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AUDIO OUT AUDIO OUT L R By Jake Rothman Demolition Derby Fig.1. Every workshop needs a storeroom, but this is out of control. (Photo: Harvey Rothman) E very so often my workshop storeroom, shown in Fig.1, has to be cleared. This is true now, more than ever, since the time for ‘downsizing’ has finally arrived – the kids have left home, the price of gas is insane and I’m moving from a huge Victorian money pit (mansion) to something much more sensible (and affordable). In the new place, large pieces of equipment will have to justify their retention, rather than just being a project I might get around to one day. This meant I had to have an unsentimental look at several decades’ worth of collecting/hoarding/scavenging and get rid of a lot of equipment – the question was, how? The following, slightly unusual article describes several approaches, which many readers may find useful when they realise their own collection of electronic ephemera and white elephants has got out of hand and some/all of it needs to go. I used a combination of selling, cannibalising parts, demolition and recycling. Surveying the storeroom, it was clear that the main space hogs were four 56 Fig.2. The huge TAC Scorpion mixing desk – does it deserve to live on? mixing desks, including a couple of TAC Scorpions. These had recently been donated by the local music festival company, which had carelessly stored them in a leaking barn. The 60+kg, two-metre wide behemoths, shown in Fig.2 cost over £5000 in 1989, but are now worthless. I noticed one sold on eBay for £183 – but it was made clear that it was collection only (no surprise!). Practical Electronics | August | 2023 Fig.4. This organ is too big, but on the plus side the illuminated latching push buttons feel like I’m using a 1960s Star Trek console. £40 intact, but it generated £150 from easily postable drive units. A huge flagship Technics SX-U90 electronic Fig.3. Sadly, these old KEF speakers were broken up and I organ (see Fig.4) also resold the driver units and crossovers as spare parts. sides in the workshop. Despite almost working and having a Loudspeakers beautiful control surface lit with soft Speakers were another major space sucker. filament lamps, it has negative value. I had a graveyard of 40 pairs of assorted I’m offering it free to a collector, bemodels – what a waste of a Victorian cause it’s eating up valuable real estate. drawing room! The collection dated back What’s the solution if nobody turns to the early 1990s when I was designing up? Strip out the spring-line reverb, crossover networks for Mordaunt-Short. bucket brigade chorus delay chips and A strange thing about speakers is that the speaker drivers and dump the rest. It’s sum of their parts is often valueless, but a shame, since there is probably an the bare drivers sell for reasonably high enthusiast who’s missing a few of its prices. This is especially true for KEF unique push buttons. speakers. Their chipboard flat cabinets are considered ugly, while their Bextrene bass units and T27 tweeters are highly Recycling valued. This kind of cannibalisation had Recycling can be effective, but it only to be done to the KEF 104aB speakers works if materials are strictly separated. shown in Fig.3. It was worth (possibly) Another big problem with recycling Fig.5. Demolition can reveal useful construction techniques. Notice the thick conductor soldered on the earth pins of the mixer’s bus board. Practical Electronics | August | 2023 electronics is that the only items that can be profitably recycled are the metals. I collected all the steel and copper cabling separately and sold what I could. Old PCBs and connectors have to be removed because leaded solder can cause contamination when incinerated. (Reselling working components, however, can be more lucrative.) Unfortunately, the downside of recycling is that it encourages rampant consumerism – it really is much better to reuse or repair whenever possible. What to do? Having found solutions for the organ and speakers, it was now the turn of the Scorpion mixers. I had two similar units – but one had to be scrapped (thanks to my local Welsh damp weather). These desks were designed to be easily maintained – fully modular with solid metalwork. I naively thought one could be repaired, played with, and then sold on, but it didn’t work. Fig.6. This old DDA mixer has the elusive 2SB737 input transistors. Vinyl aficionados pay big money for these. Nothing works better with moving-coil cartridges. 