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Practically Speaking Hands-on techniques for turning ideas into projects – by Jake Rothman Restoring old equipment – Part 4 T his month, we conclude the Offset voltage series on restoring the Leak Stereo 30, looking at the pre and power amplfers. Plus, we will suggest some interesting upgrades to the circuit design, and look at useful component substitutions. Like all old transistor amplifiers, the output is biased to sit mid-way at around half the power supply voltage. This has to be checked before the output coupling capacitor (C37) and should be around –20V. Testing the power amps Quiescent current The Leak Stereo 30 power amplifier is shown in Fig.37. Two words of warning, the power transistor heatsinks are live (connected to the collector). One brush with an earthed lead will destroy them. Ensure the volume control is at minimum in case the preamp is oscillating or has a leaky coupling capacitor. Note that the conductive ‘silver’ washers (Fig.38) for mounting the output transistors are made from solder or a soft metal to aid deformation for thermal conductivity. All discrete class AB amplifiers have a quiescent current pre-set adjustment P7 (see Fig.29). If the current is too low, crossover distortion occurs; if too high, you get thermal runaway and destruction of the amplifier. It is a fiddly fine-spot, best set up with a low-distortion 300Hz sinewave, notch filter, a pair of ears and a ‘scope to monitor the residual. If you don’t have the equipment for such a set-up, a current from 15 to 20mA is about right. R 45 R 49 R 46 0 V – 3 9V + C 3 1 2 2 µ F Iq Iq C 3 4 3 3 µ F + – 15 V Replacement transistors T R 6 G E T 5 3 8 R 5 0 – 2 3 . 6V It’s worth examining the PE Transistor Guide from the May 1966 issue, which contains a list of germanium transistors – it’s available for download from the February 2021 page of the PE website. (Pages 4 and 5 are missing – the AC and AF series. If a reader has a complete copy we would like to add them.) Also, Andrew Wylie has a fantastic transistor history site at: https://bit.ly/pe-ps21-05 T R 8 A D 140 V A 10 3 9 R 61 – 2 3 .2 V – tº Iq R 5 2 T H 2 R 5 5 0 V V R 7 T R 4 O C 44 – 4. 2 V C 3 7 10 0 0 µ F – 2 3 V to – 19V R 5 3 + C 3 2 10 µ F R 5 6 + V R 6 There are links under the PCB for measuring the quiescent or standing current in the output transistors with no signal. However, I found it easier to measure the voltage drop across the 1Ω emitter resistors. It’s prudent to set the preset (VR7) to give minimum current before turning on. Here, it’s fully clockwise or minimum resistance. I set it using the preset at 25mV corresponding to 25mA. This is assuming the emitter resistors (R56, R57) are 1Ω. After 50 years they are likely to have drifted high. Also, germanium devices exhibit a much greater risk of thermal runaway. They have higher leakage currents than silicon devices and this can increase with age. The driver and output transistors should just be noticeably warm. If they get hot, turn off. Sudden quiescent current increases as the preset is increased must also be watched for. This can be caused by an oxidised preset, faulty driver transistors (TR6, TR7) or highfrequency instability. Also, the current will rise as the amplifier warms up, so trim it back a bit after it’s run for 15 minutes. – 1. 4V R 43 R 44 C 3 3 15 0 0 µ F + 6. 3 V R 47 – 3 .8 V Scr een – tº R 48 T H 1 V A 10 5 5 0 V b C 3 5 8 2 µ F + R 5 1 c T R 7 A C 12 7Z – 2 2 .8 V T R 5 A F 118 T R 9 A D 140 R 5 4 R 5 7 0 V R 5 8 e b e c C 3 6 3 nF R 5 9 Fig.37. Leak Stereo 30 power amplifier circuit. The original Leak diagrams use an early transistor symbol shown in the inset – this is for a bipolar transistor, not a FET! 38 Fig.38. Mullard AD140 power transistor and heatsink. Note the soft metal washer is electrically conductive, so the heatsinks are ‘live’. Is the date indicated by the three-character code? Practical Electronics | July | 2021 Fig.39. (above) GET113 small-signal transistor; Fig.40. (below) The enigmatic AF118 (TR5) – a rare germanium highvoltage RF transistor. Its data sheet was in the Siemens 1970-71 Semiconductor Manual, p.228. Fig.41. Poor 1kHz square-wave response due to substituting a slow, low-frequency transistor (ASY53) for the AF118, an RF (radio-frequency) transistor (TR5). 