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SERVICEMAN'S LOG One good turn deserves another Dave Thompson A client turned up at the workshop the other day with what I would consider the perfect job for me. This bloke is a known audio ‘nut’ in my old circle of friends. Although I hadn’t heard from him for many years, he had tracked me down because he finally decided to fix up a few of the ‘vintage’ items in his vast collection of 70s and 80s audio gear. I say ‘my circle of friends’, but I didn’t really know him; I had met him briefly, and knew of him by reputation back when I was a young jobbing musician. He played bass guitar in one of the many bands I used to cross paths with while grinding out gigs on the local pub circuit. He was known for being right into his audio gear, so in lieu of any real information available to us at the time (no internet in those days), we often bowed to his supposedly superior knowledge in subjects related to sound reinforcement and instrument amplification. For better or worse, he was a fount of knowledge at the time; I had to learn this stuff somewhere, right? For those intent on writing off my experiences, put those burning torches and pitchforks down; I admit I was never a ‘rock star’ but I did play in touring bands for many years, and one picks up pertinent knowledge along the way. While I might not know what amplifier and speaker combination works best in your man-cave/ lounge room, I could suggest what sound reinforcement hardware you would choose if you wanted to sonically light up a 500-seat theatre! My point is that we all knew this bloke as an audio purist, and while we somewhat blundered on with our typically low-fi stage gear, he was the one who was really into the specifications and minutiae of the speakers and amplification that were being used back then. While I hesitate to label anyone I’ve ever known as an ‘audiophile’, he was probably as close as I ever came to meeting one. Not that he was in any way one of siliconchip.com.au these guys who proclaims that those $500-per-metre, plutonium-enriched gold and copper alloy speaker cables as the only thing to use for a home stereo setup. Or that one had to have thousands of dollars’ worth of audio hardware to have a good sound; he was just really into his audio hardware. Many of us likely know (or have known) someone like this, to varying degrees. I’ve met guys over the years who insist that speaker cabinets need to be mounted on Kryptonium-alloy Australia’s electronics magazine Items Covered This Month • • • Acoustic research turntables PA system repair HP5100 frequency synthesiser repair *Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz January 2021  61 needles embedded into unicorn-horn substrate on an isolated, rubber-floored room to really appreciate the sound of their stereos. And all this will have to be powered through some rare and obscure Class ABCDEFGHIJK+ amplifier that costs more than a small house. The marketing bumf of the time laid it on pretty thick, having us believe that unless one has this type of system, it simply wasn’t worth even firing up the turntable. This is obviously not the case, but I gave up trying to convince anyone of this a long time ago. To me, whatever sounds good to me is the best system, regardless of specs and cost. It seems my old friend has mellowed a bit; maybe because of age and wisdom, but likely also because his now-wife doesn’t share his passion for spending money on expensive audio gear. That said, he still uses and swears by his many 70s and 80s highend amplifiers and turntables. I only mention this because when he called the other day, and after the usual 30-minute, three-decade catchup conversation, he arranged to bring in a couple of his Acoustic Research (AR) turntables for me to have a look at and see what I could do with. After that is the promise of some more juicy gear to work on. Life could be worse! You turn me right ’round Most people realise that anything with words like ‘research’ or ‘labs’ in the brand name must be excellent, and therefore expensive; and back in the day, in New Zealand, these Acoustic Research ES-1s were. They weren’t insanely expensive, like the Mitchell Transcriptor turntable (featured in the movie A Clockwork Orange), or the Oracle Delphi MK1, both of which could be had here for the price of a small car (minus tonearm, of course, which they assumed you would want to add yourself anyway). But they were still relatively pricey none-the-less. The customer duly arrived with a couple of banana boxes full of bits and pieces, and I could see my work would be cut out for me. One box contained one AR ES-1 turntable, which was the most complete, but this had a damaged drive motor and spindle assembly, which made the drive spool wobble like crazy when powered up. Even if I could get a belt to hang 62 Silicon Chip onto it, playing any records would be pointless and sound reproduction would suffer badly. The second box