57 Fig.7. The hidden cause of all the trouble that resulted in the desk being discarded: the main earthing post on the chassis was oxidised and not tightened up properly. Fig.8. Another earthing disaster – this chunky copper earth bus on an Allen and Heath mixer was effectively disconnected due to rusty screws. I upgraded to stainless steel screws / shake-proof washers. Hidden gems noise transistors are unique and no longer made. They are the best device for low source impedance applications, such as moving-coil pickup and ribbon microphone pre-amps thanks to their base spreading resistance of less than 2Ω. They were wasted in these mixers, so I took them out. A good ‘swap’ is the 2SA970, which matches well with standard microphones. Careful disassembly during scrapping can reveal useful inhouse manufacturing techniques, such as the earthing in Fig.5. There can also be ‘hidden gems’ like the 2SB737 transistors (Fig.6) used in the input stages of the DDA (Dave Dearden Associates) and Allen and Heath mixing desks. These low- Faults revealed Fig.9. Oxidised ribbon IDC connector. Most mixers are full of these. They connect all the modules to the sockets on the back and are the most unreliable component in the system. Fig.10. IDC ribbon connectors cannot withstand any force. The cable is often ripped off during servicing. 58 Finding the fault that killed a piece of equipment after you’ve demolished it is more common than is admitted. The scrapped Scorpion mixer had terrible random bangs/hums. It turned out the well-hidden main ground stud (Fig.7) was loose and corroded. The Allen and Heath mixer had corroded mild steel screws on its copper main earthing busbar. Replacing these screws with new ones made it work perfectly, as shown in Fig.8. It’s smaller, so more saleable, and it’s no use to me because I can’t get my fat fingers between the knobs. IDCs, or ‘insidious death connectors’ (more correctly known as insulation displacement connectors) work by pressing a spike into a ribbon cable, as shown in Fig.9. They are a great idea in sunny California, but a disaster in damp climates unless you use the sealing jelly that phone companies use. Sometimes the ribbons even just get torn off, as shown in Fig.10. These, along with corroded jack socket switch contacts on the insert points (Fig.11) cause the most common signal loss faults on analogue mixers. Fig.11. Jack sockets frequently oxidise. If the switch contacts are employed for passing the signal through, as in insert points, it causes loss of a channel. Practical Electronics | August | 2023 Fig.12. If you’ve chewed the screw head and can’t undo it, then pliers can help, but only if you’ve got enough access room to wiggle them about. Fig.13.(left) If all else fails, drill out the screw head. Fig.14. (right) Iron filings cause havoc with crosshead screws. Keep your screwdriver clean and it’s less likely to slip and ruin the head’s slots. Tool tricks problem with socketed chips is that they tend to work loose and corrode over time, which reveals an important failure/ repair mode. I’ve often found that just wiggling chips in their sockets is good way to get failed mixer channels working again. Old TL072 op amps have proved to be very reliable and have become expensive. All the mixing desk BiFET audio op amps were pulled and sold to a guitar pedal maker. I also found Solid State Music SSM-2015 chips (see Fig.17), which were the first decent integrated microphone pre-amp. Their designers pioneered techniques used in modern devices, such as the THAT 1570. Since the performance of the SSM-2015 is almost as good, they still demand excellent (for the seller!) prices. Most electronic physical work is concerned with construction. When dismantling something, different techniques are needed. Corrosion can make it impossible to undo crosshead screws with standard hand-held screwdrivers. Impact drivers that bang the screw round can sometimes work, but the downwards pressure and noise can be injurious to hands and ears. And the screw head often ends up being chewed and mangled. When this happens, pliers can be used to grab the screw head, as shown in Fig.12. Often the result of all the bashing about is the head shears off. If decapitation is a risk then it’s best to drill out the screw head before this happens, as shown in Fig.13. Another thing to watch out for is the iron filings that build up when dealing with old screws. When these stick to the screwdriver tip (Fig.14) the bit can jump out, further mangling the screw head. Keep pulling the filings off as you go. A good tip if you want to unsolder large items, such as the gold plated connectors in Fig.15, is to use a paint stripper gun. Remember, with demolition it