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Practically Speaking Hands-on techniques for turning ideas into projects – by Jake Rothman Dissecting devices – a photographic feast T his month, the Practically Fig.1. AF118 transistor internals from a Leak Stereo 30 amplifier – the central rectangle is the mounting assembly for the germanium transistor (6×4mm). Speaking column is a little different – it is still very much about the practical side of electronics, but I’m going to concentrate on a frequently overlooked side to electronic construction and repair work – using your eyes. Back in 1971, aged nine and living in Fallowfield, Manchester, there was some waste ground where we used to play. Next to it was a wall, over which the local TV shop used to throw unwanted electronics! We enjoyed smashing valves, imploding cathode-ray tubes and bashing magnets off speakers with bricks. We were happy little vandals. Eventually, though, I developed more methodical disassembly methods. I learnt a lot about how electronic equipment and components were made, which in turn helped with physics at school. I still love taking things to bits – just like Dave Jones, who on his great Australian blog (www.eevblog.com) says, ‘Don’t turn it on, take it apart’. Here I’ll show how looking inside components can reveal interesting operations, constructions and failure modes, as well as being a great electronics education. Fig.4. A NASA electron micrograph of tin whiskers in an AF114. Image credit: https://go.nasa.gov/3wRXpwg Tin whiskers Fig.2. View looking into the can cut off from the AF118 in Fig.1. Tin whiskers can be seen growing from the lower left-hand inner edge (see also Fig.3). The white filling is a mixture of silicone grease and aluminium oxide to conduct heat away from the junction.* Fig.3. A close-up of the whiskers in Fig.3. The long one is 2mm.* 56 Following on from my last few Practically Speaking columns, where I restored a Leak Stereo 30 amplifier, I mentioned about the failure of germanium AF11X series transistors due to tin whiskers growing from the case inside. I like to see things in physical form for myself, so I decided to get out the hacksaw and cut open the failed AF118, shown in Fig.1. Dr Joe Botting, a palaeontologist next door, has a high-resolution microscope and he took some fascinating photographs of the decanned device. The transistor was indeed riddled with tin whiskers, as shown in Fig.2 and Fig.3. Fig.4 shows the result of using an electron microscope on an AF114 from NASA’s website. Fig.5 shows an AF117, commonly used in British and European radios from the 1960s. The individual device shown here was a later, more reliable version. The semiconductor junction had been specially protected with clear epoxy resin and the plating changed to tin-plus-lead, Fig.5. The internal structure of an AF117 transistor – the assembly is mechanically protected with clear epoxy resin.* Fig.6. ‘Unlimited’ currents can even burn through the steel case of a MOSFET. inhibiting whisker growth. Eventually, the manufacturers changed the packaging to the familiar TO18, such as the AF124. Practical Electronics | September | 2021 Fig.7. To open metal-can devices cut around with a hack saw. Don’t cut in too far to avoid damaging the bonding wires. Fig.8. Squeezing the metal tab of a plastic power device in a vice can remove the top epoxy revealing the die bonding site, and allowing its size to be measured. Fig.10. A Motorola MJ2955 (PNP complement of the 2N3055). This device has suffered excess current due to shorting. Note the fused emitter bonding wire towards the top. devices, a hacksaw and vice work well, as shown in Fig.7. Plastic-cased devices are less yielding of their internal secrets and have to be squeezed ‘till t’top pops off’, as illustrated in Fig.8. A hammer and chisel can also work. Fig.9 shows a plastic power transistor where the chip is still intact. The serious under-the-bonnet silicon detectives over at zeptobars.com boil devices in concentrated nitric acid, sulphuric acid and oleum (disulphuric acid) – see: https://bit.ly/pe-sep21-inside1 https://bit.ly/pe-sep21-inside2 Do not attempt anything with acid unless you really know what you’re doing – it’s dangerous, seriously dangerous. Even if you just want to crack open a device in a vice then eye protection is needed, since sharp bits can ping off sudenly. Fig.11. Top taken off a metal-can NE5534, the ever-popular audio op amp. * Fig.12. Mystery box of chips. What is a U5B YY0939X. It was not until I identified it that I realised that the ‘Y’s were an Italian way of writing ‘7’. Up the Junction Opening up power transistors can sometimes reveal the destruction of junctions, such as Fig.10, which shows a blown 2N2955 transistor from a NAD 3020 