Figure 1: Radiolette Model 27
Figure 1: Radiolette Model 27
The left hand knob is the VOLUME control, and it has no ON/OFF switch. The right hand knob is the TUNING knob, which has a spring loaded split gear drive to the under chassis tuning capacitor. The dial is a 270 degree circular type, visible through a square window. There is a toggle switch on the back for the TONE control.
DESIGN
The receiver covers the broadcast band from 550 kcs to 1600 kcs, but the dial is calibrated 200 to 550 meters. Later Radiolettes actually had station callsigns on them. All the valves are pre-octal, using 4, 6 and 7 pin valves. The receiver has an RF amplifier (Radio Frequency) using a 78 pentode valve, a frequency changer using a 6A7, and an IF amplifier (Intermediate Frequency) using a 6B7 pentode double diode. The IF frequency is 175 kcs. The 2 diodes are connected together and this rectifies the audio and produces the AGC (Automatic Gain Control) as well. This is a reflex stage, as the audio is looped back and amplified. The reflex circuit saves on the cost of one valve. It also saves chassis space. The Radiotron Designers Handbook has a section that covers the theory of reflex amplifiers. The author, Fritz Langford Smith, actually worked for AWA when he wrote the book. Following this stage is an audio power amplifier using a 42 pentode. The power supply is 240 volts input to a transformer, and provides 300 volts DC from an 80 rectifier valve. The screen supplies are provided by 3 dropping resistors with bypass capacitors. Of note is the 6B7 screen supply which is only 50 volts, surprisingly low. The small 5 inch speaker has a 1000 ohm field coil which serves as the filter choke. The power transformer also supplies 5 volts for the rectifier and 6.3 volts for the other valves.
The receiver is tall. The right hand side (viewed from the back) has 3 valves in a shielded box. Next to this are three small round cans containing the RF and oscillator coils. At the front is the dial and the speaker, with the speaker transformer on top. On the left hand side is the audio and rectifier valves, with the power transformer behind them. In the centre back is a tall can with the electrolytics inside it. There is a sheet of aspestos on it, to prevent the heat from the rectifier damaging the capacitors. The aspestos is not dangerous unless you disturb it and breath in the powder.
Figure 2: Rear View
The receiver has a deep steel chassis. The 3 gang tuning capacitor is under the chassis and driven by a spring loaded reduction gear, which also drives the dial pointer. The tuning capacitor is insulated from the chassis as it has AGC on the frame. The volume control also has a reduction drive, and a slipping clutch. There is a toggle switch on the back of the chassis that changes the receiver sensitivity for distant or local stations. It merely changes value of the cathode resistor for the RF and mixer stages. There are several capacitors and resistors tied together in a bundle under the chassis. The IF transformers are contained in screw top cans under the chassis. They have only one tuned winding each, and this uses a capacitor trimmer. The room under the chassis is very tight. When servicing, it is best to remove 4 screws in either of the chassis side panels, and fold them out. This allows access to the underchassis IF cans and valve sockets on one side, or the capacitor bundle and valve sockets on the other side. Beware that there is exposed mains terminals very near the bundle and the audio valve socket.
On the back of the chassis is a patents list, an ARTS&P label and the Distant/Local switch. At the left hand side is the power cord entry, and mains voltage selection. There are 2 fuse holders, and you can change the mains voltage tapping by placing the fuse in the appropriate clips.
RESTORATION
The receiver was working. It had been repaired, modern parts added, and original parts had been removed and lost. The main modifications were in the capacitor bundle, under the chassis. New small electrolytics had been added and a large resistor replaced with a tiny quarter watt modern type. This looked strange as it was next to a large wire wound type. I replaced it with an old style body/tip/dot resistor. An obvious bright yellow plastic modern capacitor was replaced with an old style Ducon paper capacitor, which was stuffed with a new polyester capacitor. The added modern plastic wire was replaced with woven cotton covered wire. One of the screen dropping resistors had lost all its paint and was high in value. It was replaced with a body/tip/dot style resistor. The bundle was re-wrapped and fixed in place. Behind the tuning capacitor and under the toggle switch, there is a small tag board, with 2 modern capacitors. I replaced these with restuffed Ducons. The original audio coupling capacitor had not been replaced and was leaking positive volts to the audio amplifier grid. This was replaced with a restuffed Ducon. One of the restuffed Ducon capacitors is visible in the chassis underside view, next to the toggle switch.
Figure 3: Chassis Front View
The performance was not very good, so I checked the valves.
They were all low on emission, and all were substituted with new ones,
but this made little difference. So only the 42 was ultimately replaced.
The 80 rectifier was a GT type, but I did not replace it with the larger G type.
I aligned the set, and this made a little improvement, but not a great deal.
One of the IF cans did not appear to peak at all. Upon removal it was found
to have an open circuit winding on the secondary.
This was repaired and the transformer re-installed. The performance was markedly
improved. For a 10dB signal to noise ratio, it was now
1600 kcs Local 300 uV Distant 14 uV
550 kcs Local 140 uV Distant 6 uV
Figure 4: Chassis Underneath View
While working on the set, it would occasionally click and go dead. Flexing the chassis could restore operation. An exposed wire in the bundle was shorting out, so this was insulated with old style woven spaghetti. Also when turning the set upside, it would sometimes click and go quite. This was traced to the top cap connection in the frequency changer valve, which had lost its insulation and was touching the valve shied. It was similarly insulated with old style woven spaghetti.
Figure 5: Chassis Rear View
The speaker transformer looked as though it had been replaced. It was wrapped in old style black cloth tape and soldered to the lugs. The cloth tape was old, and the solder broken, so the transformer was flopping around freely. I made some clamps out of copper strips. The chassis is spotty, dirty, and has some rust spots. I decided not to paint it, but to leave it untouched, as it would mean that the stencils and sticker would be obliterated. The mains cord was stiff and actually crackled when it was flexed, you could hear the brittle insulation breaking. It also had a plastic plug. The cord was tucked through a crack in the side of the chassis and not secured. The mains fuses were exposed, as the original cover was missing. So you could easily reach around the back of the radio to use the Local/Distance switch, and touch the exposed mains. I made up a small plate to cover the fuses, secured a new (old style) cloth mains cable to it and clamped it in place. The cover is shiny but will dull with time. I replaced the plastic plug with a bakelite plug.
CONCLUSION
The radio is very nice to look at, is easy to operate, but has ordinary sensitivity, and requires a large external aerial. The tuning has split gears and a friction drive, and has a heavy feel to it but with no backlash. The dial calibration is in reverse to what you would expect. The controls are simple but it would have been nice to have a mains ON/OFF switch. There has a 1930s feel about the whole mechanical construction. It looks nice, but also looks like it is made to a price.
REFERENCES
AWA Radiolettes 1932-1949, 2nd Edition 1998, Peter Hughes, Historical Radio Society of Australia Inc. P.O. Box 2283, Mt. Waverly, Victoria 3149, Australia.
circuit
courtesy of PH
Copyright
Ray Robinson VK2NO