doesn’t matter if you burn the PCB! No wall-warts here In the early days of electronics, many chips were put in sockets, as shown in Fig.16. So the first thing to do with old equipment is to remove these ICs since there’s a profitable market in ‘pulled chips’. It’s worth remembering that a common While the Scorpion mixers have to go, their power supplies (Fig.18) were very well made, with dual 18V rails, 10A positive and 5A negative, so I decided these were worth keeping. They also provided an output giving +48V at 1.5A for phantom power. These were solid PSUs that fitted 2U racks with massive toroidal transformers, big heatsinks and no fans. The only thing wrong was a blown 48V voltage regulator, a LM317K HV, the unobtainable high voltage TO3 version. This was replaced with a standard LM317K with added protection Zener diodes (Fig.19). Modern electronics have plastic switch-mode power supplies which only last a few years. These old linear rack-mount PSUs can be repaired forever. Fig.15. Unsoldering large multi-pinned components, such as this DIN connector, can be accomplished using a hot air gun. It wrecks the board though. Fig.16. Like most old professional audio gear, the Scorpions chips were in sockets. It was later found the sockets were less reliable than analogue small-signal chips. However, it does make for easy removal and there’s little market for unsoldered devices. Pulled chips Practical Electronics | August | 2023 59 Fig.17. The SSM2015 mic pre-amp chip. This was better than the discrete circuits at the time. It is sought after since it is used in top mic pre-amps such as the Symetrics SX202. Back to the potting shed At first sight I was dismayed to see the desks used Omeg pots (Fig.20), possibly the old version, which I don’t like. However, I was lucky; most were the current long-lasting polymer track types, which are useful. Only the gain and the sweep frequency controls were the older Radiohm versions which go scratchy. Cleaning up The mixing desks were truly disgusting with years of sticky ‘matter’ coating the front panels and knobs. The only way to clean a front panel is to pull the plastic knobs and buttons off first, undo the pot nuts, take the PCB out and wash it in the sink (Fig.21). The plastic bits can then be washed in the Fig.18. Scorpion power supplies – a solid, long-lasting, linear design. dishwasher in a stainless steel mesh tray, as shown in Fig.22. Experience has taught me that you have to use expensive Fairy Platinum dishwasher tablets, cheap cleaners won’t do the job. The difference the cleaning makes is shown in Fig.23. Repurposing A popular use of old mixer channels is to make stand-alone channel strips to be used as mic pre-amps for digital audio workstations. Many are sold mounted in a 1U rack case incorporating a power supply. I’m not very keen on this since the panel sliding pots are then sideways and the equaliser (EQ) controls go down in frequency from left to right. However, it can be profitable, with single channel strips fetching £50. Unfortunately, my Scorpion mixer channels were too long to fit in a 19 inch rack, so I had to give them a haircut and snip off the output routing end of the boards – rather brutally – as shown in Fig.24. Still, as I say above, better to be brutal and recycle rather than send to landfill. Going to the tip Once all the goodies had been removed from the failed mixer I broke up the frame in the garden (Fig.25) for scrapyard steel – it Fig.19. Using a standard LM317K regulator IC to replace the obsoleted high-voltage version. Note the extra protection components added. Fig.20. Omeg pots – a potpourri of old scratchy devices and new polymer ones. The scratchy ones were the frequency sweep controls (470kΩ anti-log). I may replace these someday, but because they are not moved much in a performance, it’s not too critical. 60 Fig.21. Washing a channel strip front panel with a lot of elbow grease! Practical Electronics | August | 2023 Fig.22. Plastic parts like knobs ‘scrub up lovely’ in the dishwasher. Fig.24. If it doesn’t fit, chop it with tin snips. I now have a stock of mic-pre EQ boards for all sorts of jobs that fit into the standard 19 inch rack case. Fig.25. Wanton destruction – if you have to use a hammer, you’ve lost the plot. Use the right sized spanner. I found the right one in my socket set… after demolition. was quite a struggle! When I mentioned this to Adam Fuest, our local music producer engineer, he said the best thing about Scorpion mixing desks was the Amek frame, which I had just destroyed. Apparently, they are so strong, people put upgraded modules in them. Oh dear! all