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SERVICEMAN’S LOG Getting amped up Dave Thompson Dave gives a bit of background on the various guitar amplifiers he built or bought, using the technology of the day. They evolved from valve-based designs, initially made from modified radiograms, through to solid-state hybrid and discrete amplifiers. All so that he could rock’n’roll! Years ago, when I was a little ‘un (or wee tacker, as you Australians might say), I got one of those 10-in-1 electronic sets from Dad as a present for Christmas. I loved it, and it became my favourite ‘toy’. It only had one transistor, a germanium diode (yes, I’m that old), a ferrite broadcast coil, a variable capacitor, a battery holder, a small speaker and a few resistors and capacitors. Still, I could eventually make more than the nominal 10 projects they published in the manual that came with it. These days, you can buy the same sort of thing from the local electronics shop, with 200 or more ‘projects’, but it was pretty rare back in the late 1960s. I believe that Dad purchased it in Australia, on one of his frequent trips to Melbourne to see my Uncle Roger (not the infamous orangeshirt-wearing cook/comedian, before anyone asks!). I loved that kit, but outgrew it relatively quickly. One of my favourite projects (besides the crystal radio, obviously) was the amplifier. It was nothing too fancy, but I could use it to amplify the audio from another crystal set I’d made the traditional (for that time) way. I used a multi-tapped coil wound on a cardboard toilet roll centre, with a germanium diode as a detector. The signal from that was puny, of course, although Dad had sourced me a pair of high-impedance crystal headphones (which I still have somewhere). That made a huge difference over the standard crystal earbud of the day. You know the earphone, cream-coloured case, clear plastic earpiece and a twisted-pair cord. It was precisely 84 Silicon Chip the type of thing Uncle Arthur had for his hearing aid. Fun fact: those earphones are still available from your local electronics store! So, with my crude crystal set and a substantial long-wire antenna that Dad helped me set up, I could get some quite far-off stations, but the audio was weak. Using my 10-in-1 kit set up as an amplifier (connected up using wire into the kit’s springs), I could now drive the small speaker in the kit and free myself of the clunky (and, to be honest, quite uncomfortable) Bakelite headphones. I often listened into the night with that setup, although I eventually built a much more advanced shortwave radio from one magazine project or another. I did use better mid1970s headphones with that, so I could listen late at night without disturbing anyone. I would regularly ‘skip’ the likes of the BBC World Service and Radio Luxembourg, which at that time had some excellent radio shows syndicated from around the world. Good memories. Learning to wield the axe While this was all going on, I was learning to play the guitar. I’d played the piano by this time for about six years, but guitar was what I really wanted to learn. I bought a rather dire electric example from a schoolmate and set about teaching myself. The first thing I discovered was that the sound it made was literally nothing, and I needed an amplifier. Boy, did that open up a rabbit hole of discovery and expense! As anyone who has ever bought a guitar amplifier will tell you, the choices are seemingly endless, and some manufacturers expected your pockets to be almost bottomless! The irony is that if I’d bought one at the time, it would now be ‘vintage’ and worth an absolute fortune. However, as a budding serviceman, I had to fashion my own somehow. I had seen a few related articles in some of the American magazines I was buying at the time about how people were modifying valve (vacuum tube) radiograms or lo-fi amplifiers to use as guitar amps. The main differences were the input impedances of the preamp and overall gain of the input stages; you needed enough gain to get some of that famous valve ‘crunch’. This was good news for a now-broke high school student. Home stereo systems were rapidly relegating radiograms and similar older console and mantle radios to the scrap heap. I recall often seeing them sitting on the roadside, offered for free. Australia's electronics magazine siliconchip.com.au Items Covered This Month • • • Home-made guitar amplifiers Repairing an Icom 551-D transceiver Fixing a car head unit Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Cartoonist – Louis Decrevel Website: loueee.com Knowing what we know now, of course, it would be amazing to be able to pick up those antiques and use or restore them – which, let’s be honest, has since become a huge part of this hobby of ours. However, back then, they were big, timber, cumbersome lumps and simply not wanted. New-fangled solid-state hifi amplifiers and separate component systems soon superseded those in every household I visited (including ours). It was good that technology progressed so rapidly in the 1970s; that meant I had my pick of old