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Vintage Radio Astor’s first transistor radio – The APN By Ian Batty While Astor was beaten to the transistor radio market in Australia by AWA, first is not always best. The circuit used in Astor’s APN became the template for many Australian transistor sets that followed it. With a smaller case and superior electronic design, the APN is a notable ‘first outing’ for the famous Astor brand. As with other Astor sets I’ve reviewed, mass production and popularity don’t necessarily mean cheap and sloppy design. This radio’s performance is comparable to other contemporary sets, while its visual design is unmistakable. While I can’t claim the misguided genius of Viktor Frankenstein, my radio (see Photo 5) is an assemblage of parts. It’s an all-transistor APN chassis in a case taken from an all-valve BRQ. 100 Silicon Chip Luckily, Astor didn’t make too many changes to the BRQ’s case when they reused it for the APN, so unless you know what you are looking at, you probably won’t notice the substitution. The APN is one of a family of radios released in the changeover from valves to transistors. These continued the valve sets’ visual designs but popped in transistor-based circuitry. A good number used identical cases. Geoff Trengove and Jim Greig published part one of a two-part series in the January 2023 issue of Radio Waves that included a complete survey of such sets. Australia's electronics magazine The only difference between the case for the APN and the BRQ is that the former lacked the hole for the On/ Off/Battery/Mains switch on the right side, and the power cord cutout in the rear flap (because the BRQ was mains-­ powered but the APN is a battery set). The APN also bears the label “TRANSISTOR” in place of the BRQ’s “SPORTSTER” beneath the speaker cutout. There’s also a label above the volume control on the BRQ labelled OFF ON VOLUME, which is missing from the APN since it has no such switch. siliconchip.com.au Internally, the APN uses a separate sub-chassis for the audio and RF/IF sections – see Photo 1. The RF/IF section is on the right, where the BRQ signal circuitry was located (Photo 2). While three transistors occupy a lot less space than four valves, the APN adds a third IF can, so the space savings are not huge. The three-­transistor audio section sits in a previously-­ unused space at the top of the case (Photo 3). The APN replaces the BRQ’s mains power supply (with its transformer and large filter capacitors) with a parallel pair of 276P 9V batteries. Aside from the transistors, all components are of similar size to those in the BRQ. Circuit description You might think that the APN circuit (Fig.1) looks pretty much like any other six-transistor set. In fact, the APN set the template for Australian transistor sets, with a self-excited converter, two IF stages and a diode demodulator with AGC to the first IF stage. The audio section comprises an audio driver and transformer-coupled Class-B output. The APN’s performance rivals that of the look-alike BRQ four-valve set. Astor’s APN showed AWA’s 897P to be a mediocre design, as the 897P needed seven transistors to give only marginally better performance. Astor drawings simply number components in order. Items #1 to #22 are resistors, #26 to #45 are capacitors, #50 to #56 are inductors, #57 is the battery and #58 is the speaker. I’ve preserved this scheme to prevent confusion; however, I’ve numbered the transistors Photo 1: the interior of the Astor APN is divided into two separate chassis for the audio & RF/IF sections. The audio section is primarily above and around the Rola speaker, while the RF/IF section is located on the right, as shown by the large IF cans mounted horizontally. Photo 2: the APN chassis metalwork is based on (and nearly identical to) the allvalve Astor BRQ, with the BRQ shown here for comparison. Photo 3: a closer look at the audio section of the APN. Astor decided to utilise the empty space below the ferrite rod antenna to house the components. siliconchip.com.au Australia's electronics magazine May 2023  101 Q1~Q6 and the demodulator D1, as Astor omitted such labels. The local oscillator uses collector-­ emitter feedback, operating the oscillator transistor in a grounded-base configuration, guaranteeing reliable oscillation across the broadcast band. As the base is not in the oscillator circuit, local