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Vintage Radio 1966 Astor “Diamond Dot” CJ-12 Car Radio By Dr Hugo Holden This car radio is a piece of Australian history. It was in such poor condition that I almost threw it away, but it has cleaned up a treat. True to its name, it has a fake diamond-like ‘jewel’ embedded in the front panel. Perhaps the most fascinating aspect is that most of its components, including the transistors, were locally made! I was cleaning out my shed and found a very old and rusty MW-band (AM) car radio with missing knobs and a broken and yellowed dial. I had acquired it for my 1966 Triumph TR4A, as it was period correct. But I ended up fitting a Motorola AM radio with an FM converter instead, and had forgotten all about this Astor radio. It had what looked like a diamond set into the metalwork. It is not a real diamond, of course; it is more like a costume jewellery variant, but it still gives the front escutcheon an eye-catching look. The radio was in such poor condition that I almost threw it away, as I was in the process of a big cleanup. But I decided to take a closer look. The 94 Silicon Chip more I looked at it, the more interesting it became, so I decided it was worthy of a complete restoration. It turned out very nicely, as I think you’ll agree from the photo. The radio’s dial is quite a piece of Australian broadcasting history, with rows of station IDs for different states: TAS, NSW, VIC, SA, NT, WA and QLD. I noticed one of my favourite radio stations listed as KQ, which is 4KQ in Brisbane. Obviously, this radio was intended to be used anywhere in Australia (given that cars were not only sold throughout Australia but also mobile, that makes perfect sense). Car radio design history I have always found the design of car Australia's electronics magazine radios interesting, especially because they commonly use permeability tuning, which permits easy pushbutton station selection. Also, my very first job out of school in the 1970s was working at a car radio factory called “Aerial Radio” in Auckland. That is where I learned about car radios. I worked in a final testing station, putting the radios through their paces and fixing any assembly errors before they were boxed up for sale. Car radios made before 1955 used valves (vacuum tubes). Generally, the HT supply was provided by a vibrator and step-up transformer; the tube anode voltages were similar to those in a line-powered domestic radio, in the 200-300V range. siliconchip.com.au In the mid-to-late 1950s, valves that required only 12V at the anode, such as the EF98 and ECH83, were devised. These were usually combined with a single germanium power transistor, typically a 2N441, in a class-A audio output stage with a collector choke. This ‘hybrid’ design was very popular until the early 1960s. The low HT voltage tubes eliminated the need for the vibrator. The hybrid radio audio stage generally used one EF98, and with a 10MW input grid resistance, this would drive a 23:1 transformer. That fed the base-emitter junction of the 2N441 power transistor, which would have a choke as the collector load, with the speaker connected directly across the choke, or to a tap on it. This hybrid design resulted in an audio amplifier system that required about 2-3V peak for full volume, with an output power of around 4-5W. Having an input impedance of 10MW at the grid and an output impedance of 4-8W was impressive, especially for just one valve and one transistor. However, it was not energy-­ efficient, and the transistor required moderate heatsinking. After all, in class-A, the idle power consumption is often a similar value to the maximum audio output power. In 1955, the first ‘all-transistor’ car radio appeared on the scene in the USA. This was the Mopar (Chrysler) model 914HR. Hybrid radios were still prevalent at that time. The 914HR was made possible by some revolutionary new surface barrier radio frequency transistors, with very low base to collector feedback capacitances. These were rivalled perhaps only by germanium RF transistors such as the OC169, which appeared later, in 1960. There is an interesting YouTube video about this revolutionary Mopar radio at https://youtu.be/Qz3JkFnvBuA Mopar all-transistor radios were fitted to the 1956 Chrysler and Imperial car models. It took about five years for other manufacturers to catch up, before the all-transistor car radios took over. So the Mopar 914HR was some years ahead of the times. By the early 1960s, most countries started mass producing all-­transistor car radios. By the mid-1960s, not only were most car radios of this type, but in keeping with other transistor radios, the audio output stages had siliconchip.com.au Fig.1: the damaged dial from the Astor “Diamond Dot” radio. Fig.2: this is what the radio looked like after being disassembled, just before commencing restoration. moved to push-pull class-AB designs. These had significantly improved efficiency over the class-A designs of hybrid radios. These class-AB designs were essentially class-B amplifiers but with enough initial bias to overcome crossover distortion. This cut the radio’s power consumption to the point that you could get away with accidentally leaving your car radio on overnight and just be able to start your car in the morning. The initial push-pull audio output stage designs used a driver and output transformer. Later, a split driver transformer was used, eliminating the output transformer and saving the cost and