Fullscreen HTML5Med Res (??MB)HTML4

Vintage Radio The Astor APK 4-Valve Superhet Radio Astor released the APK in 1958. It was available in ten two-tone cabinet colour combinations: ivory, cherry red & white, grey & white, coral & white, blossom pink & white, dark green & ivory, lime & white, tan & white, China red & white, and turquoise & white. T he APK is first mentioned in Mingay’s Price Guide for Autumn 1958. This set was purchased through the Historical Radio Society’s Victorian auction. It is similar to two other Electronic Industries sets at the time, the Astor Mickey HNQ and the Peter Pan FNQ. The Astor Technical Bulletin for the APK, dated 18/4/58, contains the circuit and alignment procedures. The valve line up is 6BE6, 6AD8, 6AQ5 & 6X4; it has permeability tuning with fixed capacitors and variable inductors, similar to most car radios before frequency synthesis. The tuning knob shaft has three brass bands around a metal cylinder. As the shaft rotates one (or two, in the other direction) of the bands push or pull a plastic sled above the chassis. The sled has ferrite cores attached to it and, as it moves, they move inside the antenna and oscillator coils to tune the radio. The technical note cautions against adjusting the cores, while providing information on how to set them if required. While there are only four valves, 96 Silicon Chip the circuit is more complex than usual since it has a reflexed intermediate frequency (IF) valve that also acts as the first audio amplification stage. This has the advantage that it provides higher output than a set with the same valve count but no reflexing, and the reduced valve count makes it less expensive and leads to less heat generated in the cabinet, which is important for plastic cabinet sets. However, there are disadvantages to reflexing: • Additional passive components • An increased tendency to overload on strong signals • A more complicated design (not as much of a concern for large production runs) • Increased distortion at high modulation levels • Play-through/minimum volume effect Play-through is the presence of an audio output with the volume control set to zero. It is caused by the rectification of the IF signal from the slight curvature of the anode characteristic (‘anode bend detection’) and amplification in the same valve. Australia's electronics magazine By Jim Greig When the volume control is set slightly above zero, the normal and the out-of-phase play-through signals roughly cancel. The audio is generally badly distorted at this point, as explained in the Radiotron Designers Handbook, pages 1140-1143. Bias to the reflexed stage is a careful balancing act between minimising play-through and preventing audio signals in excess of the bias voltage from drawing grid current. It is set to -1.8V in this set, a very linear part of the anode curve. To assist in maintaining the constant bias, automatic gain control (AGC) is applied to the converter only (see Fig.1). The converter stage employs an unusual oscillator configuration; the coil has no taps or secondary winding, and it and the capacitor are in series. The cathode is grounded and pins 6 (anode) and 1 (grid) form a triode with the capacitor from cathode to grid and the inductor from grid to anode. At resonance, the series impedance is at a minimum, and the signals across the capacitor and inductor are 180° out of phase. The triode anode has a 180° siliconchip.com.au Fig.1: Astor’s circuit for the set. It has a large number of components around the 6AD8 because it’s reflexed, handling both IF and audio amplification. Voltages on this circuit were measured with a 1000W/volt voltmeter. phase shift from the grid, so there is positive feedback, and the valve oscillates at that frequency. The IF amplifier is straightforward, but it has a very low anode voltage from the voltage drop across the 51kW anode load resistor (#28). Detected audio is filtered by capacitor 14 and applied to the volume control, while also supplying the AGC voltage to the converter. From the volume control, the audio is further filtered by capacitor 12 to remove all of the 455kHz IF signal and only pass audio, which is directed to the grid of the IF reflexed amplifier through the secondary of the first IF transformer (#46). The audio amplification stage provides reasonable audio gain, around 30 times, measured by applying a 1kHz sinewave to the volume control wiper and monitoring the grid and anode voltages. At the operating point, the anode current is 2.5mA, and the mutual conductance (gm) is around 1mA/1.25V or 800µmho, with the load resistance (Rl, 51kW) in parallel with the 470kW siliconchip.com.au grid