fortune. Most of them boasted remarkably simple, PNP transistor-based circuits, which modern manufacturers attempt to emulate with all manner of digital jiggery-pokery whilst creating newer, quieter versions for today’s noiseallergic players. However, many rollyour-own effects pedal makers – and there are a huge number of us out there – prefer those early designs, making vintage germanium PNP transistors like AC128s and OC71s and diodes siliconchip.com.au like 1N34s suddenly quite desirable (and therefore valuable). Luckily, I have parts-drawers full of them; Easy Street, here I come! The pedal I had to repair is an Ibanez WF-10 Wah-Fuzz, a late-1980s-era pedal made in Japan and now quite sought-after. It has a unique sound and this is why the owner wanted it repaired. The effect, as the name suggests, consists of a wah pedal with a fuzz circuit built-in. Switches beneath the foot-pedal activate the bypass system to switch the wah effect in and out and a fuzz on/off switch and depth control lurk on the bottom left side of the case. This WF-10 had lost its wah due to a broken linkage mechanism, leaving the pedal part of it disconnected and flopping uselessly up and down. The linkage is a clever plastic and metal arrangement that converts the up-anddown action of the foot-pedal into the rotational movement required to turn a potentiometer throughout its range. I’d need to re-create that linkage to restore wah functionality. One of the problems facing many older pedals, and this family of Ibanez pedals in particular, is with the injection moulded plastic case. It obviously seemed like a good idea back in the day, but time is the nemesis of most plastics and the WF-10’s case is no exception. Most early foot pedal effects were made using pressed-tin and/ or folded steel, which meant limited design potential and the pedals were either too light and flimsy to take stage wear or built like brick outhouses and too heavy to lug around. A moulded plastic case must have seemed a godsend at the time; they could be (relatively) easily made into any funky, fashionable shape and were very strong while remaining light enough to carry around in a gig bag or in the back of an amplifier. However, fast-forward 30 years and one of the consequences of using plastics has become all too clear. The owner of this pedal reported that he stomped on it one day and with a sickening crunch, the thing fell to bits beneath his foot, leaving fragments of plastic and distorted metal scattered everywhere. He scooped the whole lot up into a bag and brought it to me to see if I could do anything with it. I understood why he wanted to save it; it was part of his ‘sound’ and finding a substitute would be difficult, unless he could locate another WF-10 that is . . . It was easy enough to see what the problem was; the plastic mounting lugs at the foot-pedal end of the linkage had shattered due to age, leaving nothing for the metal to hold onto. These bits and the linkage then fell through the gap in the bottom part of the pedal and had been crushed by the pedal coming down on top of it, causing the metal to bend and inflicting more damage to the plastic bits directly beneath it, including the pot mounts, which were cracked and would also need repairing. Fortunately, the rest of the case was OK. It didn’t really have that much stress put on it during normal use and so would likely last a few more years yet. Any repair would entail rebuilding those mounting points and making them stronger than they were before, allowing for the fact the plastics were still going to be deteriorating and there wasn’t much I could do about that. Ideally, I’d like to swap out the whole chassis with a new one but that wasn’t an option. A quick look on eBay showed a number of these April 2017  59 Serr v ice Se ceman’s man’s Log – continued pedals for sale, but all were as old and would probably have the same issues as this one; besides that, the cost was prohibitive, so I decided that reconstructing the mounting lugs was the best way to proceed. The first thing was to split the two halves of the pedal before stripping the jack sockets, circuit board, pots and switches from the case and giving everything a good clean. Since the top of the bottom half of the case has a slot to allow the linkage through, it is open to the elements and as it sits on the floor on-stage, it tends to accumulate everything from generic dust and dirt to ash, sweat, spilt booze and other nasty stuff; Rock and roll is so glamorous! Thankfully my customer had the good sense to grab all the broken plastic bits he could find and it seems he got them all. It was simple enough to fit them all back together like a 3D jigsaw but some of the bits had been distorted and didn’t fit as well as they could, so there was a fair bit of carving and fettling and assembly to be done until everything fitted together as well as it was going to. I used an expensive, good-quality 24-hour epoxy to glue these bits, mainly because cheaper epoxies, and especially the 4 and 5-minute versions, are a lot weaker than their longer-curing cousins. I wrapped the drying brackets in grease-proof paper and clamped them using small, springloaded clamps similar to clothes-pegs on steroids, to ensure everything was straight. I left it all for longer than 24 hours, checking periodically early on to make sure it was still lined up. In the