contained a turntable identical to the first, but which had seen better days. While it still powered on and the motor ran, the veneered timber case was in a very sorry state, and the power indicator lamp was not working. This one was also missing several pieces, such as a tonearm, the tonearm mounting plate and most alarmingly, the rectangular clear Perspex cover. The cover hinges were still attached to the base, but the cover was nowhere to be found. It transpired that this particular table had been bequeathed to the current owner after the previous owner had passed away; a sorry situation to be sure, but at least the turntable wasn’t just thrown into the garbage by the grieving family. So, in summary, I had to repair two Acoustic Research ES-1 turntables; one was complete but had a wobbly drive capstan, and the other was minus a lid, tonearm and altogether incomplete and to be honest, a bit of a mess. This was no problem for a serviceman like me, though! If he wanted to pay me to restore these two beautiful machines, I’d be more than happy to oblige. However, I’m sure you see the predicament; often, when servicemen start talking about the money involved, getting vintage devices back to rude health can suddenly become a lot less urgent! I quoted the guy, and he was more than happy to pay. Darn! I knew I should have charged more! Australia’s electronics magazine Suspect #1 I tackled the most complete one first. That entailed removing the platen, which is simply held in by gravity, the drive belt, and the eight screws that held the cast metal plate to the timber chassis, the fibre-board floor underneath the turntable (which is held on with the four rubber feet screws) and desoldering the wires that go from the drive motor to the small circuit board. The tonearm cables just go straight through the mounting plate and out through a cut-out in the fibre-board floor to the amp, with just one small plastic cable clip holding them in place inside the box. Once these items are removed, the whole mechanical assembly lifts clear, leaving what is essentially a timber box with a circuit board and mains cable mounted to it. Simple, to be sure, but very effective. The only problem with this one was the bent drive shaft. God knows how this happened, but it was severely eccentric when running. Fortunately, the customer had purchased a replacement motor from an internet auction site, but hadn’t fitted it because he couldn’t get the drive pulley off the bent shaft. This proved no real problem for me, as I have the right tools for the job (woohoo!). After removing the motor (which required desoldering the two motor power leads), I mounted it in some jaws under my bench press and gently pushed the pulley free. I had to be careful because these pulleys are typically a cast alloy and are very soft. It separated from the motor shaft quite readily though, and I ran it siliconchip.com.au on a temporary mandrel in my cordless drill to check its concentricity. The pulley itself was fine. I simply mounted it onto the replacement motor with some Loctite and replaced and re-soldered it back to the drive plate. On switch-on, it ran almost perfectly, although it required a few light taps on the high side with a small rubber mallet to get it running dead true. Another problem with this turntable was the drive spindle on the platen itself. The heavy platen sits and runs in what appears to be a phosphor-bronze bearing. It had excessive play, and I suspected this to be due to the bearing drying out. These types of bearings are usually pressure- or vacuum-infused with oil when manufactured, and last for years, but this one was bone dry. I don’t have a vacuum chamber (I know, I know), so all I could do was fill the bearing with oil and leave it standing for a few days before removing the excess and trying the fit again. This time it was better, and operation was smooth and steady, but since I had no instruments to measure wow and flutter (I know, I know), I’d have to go with what I had. The owner would either have to get that bushing replaced, or re-oiled, or do some regular lubricating maintenance on it in the future; he didn’t seem too worried about this prospect when I mentioned it to him. Suspect #2 That brought me to the next pile of bits. It’s always nice to have a fullyfunctional version to compare with when assembling parts, and this time was no different. I put all the metalwork back together and laid everything out in the case, and the only part missing was the tonearm mount. I took some measurements from the working version. The plate appeared to have been made from some kind of painted hardwood, so I fabricated one from an oak offcut I had in my timber bins. I bored the four countersunk mounting holes around the periphery, but left the rest undrilled; how and where the other holes go would depend on what tonearm he wanted to use. I offered to do that for him when the time comes, and said he would let me know. After