amplifier. Fig.9. Usually, the chip is cracked apart using the vice technique, but with some transistors, such as this Sanken device, there is a resilient compound applied above the chip allowing a clean view. Here the multi-fingered emitter of a good quality device is revealed. Visual inspection Until recently, electronics was a very visual skill and a simple visual inspection of components could reveal a lot. Fig.6 shows a MOSFET from a Lee Lighting controller used in film production. It had latched hard-on, passing a few hundred amps thanks to a brass spacer that had (very foolishly) been inserted in the fuse socket. Here, the thermal damage is pretty obvious, but often a device has to be cut open to reveal the fault. With metal-cased Practical Electronics | September | 2021 Can opener Fig.13. Internal die shot of the mystery chip from the parcel in Fig.12. Old TO18, TO5 and TO99 cans are easily opened by carefully cutting around the edge with a hacksaw or Dremel powertool. Fig.11 shows the audio engineer’s favourite op amp, the NE5534 in a metalcan package. It’s worth remembering that cutting the tops off devices is not always pure destruction – topless old BC108 transistors make useful phototransistors. Mystery Chips I recently bought a box of mystery chips for a very low price (basically the gold scrap value) because no one could identify them (Fig.12). Now I’m pretty face-blind, but like many engineers I don’t forget an interesting line pattern or number. When I photographed the mystery chip, I felt I had seen the mask Fig.14. Notice the similarity? Page 119 of the March 1966 issue of Wireless World revealed that my mystery devices were just old µA709 op amps; sadly no good for audio these days thanks to their horrid crossover distortion. Maybe Bletchley Park computer museum would like them? 57 pattern (Fig.13) before. I was sure I remembered something like it in an old Wireless World magazine which I had been reading while researching the Leak Stereo 30. I went back and there it was! in the 1966 issue shown in Fig.14. Despite the cryptic in-house code number which couldn’t be Googled, it was just a boring µA709 op amp. I’ve now got 140, Fig.15. Plessey chip mystery – still unsolved.* It must be a special. Can anyone identify it? for the AO Shop (see page 52 in August 2021 PE). I’ve also got a mystery Plessey chip shown in Fig.15. Any ideas? Spam can Electrolytic capacitors suffer all sorts of problems that are often revealed when disassembled. Opening them up requires a special technique with side cutters, illustrated in Fig.16. The capacitors shown were suffering hydrogen gas buildup in the case, causing swelling. This often happens with low-ESR types when they are not used frequently enough. In this case, there was a definite hiss as the rubber bung seal was broken. There was also a bit of spray. (Safety note: Use eye protection and wash hands, since the high temp ( 125°C) types can contain DMF (di-methyl formamide) a potentially carcinogenic solvent and wet tantalum types use weak sulphuric acid). Once there is a strip of the can to get hold off, long-nosed pliers can be used to wind it back (Fig.17) like old cans of Spam. Now the contents of the capacitor can then be pulled out by the leads. Fig.18 shows the foil/paper winding which has a healthy impregnation of electrolyte. Unwinding the capacitor, shown in Fig.19, revealed no problems, so the cause of the gassing remains a mystery, (possibly too much water in the electrolyte?). Needless to say, I put that capacitor stock in the bin. A piece of foil from another capacitor suffering internal pressure and electrolyte leakage is shown in Fig.20. Breakdown of the dielectric film has taken place. Foiled again Fig.16. To open up an electrolytic, start cutting around the edge of the rubber seal. These were quite expensive Samwha devices rated at 130ºC which suffered distended tops. Fig.17. Once you have something to get hold of, then the rest of the can be ‘unwound’. 58 Continuing our capacitor demolition frenzy, I’ll attack an innocent polyester capacitor. I always get my first-year students to do this. I think if you can visualise the physical structure of a component, its function is more readily understood. Old fashioned axial film/ Fig.18. The wet guts revealed. The amount of electrolyte was healthy with no signs of being dried up. The composition of the electrolyte was suspect; maybe not enough corrosion inhibitors? Fig.19. Unwinding the capacitor element (snip the tape holding it together first) shows the typical construction of an electrolytic capacitor: absorbent separator paper and grey etched anodised aluminium foil. foil types, such as surplus SRC types are ideal. First, the capacitor has to be crushed with pliers to get the coating