oscillator radiation via the antenna circuit is minimised. Q1’s forward bias seems too low at only around 70-100mV, but that’s because Q1 runs in Class-B, giving it the nonlinear operation vital to the mixing function. Australian manufacturers generally used tuning gangs with identical sections, necessitating a padder capacitor to get the LO to track the antenna circuit. In the APN, this is #30, a fixed 310pF capacitor. The converter feeds the tuned, tapped primary of the first IF transformer, #52. Tapping the primary allows the transformer to exhibit a high Q factor without its tuned circuit being damped by Q1’s relatively low output impedance, typically under 50kW. The first IF amplifier (Q2) uses simple capacitive neutralisation thanks to 6pF capacitor #35. This eliminates the feedback effects of its inherent collector-­ base capacitance (see the panel for more details). The voltage drop across 330W emitter resistor #5 indicates a standing collector current of around 0.6mA (600μA). This will fall as the AGC circuit acts to reduce the first IF’s gain on strong stations. Q2’s bias circuit uses a high-value resistor from the supply (#12, 100kW) so that the AGC voltage (supplied via #11, 2.2kW) can effectively control Q2’s collector current and thus, the stage gain. The second IF stage (Q3) uses fixed bias, with a standing collector current of just over 1.3mA. This stage is not neutralised, perhaps due to the demodulator loading the third IF transformer (#54), giving a lower gain. Both IF stages have their bypassing (base and IF transformer) tied back to their emitters. This single-point method gives highly effective bypassing and reduces the component count by eliminating the usual emitter bypass capacitor. Diode demodulator D1 feeds demodulated audio to 5kW volume control potentiometer #13 and, via filter resistor #11, to the AGC line. The AGC line is filtered by 15μF capacitor #33, removing any audio signal and producing a simple DC control voltage. The audio signal feeds to audio driver transistor Q4 via 2μF coupling Transistor Neutralisation Some textbooks describe neutralisation in terms of feedback. Capacitor #35 applies positive feedback from the collector’s tuned circuit to the base. I verified this by increasing #35 to 10pF. That doubled the sensitivity compared to the recommended circuit, confirming that the neutralising capacitor applies positive feedback. It was tempting to ‘hot up’ the APN to equal the 897P’s superior performance this way, but I resisted. Consider the effect of the transistor’s collector-base capacitance; since the collector signal is an amplified, inverted version of the base signal, collector-­ base feedback is negative. The point of the positive feedback from capacitor #35 is to cancel this out. So you can think of neutralisation as adding a balancing circuit that nulls out the effects of collector-base feedback. I addressed the matter of anode-grid feedback in valves in my article on the Grebe Synchrophase radio in the February 2018 issue (siliconchip.au/ Article/10977). The same principles apply to transistor circuits, except that some designs account for transistor feedback’s complex nature. While a valve feedback’s phase angle is ideally 180º, transistor feedback deviates from this as the internal feedback contains resistive and capacitive elements. A simple capacitive circuit cannot totally counteract such a complex feedback effect. Full correction demands a resistive-capacitive neutralising circuit, properly known as ‘unilateralisation’. With unilateralisation, the signal in the amplifying circuit flows only from the input to the output and never in the reverse direction. Regency’s TR-1 (described in the April 2013 issue; siliconchip.au/Article/3761) uses such a design. 102 Silicon Chip Australia's electronics magazine capacitor #40. Q4 uses combination bias, with a voltage divider formed from resistors #14 and #16 and 1.8kW emitter resistor #17. There is a feedback path from the speaker connection via resistor-capacitor combination #18/#43 and series resistor #15. Transistor Q4 feeds driver and phase-splitter transformer #55, with top-cut provided by 4.7nF capacitor #41. The output stage Q5/Q6 operates in Class-B, with around 150mV of bias provided by resistive divider #20/#21 and thermally-compensated by NTC thermistor #19. Q5 and Q6 share emitter resistor #22 and drive output transformer #56, which in turn drives speaker #58. 