filling, sanding and painting it with a semi-gloss black lacquer, it looked almost the same (and as good) as the original. siliconchip.com.au The same couldn’t be said for the timber turntable case, though. It looked like it had been left exposed to the sun for many years, and all the dark external veneer had peeled and flaked, exposing the lighter timber beneath. The only way to solve this was to strip the veneer off and re-finish the timber below. I used a coarse belt on my belt sander to rip the old finish off. While messy, it came off easily; the trick is not taking any of the solid timber underneath! I then used progressively finer paper on my recentlyrepaired 1/3-sheet sander to smooth everything off. A few coats of Danish oil brought out the grain, and five coats of clear lacquer resulted in a beautiful finish which was (luckily!) very close in shade to the original model, so I was happy with it. The customer was too, when he saw it. I didn’t have a Perspex cover for it though, and couldn’t find one online. I contemplated making one from scratch, but I didn’t have any plastic heater/former/folder gear with which to do it (I know, I know). Dad had all that stuff, but I don’t know what happened to it. The customer wasn’t too bothered; he planned on putting the turntable in a rack that had a top cover anyway, so with his agreement I removed the original lid hinges and plugged and filled the holes left behind. I used plugs of similar-coloured timber so it wasn’t too obvious that it had been done, though of course, if you looked closely you could see it. However, it was on the rear of the case so the repair would be out of view for the most part. The next challenge was the indicator light. These turntables have a simple on/off switch mounted on the top (under the plastic hood) that switches the motor, well, on and off. When on, a pinpoint of orange light shows through about halfway down the front of the case and directly underneath the power switch. It is quite bright, so I wasn’t sure exactly what was being used. Whatever it was had been glued or embedded into the timber on the inside of the case, and someone in the past had tried to remove it by simply pulling on the connecting wires. They had parted company, and I could see what looked like the bottom of a peanut bulb stuck in the hole. Australia’s electronics magazine Cautious use of a dental pick from the inside of the case soon had the lamp out, and it turned out to be a peculiarly-shaped neon bulb with a pinpoint at the tip, which is the part of it I could see from the outside. I have a good selection of neons collected over the years, but I didn’t have anything like this. I decided to use a light-emitting diode instead, and the customer was OK with this. The problem was that the neon ran from a 160V AC feed from the circuit board (as measured across the broken-off leads), and I think that’s a little high for LEDs, and they prefer DC too. I installed a high-voltage full bridge rectifier across the circuit board contacts which gave me a DC voltage I could use for the LED. I calculated I’d need a resistor of around 15kW to drive the LED from this supply without blowing it. After installing the LED into the hole and soldering everything up, I tentatively plugged it in and switched on. The LED was a little dim, so I dropped the resistance to 13kW, and that did the trick. After running it for half an hour on the bench, nothing became overly hot or emitted smoke. The effect from the outside of the case was almost the same as the neon, so I was happy with it. After insulating everything with heatshrink tubing and tacking the LED into the chassis with a small dot of epoxy, I assembled everything into the case and buttoned it up with new rubber feet. This turntable suffered the same problem of excessive play in the platen spindle, so I did the oil bath trick again, and that sorted it out. After fitting a new belt the customer had supplied, and trying out the drive system, I was happy with the way it was running. The final job was to re-attach the AR badge lying in the bottom of the box to the correct location on the front of the case, and it was the job done! PA system repair R. J., of Laingholm, NZ found out what happens when amateurs and barely competent ‘professionals’ have a go at installing or fixing a large PA system. The result wasn’t pretty, so he had to spend some time cleaning up the mess... My main focus is in broadcast studios, but I do get to work at theatres and festivals and have a fair bit of January 2021  63 equipment that needs maintaining, so service jobs are part of the portfolio. In the ‘good old days’, a band or performer would turn up at a venue with a sound system of variable quality and crank it up to 11. But times have changed, and most venues now have good sound systems and some control over the volume. Bands have been replaced in some places by a DJ who can play any one of several thousand songs, making it even more important