weight of the iron core. The speaker was coupled to the power output transistors via a capacitor. Then, with an abundance of good silicon NPN and PNP power output transistors, totally transformerless circuit topologies with complementary audio output transistors appeared. After the mid-1970s, the entire audio stages often were replaced by a single Australia's electronics magazine IC, as was the trend in many domestic radios. Therefore, one could expect a transistor car radio from the mid-1960, like the Astor Diamond Dot, to be sporting a push-pull output stage, probably with coupling transformers. And that is indeed what it has. But what about the transistors? What was Astor using, and where did they come from? Inspecting my radio, I immediately noticed two grey ceramic transistors with black resin tops with the part numbers AX1130 on their sides. I was about to learn more about the sadly lost and once amazing Australian transistor manufacturing industry (more on this in the panel near the end of the article). Restoring the radio There were some interesting problems to solve in the restoration, mainly related to oxidised metalwork, missing front panel retaining nuts and missing knobs. Fig.2 shows the radio in a state of disassembly before restoration. July 2022  95 Fig.3: the radio originally used two Anodeon AT-1138 (shown opposite) transistors. Those were replaced with AD149 germanium transistors, as shown in the photo. Fig.4: The rusted Anodeon AT-1138 transistors were painted and stored in case they were ever needed later. Fig.5: I machined two new hex nuts to mount the front escutcheon. Disassembly required removing several rivets (later replaced) to separate the audio amplifier heatsink assembly from the metal lid. The dial was yellowed through its entire thickness, except where it was shaded from sunlight along its upper and lower edges. It had hardened and cracked. The metal had pitted due to surface rusting, more on the top of the radio than the bottom. The stripping processing before re-electroplating eliminates all the rust crystals. This must be done because ‘rust never sleeps’, and when I see radios that have supposedly been “restored” by painting over the rust, it makes me cringe. After electroplating, the metal pits remain, but at least the surface is plated and no longer rusting. The radio used quite a few self-threading screws, all very rusty. I replaced the common ones (eg, garden size #4 and size #6) with new screws, but for the special low-profile countersunk head types that are hard to get, I had to send those to the electroplater to be re-plated. I was able to replace all the rivets with identical geometry rivets, except for the two small ones above the AD149 on the left. I had to replace those with small stainless steel screws. The two original germanium output transistors, the Anodeon AT-1138 types, had rusted. So I replaced these with a very well-matched pair of AD149 germanium transistors with equally good performance, if not superior (see Fig.3). I kept the original Anodeon transistors and painted them, in case somebody would prefer to use them later (shown in Fig.4). The special nuts which secured the front escutcheon were missing. I searched and could not find any, so I machined two from hexagonal brass bar on my mini-lathe (Fig.5). The thread is 3/8in diameter, 32 threads per inch (TPI), and I was able to get those taps on eBay. An 8.5mm drill worked well. I took the fibre washers from some panel-mount fuse holders I had in my junk box. As for the knobs, I bought some plastic replica knobs on eBay but was disappointed with the quality. Fig.6: knobs from another Astor car radio were modified to fit the Diamond Dot. 96 Silicon Chip Australia's electronics magazine I eventually found some original metal knobs from another model of Astor transportable car radio. They were almost perfect, but the centre knob was designed to push onto a ¼in shaft. This radio had 3/16in shafts for the centre knob, so I machined brass inserts to fit into the centre knobs to make them compatible. These inserts are visible in Fig.6. The ARTS&P sticker on the radio body was moderately marked, so I scanned it (Fig.7) and made a replica. The photo in Fig.8 was taken near the end of the rebuild, after the metalwork came back from the electroplater. The upper panel (radio’s lid, #1 in Fig.8) holds the audio amplifier assembly, and a leash of wires linked it to the main radio board. For ease of restoration, I cut the wires and inserted 0.9mm gold-plated pins and sockets (from Jaycar) to make it easy to separate the audio amplifier and top plate assembly. #2 in Fig.8 points to the two interesting Australian-made Fairchild AX1130 transistors. These act as drivers for the two germanium output Fig.7: the original ARTS&P sticker, which shows the model number. A replica was made of this sticker. siliconchip.com.au transistors, in the Darlington configuration. This reduces the required drive current to the output stage. When I first powered the radio, one of these transistors was defective, so I desoldered it from the PCB. All transistors on the main board in this radio had sleeved leads, so the lead wires were not directly visible. The transistors are interesting as, in common with many of the Fairchild types of the time, they have gold-plated steel lead wires. I found that the defective AX1130 had one lead wire that was totally rusted through. But there was enough of it projecting from the transistor body to save the