valve amplifiers. A friend gave me the amp from his parent’s radiogram because he wanted to put a transistor amp into the case. That suited me – I’d be horrified with myself now if I had dumped some beautiful walnut cabinet. The antique radio people would blacklist me! My first guitar amp But suddenly, I had this lump of an amp. My experience with valves was watching Dad maintaining the blackand-white TV he’d built us in the 1960s. I still remember him showing me a nice fat arc when he held his Earthed screwdriver close to the flyback output. I was mightily impressed, I can tell you! Of course, he warned me never to try it myself. I’ll leave it to your imagination as to whether I followed that advice! So, this dusty old valve amp had several inputs that were no longer wired in, and I also made sure I got the speaker, which had an output transformer mounted directly onto the basket. First I ensured it was dust-free, then plugged it in and switched it on. This, of course, is against standard practice for valve amps that haven’t been used in a while. Still, I knew no different then, and I watched all the filaments glow, and a quiet hum came from the speaker. I did know enough to be too scared to go anywhere near it while it was running! I shut it down and looked at the inputs. I can’t recall if they were labelled or not, so I made up a lead with a guitar plug on one end and the older RCA-style connector this amp used on the other. I plugged the lead into the guitar and tried each of the inputs, but of course, it was either weedy and thin or grossly distorted, and not in the nice way we guitar players love. At the time, I knew much less about amplifiers than I do now; even the rare guides I discovered in magazines were vague or purposely omitted values and figures. I could see this wouldn’t fly, so I passed the whole thing on to a school chum who thought he’d like to play around with it. That left me with no amp (and, full disclosure, no real talent either at that point!). siliconchip.com.au I started looking more seriously into buying a commercial amplifier, but that was simply out of the question. I started asking around – these are the sorts of things people buy and then give up on, so some could be quite cheap. A neighbour down the street had just that: a Christchurch-­ made Abby 30, a locally produced clone of the famous Vox AC30. The guy who made them eventually moved to Melbourne, but he made a few of these and other styles of guitar amps back in the day. I wish I’d kept it, as they are now regarded as one of the best copies made, and likely worth a small fortune due to their rarity. I used it for my formative years though, so I got plenty of use out of it. When it came to touring, however, those valve amps became a real liability. The cabinet was solid timber with two heavy-duty 12-inch (30cm) Celestion speakers sitting in it and a solidly-made steel chassis sandwiched into the top; that thing soon broke my rock and roll spirit, not to mention my back! Making it more luggable What I needed was a solid-state ‘head unit’ and a single-­ speaker cabinet for playing smaller clubs and bars. Something a lot easier to lug around, that didn’t take up so much room in the small cars we had at the time (nothing like today’s monster SUVs!). By then, I had done a lot more research and read many more magazines, so I thought I could easily make one. The first one I made utilised a Sanken Hybrid SI-1050G 50W power module I had purchased a year previously. I’d been intending to make a small foldback amp/cabinet, but decided instead to use it as the power section of a guitar amp. The preamp I used was part of a project (if I recall correctly) in one of the English magazines of the day, perhaps Practical Electronics or Everyday Electronics. The power supply was part of yet another guitar amplifier project that was similar to what the Sanken module required to run it. I could have used either a ±33V split rail or a 66V single supply; I chose the latter, mainly because I had a nice beefy power transformer waiting for such a project. I had access to a transparency printer and made and etched my own PCBs, so it all ended up quite good-­looking, and I was pleased with the result. Australia's electronics magazine January 2024  85 At 50W, it seemed underwhelming compared to other 50W guitar amps I had used, so I played around a little with the gain of the preamp and got it running a bit hotter. Still, it was not cutting through the chaff on stage. Overall, a bit disappointing, then. The next build fared a little better. This time, I decided to add 50 more watts, doubling the apparent power but not the output sound level, giving little more than a nominal 3dB gain. However, it gave more punch and more headroom to play with. This module was another locally-produced product, and a kit to boot. It was a common-for-the-time push-pull power amplifier design using the perennially popular 2N3055 NPN and 2N2955 PNP general-purpose power transistors. Many well-liked guitar and hifi amplifiers utilised those robust and easy-to-source components. I used