52 for these. If you want a proper British Mullard transistor, the GET872 is a goodlooking substitute. Small signal Voltage amplifier Fig.42. This ASZ20 from Birkett’s was the only suitable direct replacement for the AF118; unfortunately, they only had one. except for radio use, also often works as the whiskers grow from the inside of the case. These whiskers can sometimes be electrically fused by connecting all three transistor leads together and discharging a 100µF capacitor charged to 35-50V through to the case, as shown in Fig.43. This dodge is also very useful for old radios. In my case, the AF118 had a base-emitter short, so it couldn’t be fixed. It can be hard to find replacements for failed transistors. The preamp transistors (TR1, TR2, TR3) and power amp input transistors (TR4, TR5) can all be directly replaced with silicon types, since the DC bias levels can be adjusted by pre-sets. BC559s or BCY71s (if you want an older-looking type) or other low-noise PNP small-signal example will be fine. The noise will be lower than the original germanium types. I suspect they get leakier and therefore noisier with age. (At 60 years, we may be near the wear-out end of the ‘bathtub curve’ of germanium transistor useability see: https://en.wikipedia.org/wiki/Bathtub_curve). The input transistor AC107 was originally designed for tape heads and is supposed to have a noise factor of 3dB. Even Leak had to buy specially selected ones. I replace these with the NKT214F or 2SA49 devices, which seem to be the lowest-noise germanium transistors I’ve got. The GET113 (Fig39) is not critical, almost anything rated at 25V will be fine here, such as the AC126 or CV7001. The OC44 is used because it has a 15MHz ft (upper frequency limited) and fairly low noise, but it is too expensive today. I’ve substituted germanium Japanese AM radio transistors types 2SA12, 49 or TR5, an AF118 (shown in Fig.40), is especially difficult, since it is both a highvoltage (70V), and high-frequency 125MHz alloy diffused or drift-field device. This transistor was one of the first to be designed for the video amplifier in televisions and was used in the 1963 Perdio Portarama black and white TV. (See Radio and Television Servicing, 1963-64 issue, p.523, and also https://bit.ly/pe-pe21-06). I had to get an AF118 on eBay and it cost £3.95! I tried an ASY51, which had the voltage (60V) but not the bandwidth (1MHz). The amplifier oscillated at first, so I reduced the feedback phase-lead capacitor C36 to 330pF. It worked, but the square-wave response was poor, as shown in Fig.41. Some Japanese types, such as the 2SA103 and 2SA358 should work, but that would detract from the unit’s ‘Britishness’. I did try a Mullard ASZ20 40V 40MHz switching transistor (Fig.42) which worked well, but the VCE was 40V, which gives only a small margin. This device and the AF118 use the unusual TO7 case with a fourth lead (S) for the case/screen lead. This package is infamous for internal shorts due to tinwhisker growth. Sometimes banging the unit hard will dislodge them. Disconnecting the screen lead, which isn’t needed here The two original output driver transistors (TR6 and TR7, Fig.37) are also practically unobtainable, especially TR7, the NPN device. These are shown in Fig.44. The AC127Z is an NPN small power device specially selected for high voltage (50V Vce) denoted by a ‘Z’ stamped on the top (Fig.45). A military Texas 2N388A 40V Vcb (Fig.46) could be fitted, which is better than an AC127Z and is what Tobey and Dinsdale used. There is a high voltage AC141, the AC141H. The Newmarket NKT717 could also be employed. The 2N35 (Fig.47) was used in the original Lin design, but only rated at 25V. The GET538 (Fig.48) is not a problem being PNP, and can be replaced by the ACY17, ACY48, NKT227, OC81Z or 2N1377. Note the unique tapped TO5 heatsink arrangement shown in Fig.49. The GET538 seems to be a mini power transistor rated at 1.5A Ic max and is full of copper heatsinking inside. I suspect such a highly rated device was used to allow Fig.43. The TO7 tin-whisker problem can often be fixed with a capacitor discharge. This trick has saved many classic radios using AF114/5/6 or 7 and OC170 transistors. It has to be done every five years since the whiskers continue to grow. Fig.44. The two germanium driver transistors used in the Stereo 30, TR6 and TR7. Fig.45. The rare AC127Z, the