to have a quality sound system. I have an acquaintance who runs a performing artist booking business, and before he books performers, he checks out each venue. At one venue, he found the system to be terrible. The owner who recently purchased the business claims the system cannot be that bad because it cost $50,000; that was its value on the books when he took over the business. So I was called in. It was a restaurant/sports bar/dance floor and verandah in an industrial area building which looked like it had been a warehouse or a workshop. The restaurant and bar fit-outs were well done. The stage is bigger than most, and at one time, the venue had been fitted with an excellent system which was well worth the stated value when new. The only fault I could see in the layout was the positioning of the speakers above the stage. They are mounted behind the front line and close enough to a back wall to cause feedback problems. The shock came when I was shown the equipment rack, in a nonventilated office at the back. It seems that the venue had some sort of problem with the system which caused a number of the original amplifiers to fail. The cost to have someone come and fix it was deemed too much. It seems one of the regular clients of the bar said he could fix these things. He got it going again by taking out the non-performing amplifiers and replacing them with domestic stereo amplifiers! The original amplifiers disappeared, along with that patron... The system was in a state of chaos and disrepair. The owner wanted to stage the repair process because his venue had fallen on hard times, and costs have to be managed as the owner re-invents the place to meet the current market. This suited me as I could do it as a fill-in job. 64 Silicon Chip This system has a DBX Zone Pro controller which can switch any one of six sources to one of six zones. It was not switching as expected, and randomly dropping the level to various outputs. Fortunately, the agent is Jands who have a local office and some very competent people. There is no field service. The DBX needs to be assessed on a test bench, so I took it over. Next day they phoned to report that they’ve found the fault and that it is fixable at a reasonable price. I have it back in a day, armed with a warning that the problem is definitely heatrelated. Not only is the rack in the least ventilated part of a building, but there is also a kitchen adjacent, so the ambient temperature is warm. The rack is well-made and has four fans in the top, none of which work. We agreed this was an urgent matter, so I sourced four fans. The mounting screws were different, but with some assistance from a specialist, the replacements went in. The original fans seemed to have failed due to debris falling through the vents. I put the DBX back in with an air gap above and below, and a warning to the owner that if the temperature goes up, we will have problems. I also purchased an external fan to assist with airflow, and removed most of the junk on top of the rack, which was blocking the airflow. Reprogramming the DBX with the supplied software was easy. Working through the amplifier levels, I found that they were nowhere near what the venue needs. The monster amplifier in the rack used for driving the subs had failed. It was a brand no one knows and uses components we can’t find. It seems to have been a short-lived European import brand. I contacted the company who did the original install, but they were not helpful. One good thing about this system was the speakers. They were a wellknown European brand, and they all functioned properly. They all had power-handling capabilities well above anything we could deliver. There were two line-level XLR leads from the stage back to the DBX. Whenever someone plugged into them, there was a huge hum. I was told that it had been like this since the system was put in. On the stage, a mixer of indeterminate parentage was feeding audio to Australia’s electronics magazine the DBX. Replacing this with a small Alesis ‘fixed’ the problem; the Alesis line out is isolated from the local Earth. But 99% of the visiting equipment is not such a good design, so I would have to come up with a better solution. I located a couple of 600W:600W transformers which have good audio bandwidth. Don’t be fooled by the cheap ones; they only go to 4kHz, and the proper ones are not cheap. The wall box on which the XLR sockets are mounted has enough room for a small PCB and the transformers, suitably wired and insulated. Now anyone can plug into the house and get clean audio. So the main job left was to upgrade the amplifiers. We settled on a Class-D Talon amplifier, which claims to deliver 2 x 450W. One was purchased to see how it performs, and the result was stunning. It has XLR inputs and can be run in bridged mono mode. There are two fans, and the unit is quiet and runs relatively cool. The owner was impressed and some time later called me in, and we ordered several