© Copyright Stuart Irwin and published with permission of the author
As I have mentioned before, I have done many restorations for an antique dealer over the years. Some years ago, 1999 to be exact, he approached me to do three ASAP.
At the time, I was studying at university and had exams coming up and many assignments to finish and I just couldn't do them then. Well, he came around, took them back off me and abruptly told me that he had found someone else and that my services were no longer needed. That was fine by me at the time, I was under enough stress.
Well that didn't last long. In a couple of weeks he came around saying that he had paid good money to have a set restored by his new chap but it wasn't working properly. "It only picks up two stations", was the complaint. He presented me with an Airzone chassis type 505A and the speaker for me to fix.
The Superhet that became a TRF
The Airzone 505A is a typical Autodyne type superhet dating from about 1934 and is similar to the 505 from the previous year. I promised to look at it as soon as I could.
I looked under the chassis and found that most of the capacitors had been replaced, as well as some of the resistors. I plugged it in, hooked up an outside aerial and listened for stations. Sure enough, I could receive only two stations, namely ABC (612 kHz) and Radio National (792 kHz). The tuning was broad and the sensitivity very low.
No matter how hard I tried, I couldn't find any more stations. This seemed very strange. I started to think that the oscillator wasn't working over most of the tuning range but on investigation, I found that the oscillator wasn't working at all. In fact, the oscillator coil had been disconnected! The cathode circuit of the type 57 autodyne frequency changer was simply connected to earth through its bias resistor, bypassed by the usual capacitor.
The 57 was acting as an RF amplifier, but how was the signal getting through the IF stage, tuned normally to 465 kHz in this case? I tried adjusting the IF transformer trimmers while injecting signals at various frequencies into the grid of the 57 autodyne and found that one winding of the first IF transformer was tuned to about 612 kHz and the other to 792 kHz. The second IF was also miss-tuned, and its response broad enough to accept some signal at the two frequencies.
My first job was to get the oscillator going. This was simply a case of re-connecting the oscillator coil. This wasn't very easy as I didn’t have a circuit at the time but I soon had it sorted out and the oscillator worked well. The feed to the screen grids of the frequency changer and IF valves also seemed wrong so I calculated suitable resistor values to make up a voltage divider to feed 100 volts to the screen grids at full volume. Everything seemed to be working now so alignment was the next logical step.
I first attempted the IF alignment and was able to get the second IF transformer and one winding of the first IF transformer tuned to 465 kHz. However, the primary winding of the first IF transformer would not tune down to this frequency. I took the cover off the first IF transformer and inspected the winding. It appeared that turns had been removed from it. Now it is normal for this winding in an autodyne set to be of lower inductance than the others so that its impedance at the oscillator frequency is low enough so as not to prevent the oscillator from working. However, I suspected that turns had been removed to allow it to tune to 792 kHz. I fixed this by adding an extra capacitor in parallel with the trimmer. I was now able to complete the IF alignment.
I now tuned the signal generator and radio to 600 kHz and I successfully found a peak using the padder capacitor adjustment. However, when I retuned the set to 1400 kHz to adjust the aerial and oscillator trimmers, things went awry. The set broke into a series of squeals and other unpleasant noises at the top of the dial. It seemed that the frequency converter had gone into a multi-mode form of oscillation. This is the first time I have come across an autodyne frequency converter that oscillated too well!
I had wired the cathode feedback circuit as shown on the left side of diagram one on the following page. I changed it to that shown on the right side. This was a success. Wired this way, the bias resistor is effectively connected across the feedback winding, damping it to some extent, and preventing spurious oscillation. I have since confirmed that this is the arrangement used in the 505 chassis. After completing the alignment the radio worked very well, receiving the stronger Brisbane stations from here on the Gold Coast on a short indoor aerial and with gain to spare.
So how did this radio end up in this state? I can't imagine what the original restorer was thinking when he disconnected the oscillator coil, but it seems he was well out of his depth. Perhaps he thought the set was a TRF because it had no proper frequency changer valve and was unaware of the autodyne arrangement. But it seems a curious type of logic - "I don’t understand what this part does so I'll disconnect it".
The end result of this episode is that I have received all of my antique dealer friend's work since.
Ensure you have the proper training and equipment before servicing mains-powered equipment. For more information, see our safety page.