meantime, I used my metalworking skills to straighten out the bent linkages and brackets. Once the glue was set, I tried test-fitting everything to make sure it all fitted; after a bit of glue-clearing with a hobby-knife and a drill bit, it did. However, while that repair might last, leaving it like this would invite failure somewhere down the line, so I needed to strengthen it further. This I did by using what I politically-incorrectly call “poor man’s fibreglass”. Years ago, when I was building my Lotus 7 replica, I did a lot of fibre-glassing. I used several weights of spun-glass cloth for the nose-cone and 60  Silicon Chip guards and some scraps of the lightest mat would be ideal for this pedal repair. I cut several small strips to size and mixed up a small amount of the same 24-hour epoxy, which I ‘painted’ onto the lug before wrapping one of the strips of cloth over it. I then painted more glue onto the cloth, working it well in until it was saturated. I applied the other strip the same way and after cleaning any potential runoff, I again wrapped the whole thing in grease-proof paper and held it in place with strips of rubber band material. When the glue was dry, the rubber and paper came away easily, leaving a nice, smooth finish. While the repaired mounting lugs were a bit bulkier than they were originally, we had room and they would now be many times stronger and less likely to fail again. Reassembling the pedal was straightforward and all that was required was a sound check. I put a fresh 9V battery into the pedal’s battery bay and plugged my trusty Telecaster into the input jack, with the output going to my 5W bench amplifier. I stepped on the pedal to light it up, wound up the volumes and strummed a few chords; nothing. While there was some sound, it was way in the background, even with volumes cranked up. I checked switches and wiggled cables but there was no change. This was not really what I was expecting, but no problem; I’m a serviceman! Fortunately, the WF-10 was popular enough for there to be several scans of user and service manuals online. In fact, there are circuits for effects of all types and ages because fans and experimenters reverse-engineer them in order to find out what makes them tick and at the same time draw up schematics and post them online. Very handy! The WF-10 is very much an 1980sera effects box, utilising through-hole components and commonly-available parts. Having a schematic makes things easier but the more I thought about it, the more I was convinced this was related to the broken linkage. A quick look over the board and sockets didn’t reveal anything obvious but I recalled the guy’s description of how it broke; he said the linkage gave way as he stepped on the pedal, so I took a much closer look at the area of the board surrounding the gap in the bottom case where the linkage goes through with my jewellers’ loupe and there it was, one of two diodes sitting parallel to each other near the edge of the board beside the linkage had a faint crack in the glass body. My Peak semiconductor checker told me it was open circuit, so I removed it. I used the tester on the other diode and it told me it was a standard component, so I soldered in a 1N4148, plugged everything in on the bench and tried again. This time I had full sound and after reassembling the pedal, I invoked the spirit of Jimi Hendrix and gave the pedal a thorough test. Job done. Editor’s note: Silicon Chip has published a number of effects pedals over the years and these can all be accessed by searching under the “Articles” tab of the home page of the website. Specifically, we published a wah-wah pedal (we called it “waa waa”) in September 1998. All the parts are still available although the PCB is not. Go to http:// siliconchip.com.au/l/aacc More recently, a Digital Effects Processor for Guitars & Musical Instruments was publish in October 2014. All key parts, including the PCB, are available. Go to http://siliconchip. com.au/l/aacd Dishwasher stopped by roaches B. C., of Dungog, NSW, has had a battle with a dishwasher and its eccentric owner. He managed to repair the machine in spite of the owner’s odd ways. Call it a “pro bono” job. Freda (not her real name) lives close to the beach and her house has been the preferred location for family gettogethers. During a recent visit, Freda’s dishwasher (Dishlex DX302WJ) had developed a weird problem. Unfortunately, this appliance has not had an easy life and has had (what I would call) various environmental faults over the years. In more recent times, it has been rarely used, to save on water and electricity. As I was already there on the spot, my services were enlisted. Freda said the machine had been working perfectly but admitted that she had forgotten to turn on the stop cock (mounted under the sink). Then the dishwasher would not run through the Quick cycle. She had then turned the water back on but it would now only show E10 on the display and would not fill up with water. siliconchip.com.au Freda said to me, “I am sure that all you have to do is find the blockage in the inlet hose and it will be working again”. I foolishly asked “why don’t you just leave the stop cock turned on”. Her reply was, “I always turn it off, in case a rat chews through the water hose. That way my kitchen does not get flooded”. Under these strict instructions (to only check the water