47nF capacitor #44 applies top-cut to the output transformer. Class-B operation provides better efficiency than Class-A. Of the transistor sets I’ve tested for Silicon Chip that use Class-A, only one manages even 30% efficiency (the GE P807). Terman (siliconchip.au/link/abje) quotes Class-A’s maximum theoretical efficiency as 50%, with typical values of 20-35% (p391). The same source puts Class-B’s maximum theoretical efficiency at 78%, with common values of 50-60% (p393). The APN’s Class-B output stage gives a maximum power efficiency of around 46% for full audio output, which may not seem like much of an improvement over a good Class-A stage. But the APN’s full output comes with a battery drain of around 63mA on peaks. A 250mW Class-A output stage (with an efficiency of 30%) implies a standing power consumption of siliconchip.com.au Fig.1: a redrawn circuit diagram for the Astor APN. It uses just six transistors, one less than the competing AWA 897P. The Class-B output stage and dual 9V batteries gives a typical runtime of 200 hours. 750mW. This would give a constant battery current approaching 85mA, resulting in under 50 hours of operation from the pair of 276P batteries. Such an output stage would also demand extensive heatsinking and very precise biasing to prevent thermal runaway. As the set will rarely be run at full volume, the resulting average battery drain is much lower. Average listening levels allow a battery life exceeding 200 hours. Why two 9V batteries in parallel? I suspect two reasons – first, there was enough space, given that they removed the mains power supply used in the previous valve model. It would also be a marketing point, as the APN would give about 20 times the battery life of the previous BRQ valve set. Restoration The case cleaned up nicely, with the oddity that it appeared to be a case from the previous valve model (BRQ). The electronics were another story. It did work – just. Sensitivity was very poor, and it only seemed to tune from about 700~800kHz to around 1500kHz. The original metal can (TO-5 package) 2N484 converter transistor had been replaced by an all-glass OC44, and the original CK872 TO-5 audio driver was replaced by a TO-1 package 2N406. The audio output was distorted, so I first checked the output stage bias, which was too high. I tried removing the bias thermistor #19, but one lead siliconchip.com.au broke off from the resistive body. It was not repairable, so I replaced the bias circuit with a diode-connected transistor (see Fig.2). This has the advantage of giving the correct bias voltage that tracks correctly with temperature changes. The audio output was still low, going into clipping at under 100mW, and I wasn’t getting the expected 50mW output with 5mV at the audio input. The original CK878s showed very high leakage, so I replaced both with AC128s. I was able to disconnect the CK878s and leave them in place, preserving some visual originality. The volume control coupling capacitor (#40) measured low in capacitance, so I replaced it. I could then get 50mW of output with only 4mV input – about right for a three-­ transistor audio amplification stage. I then looked at the IF channel. I’ve previously warned against using paint/wax/other stuff for sealing adjustment slugs. This set had wax poured into the tops of the three IF cans, and the slugs were held tight. Maybe it was still in alignment, and I was just being fussy. Still, I thought the sensitivity was low, and I measured 2.5V DC at Q2’s emitter. It should have been about 0.15V; the problem was excessive collector leakage in Q2. So I replaced both Q2 and Q3 with OC45s. That fixed the excessive emitter voltages, which should have meant that the IF channel was working correctly again. The remaining low gain prompted me to try removing the sealing wax, so I removed and dismantled the three IF cans. Whatever the ‘foreign’ wax was, it had a much higher melting point than the manufacturer’s wax used to seal the coil windings. My heat gun had the wax on the windings dripping while the wax in the coil cores was only just softening. Rather than overheat the windings, I boiled a kettle, poured the water into a jug, and dunked the coil. This worked well enough with IF2 and IF3 Fig.2: I replaced the NTC thermistor with a diode-connected transistor (right) to provide the correct bias voltage with respect to