off, as shown in Fig.21. Then the capacitor Fig.20. Foil from another expensive high-temperature electrolytic capacitor (Novea Secorel 125). Here, the oxide layer has decayed, some parts becoming almost bare metal. A shame because it had welded lead attachments, normally a sign of quality. Practical Electronics | September | 2021 Fig.21. First stage in disassembling a polyester foil capacitor; crush the coating off with pliers. to the case and melting the solder seal while pulling the glass sealed lead-out. This works well for metal-cased solid tantalum capacitors. It’s one of the few times when those awful pistol-grip solder guns are useful in electronics. Often, the component can be fitted into the heating loop and the trigger squeezed. Fig.25 shows the slug from a 22µF 50V tantalum capacitor used in the microphone preamp. The ‘spongy’ texture of the sintered tantalum particles provides a massive surface area for the capacitor. Inner secrets Sometimes surprises lurk inside. I was disappointed to see that the expensive non-polarised tantalum capacitor in Fig.26 was just composed of two ordinary capacitors connected back-to-back. Resistance is futile Fig.22. Unwinding a polyester capacitor. Note the interleaved foils and plastic film. can be unwound into its layers of plastic film and aluminium foil, as in Fig.22. This illustrates the massive surface area involved, even for a low-value capacitor. Fig.23 shows the effect of a pin hole in a metallised film capacitor. The metal burns away isolating the fault. Tantalum devices A problem with old silver-cased wet tantalum capacitors is shorts caused by silver deposits, as shown in Fig.24. These deposits can occur through very long storage times or reverse polarisation. Because of this problem, these capacitors have now been superseded by tantalumcased types. Opening up hermetic devices is often best done by applying a big soldering iron Carbon-composition resistors are often described as being a solid block of resistive material giving very low inductance and high peak-power handling capability. I used a bench grinder to show this construction in Fig.27. Conversely, film resistors have high inductance and poor peak-voltage rating due to their thin coated spiralled track, but they do offer high stability and low noise. The best way to see the construction is to scrape off the coating, as shown in Fig.28. This is something I think all first-year electronic students should do. Fig.24. A failure mode peculiar to silvercased wet tantalum capacitors are silver growths, sometimes called ‘Christmas trees’ (the whitish blob on the left-hand side). These potential short circuits are caused by reverse polarisation or excessively long storage. Notice the oxide film on the tantalum slug has imparted a blue colour. This indicates the thickness of the film (for, in this case a 10V unit). Potentiometers These are very easy to take apart by prising open the folded metal tabs on the front. Again, I make all my students do this because internally its function is visually obvious. I’ve always aimed to reduce abstraction to a minimum in electronics tuition. An inspection of the track on a faulty pot can be very revealing. The worn carbon track from the volume control of a Leak Stereo 30 amplifier is shown in Fig.29. No amount of Servisol contact cleaner spray is ever going to fix that – it has to be replaced. Fig.25. Sponge-like solid-tantalum slug. Here, the sintered slug is black because of the manganese dioxide solid electrolyte.* Fig.23. In metallised film capacitors, self-healing of short circuits can take place. Here the metal has been vaporised from around a pinhole. Practical Electronics | September | 2021 Fig.26. Disassembling a metal-cased non-polarised tantalum capacitor revealed it to be just two normal capacitors connected back-to-back. 59 Fig.28. A common fault with Painton resistors, one of the wires has fallen off. A bit of judicious scraping with a scalpel has revealed the spiral carbon film track. Fig.27. 1.2kΩ 2W Allen Bradley carbon composition resistors; one ground in half to show the inside construction. 