high voltage NPN TR7. Practical Electronics | July | 2021 Drivers 39 Fig.46. The mil-spec 2N388A NPN germanium transistor used by Toby Dinsdale. The late Bernard Patterson, a long-term PE subscriber, generously sent me a few. for the possibility of high-power, highfrequency testing. For audio applications, a lower-rated device can be used. The TD Towers book (Fig.50) is great for looking up data, but always check the equivalents suggested because they often have a lower voltage rating and therefore may not be suitable. In update 4, the BC153/4 are also incorrectly listed as NPN. Fig.49. Tapped TO5 heatsink used for the GET538. People took great care with transistor mounting in the 1960s when they cost as much as a beer and sandwich. Fig.50. The International Towers Transistor Selector, a vital book for old electronic restorers and job-lot buyers of NOS transistors. The old Mullard pocketbooks are also useful for quick reference. 40 Fig.47. One of the first NPN transistors by Sylvania, who specialised in NPN devices. The 2N35 in its distinctive SO-4 can from an ancient Lin/Dinsdale amplifier in an old organ. Introduced in 1953, this transistor cost $4.45 in 1955! Jack Dinsdale used a similar device, the Syl 1750 which cost the considerable amount of 25 shillings (£1.25) from Home Radio, March 1962. Selecting for voltage I’ve used the NPN transistor ASY73 (30V Vce) in a lower-voltage Dinsdale design. The same goes for the 2N1302s and 2N1304s. I use the Peak ZEN50 Zener diode analyser to test their Vce breakdown voltage to see if they are suitable by joining the base and emitter. If the ZEN50 reports a Vce breakdown voltage greater than 50V it can go in the Stereo 30. I’m not yet 100% sure about the long-term reliability of this technique, but it’s shown in Fig.51 where VZ is a substitute for Vce. If there’s no lowresistance path, say <1kΩ from the base to the emitter, or the base is left open, the voltage rating is reduced to the Vce (open) rating. The Peak’s current is limited to 2mA, so no damage to the transistor can occur. Reducing the power rail voltage below 40V (recommended for 8Ω speakers) enables easier substitution of transistors. Fig.48. GET538 (TR6), a hard-to-get PNP driver transistor, replace with an ACY17. This is achieved by setting the voltage selector to 250V (see Practically Speaking, February 2021). Transistors often have a higher actual breakdown voltage than the specification in practice. I’ve done this for ASY73s to replace the AC127Z and selected 2SA70s, which have a TO7 case, for the AF118. Fig.51.Testing transistors for collectoremitter voltage breakdown (Vce) with a Peak ZEN50 Zener diode tester. This AC188’s Vce is much higher than its stated minimum –32V rating, and could replace the GET538. Fig.52. The DC bias point presets (P6) for the power amplifiers are hidden in the top left-hand corner. Practical Electronics | July | 2021 Power transistors The Mullard AD140s, as fitted, seem to last quite well. They have a high Vce voltage rating of 55V which possibly helps. The current rating is 3A. The driver and output transistors should all be germanium. It doesn’t work pairing a silicon NPN driver with a germanium output transistor, since a nasty crossover asymmetry results. A quasi-comp output stage in silicon actually has edgier crossover distortion because of the sharper turn-on characteristic, unless a Baxandall diode is used, such as in some Naim amplifiers. There are plenty of suitable output transistors, such as OC28, OC36, AD142, 2N2147, AU103, ASZ15, NKT401, AD149 R 2 6 – 2 2 V + C 2 2 5 0 µ F C 1 10 µ F R 2 1 T R 1 A C 10 7 B C 15 4 – 12 . 8 V + R 62 R 13 – 4. 6V Set D C o nT R 2 co l l ecto r I np u t R 12 R 14 P 1 0 V F L A T Sw i tch i ng s i mp l i f i ed f o r cl ar i ty R IA A R 16 R 17 and AL103, but I have a big stock of AD140s so that’s what I use. The early Truvox amps used AD140s and later changed to AD149s. These were germanium’s last fling as output transistors, before the all-conquering silicon 2N3055 took over. OC25, OC29 and OC35 can be used in reduced voltage (35V) low-power Dinsdale designs. OC22, OC23, and OC24 have good high-frequency response for germanium (2MHz), but are only rated at 2A, so they are only suitable for a few watts. This is not sufficient for the Stereo 30. Power output Testing with sinewaves for more than