the same preamp, but as this module required a split power supply, I had to redo my 66V single supply, which was also burgled from the Sanken amp. Fortunately, the case was large enough to accommodate the new module and, with a bit of fettling, I soon had a dual-voltage power supply running at around 32V per side. That was just outside the module specs, but allowing for sag and other factors, I figured it should be OK. In use, this was a solid and reliable amplifier that really had some punch, especially after I paired it up with a tone booster, a gain/overdrive pedal and a better speaker (an Eminence driver) in the single 12-inch cabinet. I used it for many years on the circuit (pun intended!) before it was fried one night when someone hit a power pole up the road and killed the whole club’s power. I knew I had blown something, and by that time, I was in a better position to buy a commercial amplifier, which is what I did. Valve amp parts are still available I still have a love for audio amps, though, and have serviced and repaired many vastly different varieties over the years. In recent years, I’ve also re-embraced the valve amplifier scene, learning about them by building and making my own. The transformers and valves, which were always a headache for the average Joe like me to source, are now available, 86 Silicon Chip both here and overseas. While not always cheap, at least we can buy them. I also had the good fortune to stumble upon a commercial transformer winding machine that had served a local company here for decades. It sat in a friend’s garage for many more years until he passed it on to me for a peppercorn fee, and I have since tidied it up and wound several transformers with it. The different-sized cores and wire are also available (if not readily), so I’m lucky to be able to wind transformers to my own requirements and specifications. There are also many good physical schools and online classes dealing with designing and building valve amps (including an excellent one in Australia). If anyone is interested, many good books and tutorial videos are also available on the subject. Like all amplifier theory and technologies, there is often heated debate about what constitutes a good design, or even a great design. The old timers could get it right, but they also had access to high-quality, inexpensive valves and very clever people who ate, lived and breathed valve amplifier design. These days, there are still a lot of clever people about, and even those experimenting with operating valves in low-voltage applications, down to 12V, which is fantastic for the likes of stomp-boxes and portable guitar amplifiers. Editor’s note: see our ‘Nutube’ Valve Preamplifier (January 2020; siliconchip.au/Article/12217) that runs from just 9V DC! While we don’t have unlimited access to those vast quantities of valves anymore, there are still a lot of NOS (new old stock) valves available at ever-increasing prices. NOS refers to parts that have been sitting around forever, but that have never been used. As those supplies dwindle, the prices will continue to rise. While Russian and Chinese-made valves are still being manufactured, modern valve aficionados claim the quality of their valves is nowhere near as good as it was back in the days of General Electric, Philips, Sylvania and others. Sanctions are also causing supply problems for Russian-­ made valves... To take things even further, the advent of the computer and amplifier ‘modelling’ technology means that just about every ‘tone’ some legendary guitar player has come up with is now available as a patch or preset in a hardware modelling amp, selectable at the touch of a button and able to be used live at gigs, just like my own (rather crude) home-made amps. Some modellers are even built into the guitar itself! For the home recording artist, virtual instruments can be loaded into a DAW (Digital Audio Workstation), and the variety of sounds and tones available is almost limitless. Anyone with a halfway decent computer and a set of studio monitor speakers or headphones can download a free DAW and have a home studio almost more potent than many I spent time in during the ‘80s and ‘90s. It’s a whole different world in audio amplifier design and implementation now. Yet the basics remain the same – taking a signal and boosting it through several stages with minimal unwanted distortion at the output. The distortion figures on some of the amplifier designs in this magazine would have been impossible 20 years ago. I’d hate to think what that 10-in-1 amp’s figures were, all those years ago! Australia's electronics magazine siliconchip.com.au Icom 551-D transceiver repair C. K., of Mooroolbark, Vic was asked if he could repair a 50MHz all-mode transceiver, the Icom 551D, dating from the 1980s. It’s an elaborate device and capable of more than 80W output with SSB or FM and 40W with AM. I answered rather foolishly, “Yes, I can repair anything”. Little did I know what I was letting myself in for! The owner had bought it on eBay in a non-functional state. He did try to work on it, which