more. Another afternoon’s work, and we had a system which was performing as well as ever. I can’t believe the domestic stereo amps lasted as long as they did. At least one was cutting in and out on one channel; likely, its power supply was not up to 5+ years of 12 hours of daily use. The other just had trouble making steam. The two originally installed Aussie-made amplifiers are still working. But there was one last problem remaining. The venue has an AMS music system which streams music and video to each client over a proprietary system via an HP laptop computer. When this system was installed, a ‘contractor’ was booked by AMS to run the cabling. The cabling is not part of the rented system, so its maintenance is the venue’s responsibility. From what I could see, it had not been done properly. One was run over Cat5 via a balun which has one coaxial wire and a pair of wires originally intended for feeding DC. There was another balun at the end of the 40m cable with some RCA connectors that had been grafted in place of the original BNC connectors. That it worked was more by chance than design. The baluns were not designed for audio, resulting in significant losses of both low and high audio frequencies. Cat5 has varying twists on siliconchip.com.au its four pairs, and the only pair suitable for audio is blue/white (pins 4 & 5). Replacing this cabling was not an option given the cost. I tried to source some Cat5 audio baluns but had difficulty finding them. I eventually got hold of some suitable RJ45 termination. An hour or so and we have an ‘improved’ cabling system which delivers audio at high quality over Cat5. Just why this was never run with proper audio cable and terminated with XLRs or TRS jacks will never be answered. The venue now has a grunty music system which was well tested over the next weekend, and a reliable pub quiz system which fills the place on Tuesday nights. HP5100 frequency synthesiser repair R. F., of McCrae, Vic wrote in to tell us a little bit about how he got into electronics (see the Mailbag section in this issue). In that letter, he mentioned that he was fixing up an HP5100 frequency synthesiser. Here are the details of what was wrong with it, and how he fixed it… My earliest recollection of a “fabulous” electronics device was on display at an open day for prospective students in the old cream brick Electronic Engineering School at the UoM. It was an HP524A digital counter set up to measure the speed of a rotating shaft. It could measure frequencies up to 220MHz and weighed a massive 55kg. It had digital readout columns of 10 digits per decade. Somehow, it was very cool; one of my clock-building friends still has one. Years later, as engineering students, we keenly appreciated the precision and superb construction of the Tektronix 547 oscilloscope – one of the most sought-after instruments in our laboratory sessions. Hewlett Packard made a nice range of ‘scopes, but was more renowned for their general and specialised test equipment, which brings us to the devices needing repair. Back when I was home-brewing amateur radio linear amplifiers for HF and two-metre operation, I decided that I needed an accurate, high-resolution signal generator. I heard of an HP5100A synthesiser available at a give-away price, so I bought it. However, it needed an HP5110B synthesiser driver, and despite an exsiliconchip.com.au haustive search via eBay and other channels, I could only find the driver as part of a pair; the other part was a J02 HP5100B synthesiser. The HP5100A could generate signals in 0.01Hz steps from 0.01Hz to 49.99999999MHz, whereas the J02 HP5100B generated signals in 1Hz steps from 1Hz to 29.999999MHz. Fortunately, the HP5110B could simultaneously drive both synthesisers. So I bought the second synthesiser and driver as a package. To cut a long story short, I installed the two synthesisers, the driver, a Australia’s electronics magazine modified HP5061A caesium-beam frequency standard, an HP5245L electronic counter and an HP6263B DC power supply into – you guessed it – a Hewlett Packard Systems equipment rack. This massive collection of boatanchors weighs over 170 kg. The caesium beam tube of the HP5061A was unserviceable when I bought the unit for $100 about 15 years ago, but the unit had a highly-accurate 5MHz crystal oscillator in a temperature-regulated oven. So I stripped out the caesium beam assembly and related parts and replaced them with a January 2021  65 erating techniques all combine to produce exceptional performance. But inevitably, a small proportion of components do fail. Over the past few years, the performance of my units slowly deteriorated until I had no choice but to delve into them. Repairing the behemoths modern GPS receiver and controller to synchronise the oscillator. The very stable 1MHz output from the HP5061A is connected to the synthesisers, driver and counter to provide GPS-accurate synchronisation, and consequently extremely accurate and