say 20 seconds at full power into a load can quickly overheat germanium – 2 6V transistors. It’s prudent to gradually reduce the load resistance as testing progresses; say, from 100Ω to 8Ω. T o to ne co ntr o l Germanium’s maximum junction temperature is T R 2 90°C, half that of silicon G E T 113 B C 15 3 devices. The Stereo 30 will give 10W RMS into C 7 10 µ F + 15Ω and 14W into 8Ω, but not for long. Using C 8 + 8 0 µ F the reduced –38V HT voltage, it’s 10W into 8Ω. Try doing this at 10kHz into a load resistor and you’ll blow it up. In R 18 the past, loudspeakers had an inductive rise in impedance above C 5 1kHz, which provided 2 0 nF protection. Load resistors and modern speakers don’t have this characteristic. This is the time to optimise the DC bias point at around 19V using P6 for symmetrical clipping (see Fig.52). Do this into an 8Ω load quickly at 1kHz. Remember, the heatsinks on most Hi-Fi amps are only big enough for short bursts of full power. The preamplifier The pre-amp channel only uses three transistors, an exercise in minimal circuit design, reflecting their cost at the time. This is shown in Fig.53. There are two parts, the input + RIAA stage and the Baxandall tone control/filter. Once the resistors and capacitors have been checked, it rarely has faults. A tweak of DC bias pot P1 (Fig.32) is often needed, which should give a collector voltage of around 12V on TR2. The AB/ Blore-Edwards pots used are usually okay if not corroded. Locked spindles can usually be freed by using power-steering fluid, basically a light mineral oil with a detergent in it. WD40 and 151 maintenance spray will also work. The distortion and noise level are fairly high. The increase in distortion that occurs when the bass control is advanced is subjectively quite nice. The filter is useful if scratchy records from the charity shop are played. Improvements There are a few reversible improvements one can make; here are some ideas to play with. Gain structure + The preamplifier line inputs were designed for low-level 100mVrms outputs from FM R 63 C 4 tuners. For some of today’s digital sources, 8 nF such as CD players with 775mVrms level there is too much gain. Lowering R16 to 1kΩ can reduce this by Fig.53. a) (above) The input stage set for the preamplifier RIAA input. Two transistors do not give almost a factor of four, making a enough open-loop gain so the distortion is high. The AC107 was considered one of the lowesthigher-level line-input. Even this noise transistors at the time. b) (below) The tone control stage uses a single transistor. Distortion is is still too much for a CD player 1% with bass boost, but sounds interesting. and extra attenuation is needed C 15 with a potential divider on the 2 5 µ F B as s input. The same mod works R 2 3 L i near R 2 5 with tape head and microphone – 2 6V * N o te new v al u es inputs, facilities which are not i n br ack ets f o r l o w er R 2 6* mai n r ai l v o l tage s u p p l y required today. On the eBay o f ar o u nd – 40 V . C 9 C 11 O u tp u t 10 0 nF 10 0 nF amplifier I rewired the switch – 2 2 V so the line inputs went into the C 14* R 2 7 R 2 8 C 16 R 3 2 R 3 4 tone control input. However, 3 2 µ F 2 5 µ F + this mod is not easily reversible – 11V C 13 2 5 µ F and requires that C13 (polarised T R 3 C 2 4 C 2 5 G E T 113 O u t 15 nF 5 nF capacitor) is turned round. It did B C 15 3 C 12 C 10 12 nF 12 nF sound much better; even though – 4. 5 V In C 18 the high gain combined with 15 nF L i near In O u t C 2 6 N o r mal / attenuation in the preamplifier R 3 0 C 2 1 C 8 5 nF Steep R 2 9 T r ebl e 5 0 nF + 8 0 µ F R 3 3 breaks one of the main rules of high-quality audio. + + I np u t f r o m i np u t s tage 0 V O u t F i l ter s w i tch : o ny o ne f r eq u ency s h o w n to s i mp l i f y di agr am Practical Electronics | July | 2021 R 3 8 In C 2 7 12 nF Coupling capacitors Making the output electrolytics (C37, as shown in Fig.54) bigger is useful; 1000µF is only just big enough for 15Ω speakers, so I 41 Fig.54. The original output capacitors were insufficient in capacitance and voltage. I installed these big axial Philips/BC electronics/Vishay electrolytics. The top of these caps is a good place to do the output half-rail voltage check. upgraded to 3300µF 40V. I worry about the original rating of 25V with a 42V rail should the amplifier fail hard-on. It