resulted in smoke coming out. Not a very promising start. It looked like a fairly clean unit and powered up to show a display, but nothing came out of the inbuilt speaker. Also, while it showed a sensible frequency on the dial, the tuning knob did nothing. Removing a few screws allowed me to take off the top cover, revealing a large single-sided circuit board with numerous components and a rats’ nest of wires heading off to several connectors. Fortunately, I could download a full maintenance manual. After getting familiar with the various parts, I printed out the schematics on A3 paper so they were readable. All the schematics were hand-drawn; unfortunately, very few components were labelled. The diagrams of the circuit boards had all the components numbered, so together with the parts list, I could eventually identify them. I started by checking voltages. Three TO-220 NPN transistors on the main board: Q28, Q29 and Q30, in association with Q31 and Q32 plus discrete components, provide regulated supply rails. The collectors are fed by 4.7W 1/4W resistors from the 13.8V supply, which should give about 13.5V. Q28 and Q30 gave correct readings but the collector of Q29 measured about 6V (see the diagram below). On closer examination, the collector resistor looked very burnt. That must have been where the smoke came from. With the type of construction prevalent at the time, all the resistors are standing up on the single-sided board, so only one end is accessible. Replacing a resistor requires taking the board out to get at the underside. While the maintenance manual gave detailed instructions on removing the front panel, it gave no clues as to how the main board should be removed. So it was up to me to locate the many screws to be undone and all the connectors that had to be carefully unplugged. Not only were screws holding down the PCB, but two power transistors on one side were attached to heatsinks bolted to the side of the case. All the associated screws had to be removed, and I had to be careful not to damage the insulating washers between the transistors and heatsinks. Finally, after considerable time, I could ease the board out and get to the underside. If the top was a rats’ nest, the bottom was much worse! Look at all those extra components tacked on, including a 16-pin chip on a little subboard. I don’t know if these were some kind of modifications or were needed to fix design problems, but to my eye, it looked like the epitome of bad design. Editor’s note: they seem like the sort of ‘running changes’ made in a factory when they already have thousands of boards made and find that a problem needs to be addressed or an extra function included. Having located the burnt 4.7W resistor and not having that value, I replaced it with two 10W 1/4W resistors in parallel. Left: these transistors regulate the voltage rails on the main board. Below: the shaft encoder circuitry. siliconchip.com.au Australia's electronics magazine January 2024  87 After careful reassembly, I turned on the power and checked that the output on the emitter of Q29 was 9V. When I turned the volume up, hiss came out of the loudspeaker. Attaching a signal generator gave a good signalto-noise ratio with an input level well below 1μV. However, the tuned frequency was well off the indicated frequency on the dial. There is a calibration procedure in the manual that should correct this. The phase-locked loop (PLL) module has a 10.24MHz master crystal oscillator from which the VFO frequencies are generated. A trimmer capacitor on this oscillator sets the exact frequency. However, on measuring this frequency at the test point specified in the manual, I found it was too low and, even with the trimmer capacitor at its minimum value, was a long way off. The crystal had obviously aged and dropped in frequency. I decided to ignore that for now, not having a crystal with a suitable frequency in my collection. The next major problem was the tuning knob. Accessing that meant removing the front panel assembly by undoing numerous screws and carefully unplugging the many connectors. Attached to the tuning shaft is a disc with slots around the circumference. On one side are two phototransistors, and on the other side, mounted on a small subboard, are two LEDs. These are an early SMD type of LED, TLR121, made by Toshiba. They have a clear lens and provide a point source at 700nm, a red wavelength. The LEDs and phototransistors are positioned so that the outputs on the collectors are 90° out of phase, meaning the rotation direction can be ascertained by the logic. No light was coming out of either LED and, on removing the sub-board, they both measured close to a dead short. Not having any LEDs of the same size, I jury-rigged two 3mm red LEDs. It was an ugly workaround, but on reassembly, I was surprised to find that it worked; rotating the knob changed the frequency smoothly in 100Hz and 1kHz steps. Not being too happy with the long-term stability of such an arrangement, I ordered some M3216/1206-size SMD LEDs with clear lenses for a more permanent fix. They arrived a week