24 of these resistors in parallel make an excellent 50Ω RF dummy load because of the low inductance. Fig.29. A volume control track so worn the carbon track has disappeared in places. Fake news Visual inspection is important for identifying fakes, sometimes simply revealed in the device label printing with font errors or even bad spelling. However, it can be more dramatic, such as the fake iPhone charger which blew its top across a room (see Fig.30). Expensive audio semiconductors are often faked, especially Toshiba devices, such as the 2SC5200, shown in Fig.8. The usual clue is that the die is much smaller than the original. Good audio power transistor dies are normally at least 4×4mm. Another common fake is the 2SK170 lownoise FET. eBay Hong Kong suppliers are the usual culprits, shown being checked in Fig.31. I’ve had similar issues with 2SA970s. The fake device is shown in Fig.32 and the real deal in Fig.33. Notice the different print styles. When I’ve sent photos and test results back to suppliers, I’ve always got suspiciously instant refunds along with pleading not to leave negative feedback. I tell them I’m always applying negative feedback in my work. Voltage in vitro Older components were often packaged in glass packages; after all, it was the Fig.30. This fake iPhone charger nearly burnt down the house. 60 dominant encapsulation technology in the valve days of the 1960s. Because you can see inside, these glass components are great educational props, as well as objects of beauty. The triode valve shown in Fig.34 is a gem, with all the electrodes visible. I can’t believe us kids used to throw them against the wall back in 1971. Old germanium transistors were often supplied in glass SO2 encapsulations. There was a photosensitive type, the OCP70, shown in Fig.35, where the junction could easily be seen. Quartz crystals normally come in metal cans but occasionally glass ones are found, such as the one in Fig.36. Finally, one of the few components to still use a glass encapsulation are some thermistors. The classic RA53 in Fig.37 used in Wien bridge oscillators, employs an evacuated glass envelope, like a vacuum flask, to minimise heat loss from the tiny bead of resistive material. Looking through the glass Every so often I get a fault that has me going round in circles. One was an open-circuit OA70 diode inside the intermediate frequency transformer Fig.31. This bunch of 2SK170 JFETs from eBay were fakes. Watch out for odd laser writing and flashing between the legs. CA3080 and MN-series bucket-brigade delay chips fakes are also common. Practical Electronics | September | 2021 Fig.32. The fake device from Fig.31 – these constantly re-circulate on eBay at ever increasing prices. Fig.33. The real deal from Toshiba, note silver print and copper portion on leadout wires. There’s no side seam and the break-off tab at the top is central and 2.3mm wide. The dimple is 0.005-inch diameter, shallow and circular. This is the detail we have to check for in audio parts off eBay. can of a classic Bush TR82 radio. It had languished undiagnosed on the back shelf of a local electrical shop for 20 years before I was able to fix it. The guts of the IFT, which contains the hidden black diode, are shown in Fig.38. A recent fault was in a Fender De Lux guitar amp where no signal was getting through the input valve, an Fig.35. OCP70 phototransistor – it’s shocking how imprecise and blobby the junctions were in old germanium transistors. ECC83. Tapping the valve elicited loud bangs, so I changed it. The problem remained, so the socket appeared to be the problem. I laboriously changed the socket and it worked for three weeks. Then it stopped passing signal again. This time the valve was the problem. Two intermittent faults in the same place add up to a fault-finder’s nightmare. Eventually, I found that it was a faulty internal weld linking the valve’s pin to the cathode. I could see it through the glass base with a magnifying glass. I proved this after carefully crushing the glass envelope in a cloth and the link just fell off. I looked at a load of ECC83 valves and noticed the old Mullard ones had much better welding than the newer types. The moral of this story is that in electronics, using your eyes can be a very helpful technique – but do remember to protect them in the lab. Thank you Last, a ‘thank you’ to my neighbour Dr Joe Botting. All the photos marked with an ‘*’ were taken by him. Fig.36. 35MHz quartz crystal in glass encapsulation – low-frequency examples were often made in B7G valve envelopes. Fig.37. A glass RA53 thermistor, much loved by builders of audio oscillators. WARNING! When disassembling electronic equipment or components, you must work with: • Eye protection • Fume extraction • Safe wiring/earthing • Comprehensive understanding of any chemicals and materials in use These are not nice-to-have optional extras – you must follow all safety guidelines to protect yourself and those around you. Fig.34. A beautiful T20 triode valve from 1930. The heater, grid and anode structures can be clearly seen. Fig.38. The final IFT (intermediate frequency transformer) of a Bush TR82 radio with its screening can removed. The open-circuit OA70 diode that had eluded discovery for over 20 years was hiding inside. I now install replacement diodes on the pins under the chassis rather than taking the IFT out and opening it up. Practical Electronics | September | 2021 61