could take the speakers out if the capacitor breaks down. Modern electrolytics are much smaller and better, so this is an improvement recommended for all capacitor-coupled amplifiers. Make C1 bigger: the circuit diagram in Fig.53 specifies 10µF, but the 1964 amplifier originally had just 1µF fitted. The rise in impedance at low frequencies increased the noise on the turntable magnetic pickup input. I further raised the value of C1, and put in a 100µF 6V metal-cased solidtantalum capacitor of 1969 vintage. These old types of capacitor do not deteriorate with time, so are an ideal NOS ‘antique’ replacement, as shown in Fig.55. Silicon sacrilege Leak did one improvement themselves a few years later. They replaced all the germanium small-signal transistors with what were then the only low-cost PNP silicon transistors, a BC154 for TR1 and BC153s for TR2, 3 and 4. They had unusual TO106 epoxy cases (Fig.56). This reduced the noise level. TR5 could also be replaced with a silicon transistor, such as a BC327, with no real difference in sound quality. It’s very tempting to try some distortionreducing tricks developed in the silicon era, such as the Baxandall diode, going back and applying them to germanium. A simple mod to the tone control would be a bootstrapped collector load and emitter follower. (See G Hibbert, Circuit Ideas, p,89, Wireless World, April 1980). Fig.56. Later models of the Stereo 30 dispensed with the small-signal germanium transistors and used these early PNP silicon BC153 and BC154 transistors. 42 Fig.55. Old axial solid-tantalum capacitors make a good replacement for old axial electrolytic types; they seem to last forever. These ones were in an RAF store for 50 years, all perfect. Note the much larger, gold-coloured C14 decoupling capacitor. Constant-current sources Another ‘high-tech’ addition which I’ve yet to fully analyse would be to replace the collector load resistors R21 and R28 with 2.7mA current-regulator diodes (CRD) reducing distortion by increasing the openloop gain. Another 2.7mA CRD in the collector load (R50) of TR5 may reduce the dreaded crossover distortion. It’s essential to get these CRDs the right way otherwise they are almost a short circuit and you will blow your transistors up. sound as good as a complementary silicon amplifier, having its own unique slightly ‘woolly’ sound, but it’s good enough for a second Hi-Fi system used at low volume for the home office, especially a 1960s-themed office with an Olivetti typewriter! I’ll be using it in lectures on amplifier design, so it will earn its keep. Next time we’ll have a look at restoring older silicon designs, such as the millionselling NAD amplifier series. New sockets On the eBay amplifier the power cord had been cut off, so I put in an IEC socket in place of the mains outlet sockets, another facility no one uses today. The new socket can be fitted in the exiating rectangular hole by just enlarging the screw holes to 4mm. This mod is shown in Fig.57. New phono connectors were also fitted with simplified switching, shown in Fig.58. Odd distortion I noticed a DC bias shift with signal level causing lower cycle clipping after a loud burst. This was a strange form of ‘delayed distortion’ that took a lot of tracking down. The cause of this was power supply droop on the preamplifier. There is no regulation, only RC decoupling, so when a loud transient is reproduced the preamplifier DC bias can drift about as well. This effect was reduced by altering the decoupling components, reducing R26 to 150Ω and increasing C14 to 1000µF (Fig.53b, Fig.55). The lower resistance was necessary to maintain the voltage to accommodate the reduced rail voltage of 38V. Of course, a regulator could be installed. Fig.57. A lot of dumped equipment will have had their power cords cut off by overzealous technicians, especially in the public and educational sectors. It’s a good excuse to then add a proper IEC mains connector. Was it worth it? Economically, it doesn’t make sense. For several days work the value has increased from £26 to £60. However, I love it for its physical embodiment of early transistor technology and its build quality. It doesn’t Fig.58. New phono connectors were required on the eBay amp, but were a pig to fit. I had to use the insulating plates from the old phonos. Practical Electronics | July | 2021