transistor by joining another wire. I decided to inspect the other transistors on the main board in the radio frequency sections. All the lead wires had grossly rusted, extending right up to the transistors’ plastic bodies. Ultimately, I elected to replace all of them with high-quality mil-spec 2N2222A transistors to avoid any future troubles. This radio must have been in a very moist environment, possibly even saturated with water at one point. After replacing the radio’s electrolytic capacitors, powering the radio and adjusting the output’s stages quiescent current, I tested the audio output stages with a signal generator. I then moved onto the radio-frequency sections. The radio was stone dead, with just a faint hiss from the speaker. I quickly determined that the local oscillator (LO) was not operating. I checked the transistors’ DC conditions, and they were normal. I worried that the oscillator coil in the permeability tuning unit could have gone open-circuit. Testing showed that the oscillator started when a 47pF capacitor was placed in parallel with the existing 56pF feedback capacitor in the oscillator circuit (#3 in Fig.8). Fig.9 shows this capacitor in the circuit. It provides positive feedback from the tank circuit to maintain oscillation. At first, I thought that the requirement for more feedback capacitance indicated the transistor stage gain had dropped or the coil losses had increased. I tested the 56pF polystyrene capacitor shown in Fig.11; it had zero leakage and read 57pF on my YF-150 capacitance meter. Yet, I found when I replaced it with a new 50pF capacitor that the oscillator ran normally. How could that be when the 56pF capacitor tested fine? I siliconchip.com.au #1 #2 #3 #4 Fig.8: the internal topside of the chassis is marked with four locations: #1 radio lid and audio amplifier assembly; #2 two Australian-made Fairchild AX1130 transistors; #3 local oscillator; #4 permeability tuning mechanism. Fig.9: a section of the oscillator circuit with a 56pF capacitor shown. This 56pF capacitor provides feedback from the tank circuit. Australia's electronics magazine July 2022  97 Fig.10 Fig.11: the 56pF capacitor from the radio was faulty, despite having zero leakage and reading fine on a capacitance meter. It was replaced with a new 50pF capacitor. have never seen this defect in a polystyrene capacitor before. Of course, when a technician finds a faulty part, it most often gets thrown in the bin, as it is not cost-effective to investigate it. But I decided to attempt to find out what was wrong with this 56pF capacitor, in light of the disturbing fact that it tested as normal on my meters but didn’t work. Testing it with a signal generator and a scope, I determined that its ESR had increased massively, to around 22kW. Of course, ESR meters cannot measure low-value capacitors like this. I then tried measuring known-good lowvalue capacitors in the range of 50 to 100pF with 22kW resistors in series on my YF-150 capacitance meter; it was unable to detect the significant series resistance. Presumably, inside the capacitor, the bonds or connections between the lead-in wires and the foils have become oxidised or corroded. The implications of this sort of failure are interesting. If a capacitor with this fault were used instead in a tuned circuit in an RF amplifier, it would not throw the centre frequency off to any significance. Still, it would certainly lower the circuit Q, lowering the gain and increasing the bandwidth. Since, after alignment, this radio is now working properly and is sensitive, I have not removed any of the other polystyrene capacitors for testing. The permeability tuning mechanisms of vintage car radios (#4 in Fig.8) are fascinating. They have continuous tuning by the control knob and preset pushbutton tuning, which acts as mechanical memory for preferred stations. When a button is pushed, a sliding arm disengages a clutch mechanism to mechanically isolate the tuning knob. With time, these rubber clutches have a habit of slipping, even with an otherwise well-lubricated mechanism. The rubber ages and hardens, its surface becomes glazed and the metal disc it runs against can become quite polished. Disassembling it and replacing the rubber disc requires pressing off a gear from the assembly’s shaft, which is better avoided. Cleaning the rubber disc with isopropyl alcohol (IPA) helps but often won’t solve the problem. I developed a method to fix these clutches using some very thin cardboard, similar to thin transformer card with an adhesive on one side. A washer is made the same size as the rubber disc, and the central hole is opened to the disc perimeter. The clutch is opened manually or by pushing a button, and the disc is inserted with the adhesive facing the metal disc surface, and it sticks to that. The rubber face then runs on the card face rather than the shiny metal surface, increasing the friction and preventing slipping. As an aside, my view is that the continuously variable tuning knob is the safest method to use a radio while driving a car. The driver could keep their eyes on the road while turning a knob, and stop on the station they liked the sound of. Other radio tuning methods could require the driver to take their eyes off the road. Circuit diagram The circuit diagram (Fig.10) and PCB layout (Fig.12) are reproduced here. This diagram, the manual for this radio and other relevant documentation is available from Kevin Chant’s website at siliconchip.au/link/abek The