hose), I rolled up my sleeves and faced up to the challenge. Checking first that both the power and the water were turned off, I put a hessian bag on the floor and slid the dishwashing machine out. I found a position where I could inspect it underneath and still have access to do the necessary testing and repairs. I could see the wiring harness around the wash motor had been repaired and the water inlet hose appeared to have been re-joined near the back of the machine. Perhaps in the past, a rat did chew through the water inlet hose, resulting in the flooding of the kitchen! I then removed both the rubber hoses that were connected to the water inlet solenoid valve. I first removed the water filter gauze from the end of the inlet hose and found it to be clear of debris. Then the free end of the water inlet hose was put into a plastic bucket. The stop cock was turned on briefly and plenty of water gushed out. Blowing through the water outlet hose (from the solenoid valve) proved there was no blockage. So it was now time to test the solenoid coil. The original siliconchip.com.au two harness wires were disconnected, ready for this test to be done. Set on a low Ohms range, the multimeter showed that there was continuity in the coil and I then proceeded to test the solenoid valve on mains power. A suitable test lead was made up from a discarded figure-8 power lead, with a lamp holder wired in series and then fitted with some fast-on connectors. A 100W globe was put into the lamp holder and this test lead was connected to the coil terminals. When the mains power (current limited by the light globe) was applied, the globe lit up but the solenoid valve made no noise at all; it appeared to be stuck in the closed position and needed to be replaced. I managed to salvage one from another dishwasher, sitting in the backyard appliance graveyard. I tested it first with my mains power test lead. It made a noise as it operated and the globe went back to a dim glow. This solenoid valve was then installed into the machine and the water hoses and cables were reconnected. I put the machine onto the quick cycle and I could hear the water flowing into it. But I could also hear the drain pump motor running as well. As fast as the water was flowing in, it was getting pumped back out again! Over a period of time, dishwashers can have debris accumulate down inside the sump housing, particularly when some owners do not bother to regularly clean and refit the filters correctly. To get access to the lower section of the sump housing, it would be necessary to take it completely out of the machine. I disconnected all the top parts and all the attached hoses underneath, and only then it was possible to remove it. Sure enough, there was enough accumulated debris to nearly block up the waste water outlet, which connects to the drain pump. Still left in the machine was a clear plastic rectangular box, the pressure switch assembly. It had some internal galleries and two rubber hoses that went back to the sump housing. It also housed the water level and the water overfill level pressure switches. I found one gallery and its rubber hose full of black sludge. There was also black sludge in a side compartment of the sump housing. Perhaps this was an accumulation of coal dust, blown in through the back gauze door over the years. I remember Freda had a habit of leaving the dishwasher door open, until the next load was ready to be put through. Eventually all the black sludge was cleaned out and the sump housing and pressure switch assembly were refitted back into the machine. April 2017  61 Serr v ice Se ceman’s man’s Log – continued The water sensors would now faintly click when their rubber connecting hoses were gently blown through. All the rubber hoses were reconnected and all the upper parts were refitted to the sump housing. The dishwasher was turned on again but the drain pump was still pumping out water at the same time as it was trying to fill up with water; very strange indeed. I went onto the internet but I was unable to find any reference to this particular fault but I found the correct method of entering into the test mode. This involves pressing the “Program/ Clear” and the “Start/Pause” pushbuttons together for two seconds while switching on the Power switch. Then you can use (mainly) the ‘Program/Clear’ pushbutton to step through the various sections of the test schedule. Earlier on, you will find (stored in the memory) up to the last three error codes. As you go further down the schedule, each machine function can be directly turned on. These are displayed as item numbers 4 through to 10. The numbers of interest to me were item 6 – inlet valve open and item 5 – drain pump on. I stepped through to (5) and found that the drain pump would run. However, when I went to 6, the water inlet solenoid valve would operate together with the drain pump. So it also appeared to have a fault on the control PCB, perhaps around the microprocessor. With the power turned off again, I removed the control module out of the dishwasher door and there was evidence of bush cockroach ingress. Both the internal PCBs were removed out of the housing and carefully washed down with alcohol and then air dried out in the sun. But after reassembling and refitting the module, the fault was still there. So back to the internet and there were two options available on