later. Barely large enough to straddle the hole in the PCB, it was a fiddly job to fit them and I had to use solder blobs to affix them. Unfortunately, on reassembly, the tuning knob was not working correctly. It turned out that the brightness of the LEDs was insufficient to saturate the phototransistors. Reducing the LED series resistor from 560W to 330W fixed the problem. Why both LEDs had failed in such a manner was a mystery. When I mentioned the problem to another contributor to the magazine, Andrew Woodfield, he knew about this problem. Apparently, those LEDs were used in many instruments and were notorious for their failure rate. Warranty failures due to those LEDs clogged the workshop. As for the 10.24MHz crystal, local suppliers did not stock such a value, but I could get them online from the likes of AliExpress as long as I ordered a batch of ten and waited for weeks. However, an associate of mine came to the rescue and gave me a suitable crystal. Having replaced that, I could now tune it to the correct frequency. What about the transmitter? My power supply can only provide 3A at 13.8V. Also, my 50W dummy load is rated at only 15W. I plugged in the microphone, pressed the switch and whistled. This pinned the 3A supply meter, the dummy load got warm, and a pickup loop on the scope showed a clean sinewave. This gave me confidence that the transmitter part was working. Fortunately, the unit’s owner lent me a large power supply and dummy load, enabling me to check that full power was available. Repairing a car head unit These photographs show the Icom 551-D transceiver at various stages of repair. 88 Silicon Chip S. G., of Bracknell, Tasmania found that when you can’t get replacements any more, you need to have a go Australia's electronics magazine siliconchip.com.au at fixing the fault, even if it’s a bit outside of your comfort zone... One of my radio club members was selling off one of his amateur radios, a little Any Tone AT5888. This dual-band radio covered the 2m and 70cm bands, with a power output of 50W on the 2m band and 40W on the 70cm band. It was in very good nick. The price was right, so I soon struck a deal to buy it. This radio was to go into my car; one of the things I liked about it is that it had a remote head. The whole front of the radio can be removed, and one can use a Cat 5 cable to interconnect the main part of the radio and the head unit. This meant I could mount the radio in the boot and the head unit under the dash of my Ford Falcon. The antenna was mounted off the side of the boot. The whole system worked well for around 12 months until I had a car accident, and my car was written off (I was OK; it was just my pride that was hurt). It took me around a week to remove the radio; I had to go to the storage yard to remove all my belongings. I soon had a new car, a 2017 Hyundai Elantra. Mounting the main radio was not difficult, but running the power, Cat 5 cable and external speaker wiring was more challenging. I had to remove several plastic trim panels to run the cable. I ended up not mounting the head unit, as the only spot was just forward of the gear lever and under the air conditioning controls. I gave that job to the local car radio installation company as I did not want to break the plastic trim. This took the company three hours, and I paid the going price at the time, but I got what I wanted. Several weeks later, I noticed that some of the digits in the display were missing, making it hard to understand the letters. The only thing for it was to try to get a replacement head unit. This was a total waste of time; after contacting several retailers in Australia and overseas, I came up blank. It seemed like I would have to put up with the faulty LCD screen. A few months later, I thought to carefully push on the display while the radio was working, and some of the letters returned, only to go missing again once I took my finger off it. I realised the LCD’s edge connector might have gone out of alignment. I figured that taking it apart to check would be unlikely to make the situation any worse. I removed the head unit from the car mounting bracket and took it to my workshop. I soon had the unit apart and carefully removed the LCD glass and its rubber membrane. I ended up cleaning the surface edge area of the membrane with a little bit of contact cleaner, and did the same to the contact edge of the LCD glass. Putting it back together was not that hard; I just had to take my time. The actual LCD glass sits in a tin bracket that also holds in the rubber membrane that interconnects the LCD glass to the circuit board. The tin bracket has four little tabs that allow the alignment between the display and the circuit board to be adjusted, and it can also put a bit of tension on the display. Putting everything back together and reinstalling the remote head unit to the dash bracket, it was working again. The display had no more missing letters or numbers. That was nearly nine months ago, and the display has not faltered yet. SC siliconchip.com.au Australia's electronics magazine January 2024  89