transistors were drawn in a way typical of some early 1960s vintage Australian transistor manufacturing Bardeen, Brattain and Shockley invented the point-contact transistor at Bell Labs in December 1947 and announced it to the world in 1948. Shockley’s junction transistor was also announced that year. Within a decade, four companies came to invest in Australian transistor manufacturing: AWA, STC, Philips and Ducon. All came to manufacture germanium-alloy junction transistors in Australia in the late 1950s to early 1960s. But what about silicon transistors, specifically, the AX1130 in the 1966 Astor radio? I looked in my parts inventory for similar transistors and came up with the devices shown in the accompanying photo. These transistors, all with the A prefix, were manufactured by Fairchild’s Australian division. They are relatively rare now, unlike most transistor types. If you search for them on eBay trying to find a spare part, you do not get any hits, as these transistors are ‘unique Australiana’. In June 1964, Radio Television and Hobbies magazine carried the following announcement: “A new Australian company to produce heat resisting silicon transistors has been formed in Melbourne. An offshoot of the Fairchild Camera and Instrument Corporation of New York, the Australian company will be known as Fairchild Australia Pty Ltd”. siliconchip.com.au In 1966, the company opened its laboratory facilities (see the EA article on page 102). The factory closed in 1973, and the AY/AX series of transistors unique to Fairchild in Australia became obsolete. For more on the history of transistor manufacturing in Australia, see this fascinaring website: http://siliconchip. au/link/abel A short list of some Australian-made transistors from Fairchild Semiconductors. Australia's electronics magazine July 2022  99 Fig.12 radios. One interesting thing is that the audio driver transformer does not have a primary winding. Due to the Darlington output devices made from the combination of the AX-1130 and AT-1138 transistors, the output stage has a fairly high impedance. Therefore, the driver transistor can simply capacitively couple into one side of the driver ‘transformer’, which is essentially a centre-tapped choke, and acts like an auto-transformer. Upper transistor #144 gets its drive directly from the previous stage (via an AC-coupling capacitor) while lower transistor #144 gets its phase-inverted drive from the other end of the centre-­ tapped autotransformer. The centre tap is held at a mid-rail voltage point due to the action of Vbe multiplier transistor #143. I measured the properties of this transformer, as well as the output transformer, in case others need to wind replacements for faulty units. The driver transformer is bifilar wound on a 7.5 x 7.5mm cross-section core and each winding measured 195W and 2.3H. The output transformer is designed for a 15W speaker and it is wound using 0.5mm diameter enamelled copper wire on a 15.4 x 15.4mm cross-section core. Its two primary windings measured 1W & 66.5mH with the single secondary measuring 2W & 190mH. The windings ratio is 1.7:1. Performance This radio is a good performer, sensitive in the RF circuitry due to a tuned RF stage, one mixer stage, separate local oscillator injection and two IF stages. On the audio side, it’s a good performer with a push-pull class-AB output stage, with plenty of audio output power for use in a car. The audio amp in the Astor radio is pretty good. The use of a 15W speaker is unusual in latter days for a car radio; most became 4W. But of course, when you have an output matching transformer, it is easy to use higher-­ impedance speakers, if more costly. Astor don’t mention the maximum audio output power in their manual. With a 12V supply, you end up with about 10-11V swing before peak clipping in the collector load (half of the output transformer primary) because of the collector-emitter saturation voltage of the Darlington pair, and their emitter resistors. siliconchip.com.au Fig.13: the internal underside of the chassis shows just a few discrete components attached via point-to-point wiring. So the power delivered to the 15W speaker just on clipping can be calculated as about 6-7W, allowing for transformer losses. It is more like 8W, given that the radio’s supply voltage creeps closer to 14V while driving, as the battery is charging. That is plenty of audio power, even in a noisy car. It is physically very well made, and rivals any MW-band car radio made in any other country. I am glad I could see the potential in this radio, to become something beautiful again and took the time to restore it. It would make a fine addition to a vintage car of the same period. This radio is a reminder of how advanced Australian electronics and transistor manufacturing was in the Australia's electronics magazine mid-1960s. This saddens me, as we were once able to make our own transistors and ICs. The worst thing about this is the strategic significance of this, with the inability to build our own electronics, and the impact of disrupted supply chains for electronics, medicines and other vital products that is now quite apparent. This has exposed how dependent we have become on overseas-­made products. When high-tech manufacturing infrastructure and ability is lost, it takes decades to rebuild it. The human skill-base and required engineering experience get lost along with it. The problem goes much deeper than derelict factories and unemployment. SC July 2022  101