temperature. Australia's electronics magazine May 2023  103 Photo 4: the LO coil is not easily adjustable on the APN (shown at far right), despite it having an adjustable slug. I had to spend quite a bit of time cleaning the wax out of the three other coils that someone else had added, so that they could be adjusted. to let me carefully extract the adjusting slugs with several ‘treatments’. I visited a machinery shop and came home with a ¼-inch, 26 thread-perinch (TPI) tap and die. The tap worked a treat. Held with no more than finger tension, I was able to clear the coils’ internal threads of wax gradually. Heating and swabbing the threads with cotton tips was not an option because I didn’t want to risk damaging the coil windings, and heating the wax would have allowed it to coat the internal thread evenly, worsening the problem. Curiously, although the tap seemed a correct fit to the coil thread, the adjusting slug would not drive into the matching die. There was definite interference, so I resorted to a fine wire brush to clean the slug threads. I managed to get IF2 and IF3 adjustable, but IF1 defied all my attempts. Luckily, the slug was well out of the coil, with the IF resonating at close to 520kHz. I first tried the easy way – bridging an extra capacitor across the primary of IF1 (in this case, 68pF). While this brought the resonance down to a bit below 460kHz, the resulting gain appeared low. That makes sense; Q = (1 ÷ R) × √L ÷ C, so a larger C, for the same L, reduces Q and thus, stage gain. My back-ofthe-envelope shows an expected Q reduction of about 15%, close to what I measured. So instead, I recovered a suitable slug from an old TV coil and popped it in. Luckily, the jammed slug was at the bottom of the IF, so the new one screwed easily into the top of the winding, and I could bring the IF down to 455kHz. I still didn’t have the gain I expected. 104 Silicon Chip The AGC filter capacitor (#33) was open-circuit, so I replaced it. The second IF amplification stage showed a low gain; the culprit was #37, the emitter bypass. It’s unusual to find a paper capacitor open-circuit, but I did, so be alert to that possibility. With the IF going, I looked at the converter stage. The ferrite rod’s leads must have broken at some point, as they were soldered to single-strand hook-up wire. I replaced the connections with flexible stranded wire and protected the joins with heatshrink tubing. After replacing the existing OC44 with one from my spares box, I found that the local oscillator would not work. I suspected the emitter coupling capacitor, #29. Remembering the faulty capacitor #37 in the IF strip, I replaced #29. The oscillator would still not work, and after much faffing about, I pulled my substitute OC44 and tested it. Its current gain (β or hfe) was only about 30. So I tested all the OC44s I had on hand and selected one with a β over 100. That got the set going at last. I was surprised to find that the oscillator transistor’s gain was so critical. OC44 specifications show a β range of 45~225, with 100 typical. Yes, my replacement had a β of only 30, but I’d have expected the designers to be pretty liberal and allow for low transistor gains. As with valve sets, it looks like the converter is the stage most sensitive to device performance. Perhaps they selected the OC44s for gain at the factory. With all that done, I was able to finish the alignment and complete my tests. The ferrite rod has a small Australia's electronics magazine auxiliary winding that can slide along its length to adjust the antenna circuit at 600kHz. While this works, I’d be careful not to ‘exercise’ it too much, as I expect the coil wiring to be delicate. One final niggle: the LO coil cannot be adjusted on this set. Yes, it does have an adjusting slug, but it’s obscured by a ferrite rod mounting bracket (see Photo 4). Transistor failures I’ve never had to replace every transistor in a radio. The APN is a reminder that transistor technology was advancing rapidly in the 1950s and 1960s, and didn’t really mature until silicon transistors became mainstream. You can still buy OC44/45s online, but you’ll likely get a better deal from the HRSA’s Transistor Bank (visit hrsa.org.au for more information). While it’s often possible to rejuvenate valves by over-running the filament/heater, I’ve not found any similar technique for transistors. That makes sense: valve