© Copyright Stuart Irwin and published with permission of the author
This repair is not so interesting for the faults that had to be repaired, but for the radio itself, which was quite a departure from usual Australian practice. It was a Philco model 41-290 ten-valve console, manufactured in 1941. The most unusual aspect of the circuit, by Australian standards, was the frequency changer section, which consisted of a separate oscillator and mixer, both triodes! The triodes are type XXL, which are Philco/Sylvania Loktal type tubes. These valves were later given the RMA type number of 7A4. Electrically, they are the same as the 6J5, or one half of the 6SN7GT. As can be seen from the circuit on the next page, the set had two IF stages, using 7B7's and the detector is a 7A6, all Loktal tubes. The set also used a loop antenna and push button tuning.
The audio section consisted of a Loktal 7C6 audio amplifier, a type 37 phase splitter, two type 41's in push pull output and a type 84 rectifier. The last four tubes puzzle me somewhat, because, unlike the Loktals, which were state of the art in 1941, the remaining valves seem somewhat out of date, especially the type 37. The 37 was in fact superseded by the 76 way back in 1933. Another puzzle is that the 84 rectifier is only rated at a maximum current of 60 mA, and yet has to supply 9 other valves. In actual fact, the HT is only 180 volts and I measured the supply current at 62 mA, so the 84 is only slightly overrun.
I started the repair of the chassis by replacing the usual suspects as well as one of the 41's and the 84, which were weak. I then checked out the speaker, which proved to have an open circuit field coil. Unfortunately, it was impossible to remove the field coil and my attempts to do so merely ruined the speaker. I had to use another speaker, and another output transformer because the replacement speaker has a different voice coil impedance.
Eventually I had the radio working, but not very well. There was a prominent scratchy noise that after much probing I tracked down to the second IF transformer. I could not readily replace this because it was a special type having only one tuned winding so out it came for inspection. The primary resistance measured higher that I felt it should have been (I was working without a circuit diagram at the time) so I disconnected one end and started unwinding. Well it was my lucky day because after about five turns the wire came away revealing a break. I was able to pick out the end and attach it to the terminal. I measured the primary resistance as about 9Ω which I subsequently confirmed from the circuit is about correct. The set was now working better and just needed an alignment.
Unfortunately, the alignment did not go smoothly. Try as I might, I couldn't get the oscillator and aerial coils to track properly. If I adjusted for maximum gain, the dial calibrations were way out, and if I tried to calibrate to the dial, the gain was well down.
After much fiddling I arrived at a compromise. I don't know what I was doing wrong here, but in light of the subsequent 14 years of experience, I've no doubt I would do a better job now, or figured what the fault was. A circuit diagram and alignment instructions would have been a great help at the time.
The push-button tuning worked fine, with tracking not being an issue of course. As a comparison, I adjusted the push-button tuning to receive the ABC on 612 kHz and compared this to the manual tuning. The push-button tuning generated –5.3 volts on the AGC line, compared to –4.0 volts with the station manually tuned in. This showed the degree of mistracking evident using the tuning gang.
As the radio was to be used up in North Queensland, I had to write out detailed instructions to set up the push-button tuning for local conditions.
Anyway, I received no complaints.
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