the way back to November 1964 is to be entrusted to our editor’s archive. That’s seven cubic feet of Brighton real estate. Magazines Fig.23. Comparison of clean vs. dirty. Audio gear does not sell if it’s dirty – I learnt that from second-hand car dealers. For an old analogue person like me, most of my information is paper based. Sadly, some of this has to go as well. Luckily my collection of Practical Electronics, dating Back to mouse clicking I’ve enjoyed using the remaining big mixer with it’s padded arm rest and ease of adjustment. It’s a superior tool for composition and live group playing compared to using a laptop. It’s also fantastic for teaching mixing techniques. But, it has to go. Anyone want to make me an offer I can’t refuse? jrothman1962<at>gmail.com STEWART OF READING 17A King Street, Mortimer, near Reading, RG7 3RS Telephone: 0118 933 1111 Fax: 0118 933 2375 USED ELECTRONIC TEST EQUIPMENT Check website www.stewart-of-reading.co.uk Fluke/Philips PM3092 Oscilloscope 2+2 Channel 200MHz Delay TB, Autoset etc – £250 LAMBDA GENESYS LAMBDA GENESYS IFR 2025 IFR 2948B IFR 6843 R&S APN62 Agilent 8712ET HP8903A/B HP8757D HP3325A HP3561A HP6032A HP6622A HP6624A HP6632B HP6644A HP6654A HP8341A HP83630A HP83624A HP8484A HP8560E HP8563A HP8566B HP8662A Marconi 2022E Marconi 2024 Marconi 2030 Marconi 2023A PSU GEN100-15 100V 15A Boxed As New £400 PSU GEN50-30 50V 30A £400 Signal Generator 9kHz – 2.51GHz Opt 04/11 £900 Communication Service Monitor Opts 03/25 Avionics POA Microwave Systems Analyser 10MHz – 20GHz POA Syn Function Generator 1Hz – 260kHz £295 RF Network Analyser 300kHz – 1300MHz POA Audio Analyser £750 – £950 Scaler Network Analyser POA Synthesised Function Generator £195 Dynamic Signal Analyser £650 PSU 0-60V 0-50A 1000W £750 PSU 0-20V 4A Twice or 0-50V 2A Twice £350 PSU 4 Outputs £400 PSU 0-20V 0-5A £195 PSU 0-60V 3.5A £400 PSU 0-60V 0-9A £500 Synthesised Sweep Generator 10MHz – 20GHz £2,000 Synthesised Sweeper 10MHz – 26.5 GHz POA Synthesised Sweeper 2 – 20GHz POA Power Sensor 0.01-18GHz 3nW-10µW £75 Spectrum Analyser Synthesised 30Hz – 2.9GHz £1,750 Spectrum Analyser Synthesised 9kHz – 22GHz £2,250 Spectrum Analsyer 100Hz – 22GHz £1,200 RF Generator 10kHz – 1280MHz £750 Synthesised AM/FM Signal Generator 10kHz – 1.01GHz £325 Synthesised Signal Generator 9kHz – 2.4GHz £800 Synthesised Signal Generator 10kHz – 1.35GHz £750 Signal Generator 9kHz – 1.2GHz £700 HP/Agilent HP 34401A Digital Multimeter 6½ Digit £325 – £375 HP 54600B Oscilloscope Analogue/Digital Dual Trace 100MHz Only £75, with accessories £125 (ALL PRICES PLUS CARRIAGE & VAT) Please check availability before ordering or calling in HP33120A HP53131A HP53131A Audio Precision Datron 4708 Druck DPI 515 Datron 1081 ENI 325LA Keithley 228 Time 9818 Practical Electronics | August | 2023 Marconi 2305 Marconi 2440 Marconi 2945/A/B Marconi 2955 Marconi 2955A Marconi 2955B Marconi 6200 Marconi 6200A Marconi 6200B Marconi 6960B Tektronix TDS3052B Tektronix TDS3032 Tektronix TDS3012 Tektronix 2430A Tektronix 2465B Farnell AP60/50 Farnell XA35/2T Farnell AP100-90 Farnell LF1 Racal 1991 Racal 2101 Racal 9300 Racal 9300B Solartron 7150/PLUS Solatron 1253 Solartron SI 1255 Tasakago TM035-2 Thurlby PL320QMD Thurlby TG210 Modulation Meter £250 Counter 20GHz £295 Communications Test Set Various Options POA Radio Communications Test Set £595 Radio Communications Test Set £725 Radio Communications Test Set £800 Microwave Test Set £1,500 Microwave Test Set 10MHz – 20GHz £1,950 Microwave Test Set £2,300 Power Meter with 6910 sensor £295 Oscilloscope 500MHz 2.5GS/s £1,250 Oscilloscope 300MHz 2.5GS/s £995 Oscilloscope 2 Channel 100MHz 1.25GS/s £450 Oscilloscope Dual Trace 150MHz 100MS/s £350 Oscilloscope 4 Channel 400MHz £600 PSU 0-60V 0-50A 1kW Switch Mode £300 PSU 0-35V 0-2A Twice Digital £75 Power Supply 100V 90A £900 Sine/Sq Oscillator 10Hz – 1MHz £45 Counter/Timer 160MHz 9 Digit £150 Counter 20GHz LED £295 True RMS Millivoltmeter 5Hz – 20MHz etc £45 As 9300 £75 6½ Digit DMM True RMS IEEE £65/£75 Gain Phase Analyser 1mHz – 20kHz £600 HF Frequency Response Analyser POA PSU 0-35V 0-2A 2 Meters £30 PSU 0-30V 0-2A Twice £160 – £200 Function Generator 0.002-2MHz TTL etc Kenwood Badged £65 Function Generator 100 microHz – 15MHz Universal Counter 3GHz Boxed unused Universal Counter 225MHz SYS2712 Audio Analyser – in original box Autocal Multifunction Standard Pressure Calibrator/Controller Autocal Standards Multimeter RF Power Amplifier 250kHz – 150MHz 25W 50dB Voltage/Current Source DC Current & Voltage Calibrator £350 £600 £350 POA POA £400 POA POA POA POA Marconi 2955B Radio Communications Test Set – £800 61