stable frequency synthesis and measurement. It still seems like a small miracle that all this stuff can work so reliably as a total package. There are thousands of components in the fully solid-state system, hundreds of discrete transis66 Silicon Chip tors, and just a few integrated circuits. Fortunately, the total power consumed with all devices on-line is just over 200W, so heat – the enemy of long-term reliability – is at a minimum. Clearly, the original HP designers realised that component quality and assembly workmanship had to be of the highest order to ensure a high MTBF rating, and this is part of the ‘wow factor’ associated with these early behemoths. Gold plated circuit board traces and pins, beautifully loomed cables, modular construction and simple opAustralia’s electronics magazine The front panel of the HP5100A synthesiser has an array of 103 pushbuttons to select the desired frequency. At the rear, it has 46 coax cables going to the HP5110B driver. A harmonic generator takes the output of the 1MHz master oscillator and feeds them through a series of filters. These produce signals with discrete frequencies of 30-39 MHz (in steps of 1MHz), which are fed to 10 of the BNC sockets on the rear panel. Each of these signals is also divided in decade dividers to produce 3.0-3.9 MHz signals which also go to rearpanel BNC sockets. A further three signals at 1.0, 3.0, and 24.0 MHz are generated as driver outputs. All output signals are extraordinarily stable and spectrally pure. An elaborate power supply produces voltages of +6.3V DC and -12.6V DC, and is always on to supply the temperature-controlled oven of the 1MHz master oscillator and the oscillator circuitry. A DPDT toggle switch with Standby and Operate positions supplies the rest of the driver electronics. I discovered that this switch was faulty, with a defect that I hadn’t previously encountered. The switch toggle operated and felt completely normal, but only one of its poles was actually switching, thereby leaving the -12.6V supply disconnected from the instrument in both standby and operate modes. A replacement switch brought the driver back to life. With the HP5110B driver working again, the HP5100A synthesiser performed well, but the J02 HP5100B stubbornly refused to generate an output signal. This near-twin of the HP5100A is not quite as complicated, having a reduced frequency range and lower resolution. “Only” 73 push-buttons are required to cover this range and resolution. The output frequencies from the synthesiser are all derived from the single 1MHz precision oscillator in the driver, and its 23 output signals. The synthesiser outputs are produced through the processes of fresiliconchip.com.au quency addition, subtraction, multiplication and division. The array of 73 push-button switches supply -12.6V to a matrix of diode switches. Each diode switch comprises one silicon and two germanium diodes, normally biased off by 6.3V applied to the silicon device. The -12.6V overrides the 6.3V through a resistive network and turns on the germanium diodes, allowing an RF signal to pass through the matrix with low loss and negligible delay. A full description of the synthesiser is beyond the scope of this article, but through an extremely ingenious combination of mixers, multipliers and dividers, the signals from the driver can be synthesised into any frequency between 1Hz and 29.999999MHz. The synthesiser function is implemented by a low-frequency section (3.0 to 39.0MHz) and a UHF section (370 to 390MHz). The latter section is mounted on an internal sub-frame which swings outwards from the instrument, to provide access to the various modules and their interconnecting cables. All told, there are 25 modules, many of them plug-ins, and 125 germanium PNP transistors. Despite its complexity, troubleshooting the synthesiser is relatively straightforward. If a single module fails, the device produces no output signal. Locating the faulty module, and confirming it is indeed at fault, is difficult; but access is easy, and all the modules contain discrete components (1964 was before ICs had hit the market). I reflected that even the transistor had only been invented 15 years beforehand, yet the mainly germanium transistors in the instrument were capable of reliable operation up to 390MHz. That led me to the 39 to 390MHz multiplier module, which indeed was faulty. I found that a 2N2402 transistor had failed. This PNP Ge device was rated at 18V maximum between emitter, base or collector and had a transition frequency of 220MHz. The nearest equivalent in my parts bin was a 2N3906 Si device, rated at 60V and with a transition frequency of 250MHz. It seemed to be operating as a ClassC amplifier, and biasing differences between germanium and silicon devices proved to be unimportant when its replacement with the 2N3906 brought the module and the entire instrument back to life. SC siliconchip.com.au Australia’s electronics magazine January 2021  67