resistance on the 6AQ5, giving an overall load of 46kW. The quick formula for a high internal resistance valve, gain = gm × Rl, gives a gain of 37 times, but it measured as 30 times. A more accurate formula for gain includes the valve internal impedance. The curve of anode voltage vs current for constant grid 1 (and 2) voltages for a pentode is very flat (see Fig.3). The (variational) anode resistance is “the incremental change in anode voltage divided by the incremental change in anode current which it produces, the other voltages remaining constant”, per the Radiotron Designers Handbook (page 14). To calculate Ra, the current was Table 1 – Anode current vs voltage Anode voltage (Va) Current (Ia) 60V 2.502mA 65V 2.510mA 70V 2.514mA 75V 2.518mA 80V 2.524mA Australia's electronics magazine measured at anode voltages around the nominal 71V, with the actual operating voltages on Grids 1 (-1.74V) and 2 (43V) in-circuit – see Table 1. I a increased by 8µA while V a increased from 65V to 75V. Ra is therefore 10V/8µA or 1.25MW. Gain = gm × Rl × Ra ÷ (Rl + Ra), so the calculated gain is 35, still higher than what I measured. Could the input impedance of the 6AQ5 be reduced by the negative feedback from the 100pF capacitor shown in Fig.4? It seems unlikely at 1kHz, but I checked that by adding a 68kW resistor in series with the 2.2nF (0.022μF) coupling capacitor and measuring the AC voltages around it. The 6AQ5 AC input impedance calculated from the voltages measured is 141kW. RLl is then 37kW (51kW || 141kW). The gain is now calculated to be 29.5, which is close enough to the measurement. Using the same test method and removing the 100pF capacitor increased the input impedance to 330kW, so the capacitor has a definite effect. April 2025  97 The audio gain from this stage allows the use of overall negative audio feedback (to the bottom of the volume control), reducing distortion and effectively increasing the audio bandwidth. The circuit shows a resistor (#22) and capacitor (#16) connected to the diode on pin 8 of the 6AD8. After some time looking for an electrical reason for the diode and finding none, it seems likely that the pin is used as a convenient tag for the connections, and saves adding a ground wire to it. The link has the effect of slightly increasing the bias on the 6AD8 for strong signals. It varied from -1.78V to -1.97V, possibly to allow for a greater voltage swing. When operating normally, there is around 8V (0.16mA through 51kW) deviation of the 6AD8 anode voltage from the nominal 71V; the valve is operating comfortably on the linear part of the transconductance curve (see Fig.2). Audio is coupled directly to the 6AQ5 output valve, which operates with -8V of fixed bias. The relatively low anode voltage (185V) reduces the heat dissipated (again, important in a plastic cabinet) and lowers the power transformer requirements. Values from the RCA Receiving Tube handbook (Frank’s electron tube data sheets, RCA 6AQ5A) show comparative anode dissipations of 11.2W (250V × 45mA) and 5.2W (180V × 29mA), with the audio power output reduced from 4.5W to 2W, which is still quite sufficient for the set’s intended use. Fig.2: 6AD8 valve mutual conductance plots from Frank’s Electron Tube Pages (black) and my measurement (red). Restoration The chassis is mounted diagonally The under-chassis view with major components labelled. C6 22nF C16 50pF R28 51kW 47kW 47k W screen resistors C15 100pF 98 Tuning shaft 3 brass bands Australia's electronics magazine siliconchip.com.au Fig.3: the 6AD8 pentode’s anode characteristics (measured) for varying control grid voltages. Fig.4: some voltage measurements I made to help determine the 6AD8’s gain was as expected, or low. in the cabinet, so the tuning shaft connects directly to the large centred dial, and the volume control is on the lower left. The cabinet was in good condition; a wash with soapy water and a little polishing had it looking in a reasonable state for its age. The chassis was clean with no rust and a small amount of accumulated dust. Note that the speaker is held onto the front panel with metal tags on plastic posts. It is hard to remove them and keep the posts intact. Careful work expanding the jaws of the clips before removing them cut the breakages to one in four. I regarded all paper and electrolytic capacitors in the set as potentially bad, so I replaced them. Work had been carried out on the radio at some point; the first filter capacitor (#18) was a newer 47µF type, not the 24µF specified; I replaced it with 22µF, which is closer to the original value. I replaced the other filter capacitor (#17) with a 16μF electrolytic that I placed