eBay: Purchase an exchange control module for about $94 or a new one for about $170. It was now time to consult the management, ie, Freda. Apparently the existing control module was already a replacement unit. This was done a number of years ago (as a goodwill gesture) beyond the normal warranty time. Ironically, the previous control 62  Silicon Chip module had also failed due to bush cockroach ingress. Freda then told me “I don’t see how leaving the Stop Cock turned OFF could have caused all these problems you have found. I don’t want a reconditioned Control Module put into my machine, as it probably won’t last, and I don’t want to spend the money on a new one”. However, there was another possible option to save face. Find a writtenoff machine at the recyclers and take out the control module, with the hope that it was still functional. That way I might have a good chance of getting the dishwasher fully operational again. After several months, an Electrolux/ Dishlex dishwasher did turn up at the recyclers and since it had been assigned to go onto the steel scrapheap, I was allowed to remove the control module. After getting it back home, I dismantled and cleaned the module and any suspect joints were resoldered on both the PCBs. On the next trip to Freda’s house, I fitted this module into the dishwasher door and put the machine into the test mode and now correctly started to fill with water. The dishwasher was then put through the quick cycle successfully. Freda eventually came out from her afternoon nap and wanted to know “what are you doing?” I said that I was testing the dishwasher and it is now working properly. ”See, I told you there wasn’t much wrong with it”! Now that’s a real love job! Gas igniter repair This story from Geoff H., in Littlehampton, SA, involves the repair of a gas igniter for a 4-burner gas stove-top. My son asked if I would have a look and see if it was possible to repair the gas igniter. There was no brand name on either the hotplates or the igniter box. So the first challenge was to dismantle the hotplates. Often it is as easy as pulling off the control knobs, removing the trivets, lifting off the gas burners and unscrewing a large nut off each burner so the complete top can be removed. This exposes all the gas pipes, the igniter box and the wiring. The main thing to be careful of is lifting the top over the ceramic plugs that feed spark to the burner so as not to damage them. Normally it is easy but this one was not like that. For this one you have to be a contortionist inside the cupboard below to remove two screws from each side then everything tends to fall down on top of you. It was definitely a two-person job. Anyway we got the igniter box out and I took it home to attempt a repair. The first thing I noticed was that the momentary switch which activates it was stuck down. I was expecting a small transformer powering a simple timer type circuit to generate a pulsing low voltage spark into an ignition type coil, as these hotplates would be about 25 years old. Instead, 230VAC was connected to a bridge rectifier to charge a capacitor via a resistor. From the capacitor the supply was connected a gas discharge surge arrestor to the ignition coil. So when the switch was pressed the capacitor charges up to the flashover point of the arrestor, induces a spark in the coil and the process continues while ever you press the button. It did not take long to replace the stuck down momentary-contact switch. I was looking for a ground return for the spark but there wasn’t any. Instead it uses the other leads as its return. Clever. I then tested it using my small 12V DC to 230VAC inverter. I did this as I want to isolate it from the 230VAC mains supply, being aware that it’s potentially dangerous. Anyway it worked so I returned it to my son and he installed it back into the hotplates. But it still didn’t work. What was going on? After further investigation I found that someone must have disconnected the Active lead in the junction box to stop it working continuously because of the stuck momentary contact switch. I wonder how many years it had been in that condition. Technics SU-Z400 amplifier Japanese hifi gear from the mid1980s was certainly built to last but 30 years later, it’s not unusual to encounter faults. J. L., of NZ recently brought a dead Technics SU-Z400 stereo amplifier back from the dead but it was quite a battle . . . I had been looking for a basic amplifier to play music in our games room and recently came across a Technics SU-Z400 power amplifier. It looked like it would do the job, it cost nothing and it still had the original owner’s manual with it. This amplifier is a pretty solid unit with a large, heavy power transformer siliconchip.com.au and a hefty heatsink. In short, it was typical of the well-built Japanese electronic gear from the 1980s. Inside it is based around an STK2058-4 stereo power amplifier IC and the manual claimed around 60W RMS per channel at 0.02% THD. I duly plugged the unit in, applied power and got nothing; no sound, no indicator lights and no signs of life whatsoever. There wasn’t so much as a sausage from this rather nice-looking amplifier that still appeared to be in good nick. Now I’m the sort of person who will have a go at fixing virtually anything before writing it off as scrap, especially seeing how much gets thrown away these days due to