emission depends on the chemical composition of the cathode coating, so it’s possible to ‘boil off’ contamination by overrunning. However, a semiconductor junction is intimately fused in manufacture, and degradation that increases leakage is unlikely to be remediable. There are two significant measures of leakage, ICBO and ICEO. ICBO is the current flow measured from collector to base (“CB”) with the emitter not connected (“O”), while ICEO is the current from collector to emitter (“CE”) with the base not connected. An ICBO of, say, 10μA might seem trivial, but it’s a base current, so the transistor’s current gain can magnify this to a collector current of 100μA or siliconchip.com.au considerably more. This would affect the ICEO. The leaky 2N484 in this set had an ICEO exceeding 10mA with a Vce of 10V. How good is it? For a first outing, it’s pretty good. The best comparison is AWA’s first transistor set, the 897P, which I previously reviewed (April 2015 issue; siliconchip.au/Article/8458). The 897 used seven transistors, with two interstage audio transformers for maximum gain in the four-transistor audio channel. This appears to be in compensation for the low overall gain of the RF/IF section. The 897’s audio gain is over ten times higher than that of the APN, so let’s keep that in mind. The APN’s RF sensitivity is 375μV/m at 600kHz and 200μV/m at 1400kHz. Both readings showed signal+noise to noise (S+N:N) ratios better than 20dB. Compared to the 897’s 250μV/m and 150μV/m, and discounting for the 897’s extra audio gain, the APN’s RF/ IF section has more gain overall. The APN’s actual performance is certainly on par with valve portables of the day. My favourite ‘distant’ station, Horsham’s ABC Western Victoria (3WV) on 594KHz, rocked in at full volume. The maximum audio output, at 10% total harmonic distortion (THD), is 260mW. At 50mW, THD is about 5%; at 10mW, it falls to 3%. The -3dB audio response from the volume control to the speaker is 260-4600Hz, with a peak of about +4dB at 1050Hz. From the antenna to the speaker, it’s 150-1900Hz. For a +6dB output rise, the signal increase was around +25dB, about as good as can be expected with the simple AGC used in the APN. It went into signal overload at around 25mV/m, which is a strong signal. -3dB selectivity is ±2.5kHz; for -40dB, it’s ±14.5kHz. This selectivity compares well with the 897’s figure of ±13kHz at -60dB, allowing for the 897’s double-tuned IF transformers. I tested it at only -40dB because much over this put the APN’s converter into overload. It gave reliable results at -40dB, and the ±14.5kHz skirt selectivity is enough to reduce interference from any adjacent channel radio station. Its low battery performance is good. Although its sensitivity reduces with a supply voltage of 5.5V, it still exceeded 50mW output with low distortion. This low distortion justifies my bias diode replacement for the failed voltage divider/thermistor circuit. Purchase recommendations I’m looking for a good original case with a wrecked chassis to de-­ Frankenstein my example (see Photo 5). If you have AWA’s 897 in your collection and don’t have an APN, consider getting one. It’s a bit smaller, with – to me – a more interesting visual design. As an engineer, I appreciate its comparable performance to that of the AWA, especially given that it has one less amplifying stage. Jim Greig restored a genuine APN (described in the HRSA Radio Waves magazine, October 2020) and found similar faults to mine. Jim’s method of fault-finding is a valuable reminder that different repairers use the basic principles differently. According to the Radio Waves article in January 2023 referenced earlier, Geoff and Jim have only discovered one issue of this radio, in the red case (see the lead photo). So if you see an APN in a different case, it’s likely another Frankenstein’s monster. Special handling The tuning and volume knobs are a press fit onto the capacitor shaft. I recommend that you don’t use screwdrivers or other levers to remove them. I was able to use finger pressure; if you can’t get them off that way, run strings under the knobs and use a gentle pull SC to remove them. Photo 5: my ‘Frankenstein’ Astor APN came in a leather case originally for a similar valve set. It is different from the ‘original’ red leather case shown in the lead photo. siliconchip.com.au Australia's electronics magazine May 2023  105