inside the original can. I also replaced the 100pF mica capacitor (#15) on the output anode, as it is subject to high voltages, and there is a history of mica capacitors in this position breaking down. Any faults on powering on would not be from these components, and hopefully not from a wiring error while replacing them. The original power cable was a twincore cord knotted behind the plastic back, so I replaced it with a threecore cable, with the Earth connected to the chassis and the cable properly restrained. I carefully enlarged the small hole in the cabinet’s rear to fit the new the cord. The power transformer sits on the chassis, and the mains and HT lugs are exposed and uninsulated; a clear safety hazard. Now that the chassis was Earthed, there would be 230V AC from the mains Active to the chassis, around 440V AC from the out-ofphase HT secondary and 380V across the secondary. The back of the volume control also has exposed mains wiring. Beware if you are working on one of these radios; cover the exposed terminals before powering it on! Having powered the radio on, there was no smoke but its performance was poor. A check of the DC voltages showed some anomalies. I measured 146V on the 6AD8 anode, not 71V. Its screen was at 28V. The converter screen supply was also low. So I powered it off and checked the resistors. The ½-watt resistors were within tolerance, but the 1W types This top view of the chassis shows the permeability tuning system, which is attached to a plastic sled. 2nd IF Transformer 1st IF Transformer 6AD8 6AQ5 6BE6 6X4 siliconchip.com.au Australia's electronics magazine April 2025  99 Scope 1: the converter oscillator grid voltage (red) and its anode voltage (yellow). Scope 2: the 6AD8 reflexed audio amplifier’s grid (red) and anode (yellow). You can see some of the IF signal superimposed on the red trace. Scope 3: the small variation with signal in the 6AD8 anode voltage. that were connected to the B+ were all high in value: the 6AD8 anode resistor (#28) was 60kW instead of 51kW, the 6AD8 screen resistor (#29) was over 100kW rather than 47kW, and the converter screen resistor (#30) was 60kW instead of 47kW. I replaced them all and then the 6AD8 screen measured 44V but the anode was still over 100V. A new 6AD8 bought it back to 70V. The radio could now tune a Melbourne station, and a tweak of the IFs bought it in reasonably well. Many of the Vintage Radio articles include information on receiver sensitivity. I have no experience making these tests and no screened room. However, I built a dummy antenna based on Graeme Dennes’ in Radio Waves, October 2020 and set up my signal generator, oscilloscope and voltmeter. I measured the audio power across the speaker; ideally, a resistor would be used instead. To obtain the standard 50mW of output, I needed 1.5mV of modulated RF. Assuming the dummy antenna to be part of the receiver per the 1995 British Standard, the aerial voltage to achieve the standard output was 1500μV, which is way too high. I replaced the 6AQ5, which made no difference, but a new 6BE6 converter dropped the required signal level to 500μV. No doubt this is still too high, but the AGC level increased from -0.51V to -1.77V as the signal strength was increased from zero; this change would have reduced the sensitivity. The result shows a lack of knowledge of the testing process rather than the absolute performance of the receiver, but it did help to diagnose a weak converter valve. In Bendigo, this set receives 774 Melbourne with some noise. A Panasonic R-399 12-transistor set with an RF stage performed better, but there is still noise; clearly, my location is not ideal for receiving that station. Overall, this set is typical of the era, in an attractive shape and available in numerous colours. It is well-made and achieves quite good performance with SC a reasonable price tag. An advertisement from The Biz (Fairfield, NSW), 24th of September 1958, page 18. References ● Frank’s Electron Tube Pages (https://tubedata.wernull.com/ index.html) ● The Valve Museum (www.rtype.org) ● Radiotron Designers Handbook, F. Langford-Smith, Fourth Edition 1963, Wireless Press ● Vintage Radio March 2019 (The Astor HNQ Mickey; siliconchip.au/ Article/11451) ● Astor Technical Bulletin Mantle Model-APK (www.kevinchant. com) ● HRSA Radio Waves, October 2020, Ferrite Rod or loop Antenna-­ equipped Receiver Testing ● Advertisements from the National Libraries Trove (https:// trove.nla.gov.au/newspaper/) I added insulation around the power transformer terminals after Earthing the chassis (and thus the transformer frame). 100 Silicon Chip Australia's electronics magazine siliconchip.com.au