simple faults. So no problem, I thought, it’s probably just a blown fuse. The unit came apart easily and I quickly discovered that the fuse was OK. What’s more, power was reaching the primary winding of the hefty power transformer but nothing was coming out from the secondary leads. I immediately switched it off and reached for my multimeter. A quick continuity check showed that the primary winding was open circuit. Ouch! The transformer carried an SLT5­ M­408 part number and appeared to have multi-tapped secondary windings. In addition, there is a switch on the rear of the amplifier that appears to change the secondary voltage depending on the impedance of the speakers connected. I figured that the transformer was rated at somewhere around ±35V and possibly up to 300VA. I had some similarly-rated parts in my junk box but unfortunately none of them fitted into the confines of the chassis. At that point, I mentally wrote the unit off as junk. And then, some time later, I recalled that some transformers I’d come across had thermal fuses built into them. Could that be the case here? I removed the transformer and unsoldered the PCBs from its terminal pins. Some very careful cutting into the transformer’s insulation then revealed a small thermal fuse tucked inside (without any markings) and sure enough, it was open circuit! Fortunately, the primary winding beyond that appeared was intact, according to the multimeter. I took a guess and replaced the fuse with one rated at 125°C. I then reassembled everything, including tedisiliconchip.com.au ously wrapping the windings in a new layer of tape. This time. when power was applied, the input selector display and source LEDs lit up and all looked to be well! And so, with the transformer now transforming and the lights lighting, I duly connected a signal source and a pair of speakers and got . . . nothing. There wasn’t even a faint hiss from the speakers with the volume turned all the way up. At that point, something in my mind recalled the law of diminishing returns but I’d already come this far and after all, it was just a simple amplifier. How hard could it be? Studying the main PCB showed a pretty conventional power amplifier and power supply, along with some other parts surrounding a relay. I traced the PCB tracks from the relay and this revealed that the relay switched the speaker outputs, so it was likely to be a form of protection circuit. The relay itself was controlled by IC601, a TA7317P. A quick Google search revealed that this was indeed an amplifier protection IC with de-thump and DC detection. Connecting a speaker to the input side of the relay (accessible from the top of the PCB via R411 and R412) resulted in crystal-clear audio, so the protection circuitry looked like it might be the culprit. Further PCB track tracing now revealed that some of the pins on IC601 weren’t actually used. The DC detection pin was connected via isolating resistors to both outputs of the power amplifier IC (IC401), as well to the emitters of transistors Q621 and Q622 which appeared to make up a current detection circuit on the output of each channel. The current detection outputs were then both fed via D602 into Q601 and Q602 which formed a latch, the output of which was also connected to IC601’s DC detection pin (pin 2). This arrangement was likely there to ensure that the load remained disconnected once an overload condition had been detected. I checked the voltage at the input to this latch circuit (collector of Q601 and base of Q602) and it was at -30V DC, as was the output. I figured that this -30V DC was likely to trigger the DC detection circuit in IC601 and cause it to disconnect the speakers. In order to check if this really was the problem, I desoldered R605 which effectively disconnected the overload protection circuit and switched on. This time, the relay clicked in after a few seconds and sound burst forth from the speakers! At that point, I took a punt and replaced both Q601 and Q602, figuring that one of them was probably leaky. I didn’t have the original types on hand (2SA1015 and 2SC1815) so replaced them with a BC556 and a BC546 respectively, as they were the closest equivalents I had on hand. The only trick here was bending their base and collector pins into new positions to match the PCB. Finally, with everything back in place the amplifier fired up and worked perfectly. Job done, you say? Not quite; Murphy made sure that the problems didn’t end there! After it had been running for about half an hour, the speakers suddenly crackled and then cut out completely. As I investigated the cause, I noticed the sound intermittently returning, along with accompanying relay chatter from the amplifier. My first thoughts were that the transistors I had swapped in weren’t quite right but on further reflection, a temperature-related cause seemed the more likely at this point. Close examination revealed some slightly dodgy solder joints around the protection circuit. I reworked all of them, along with some other suspects around IC401, the main amplifier IC. After all that work, the amplifier hasn’t skipped a beat since! Was it worth it for such an old unit? I think so – it sounds good and hasn’t me cost anything apart from some time. The resale value of some of this vintage gear is on the rise too. SC 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. April 2017  63