CONTROLS
The BFO has a main frequency tuning knob with a slow motion drive. The outer ring has a 1:1 control, and the centre knob has a 56:1 reduction. The calibration marks are continuous from 10 cps to 13 kcs, which is over 3 decades. Most modern audio oscillators have to switch bands to cover this range, with usually one band per decade. On the right hand side of the front panel there is a single potentiometer for the output level labelled OUTPUT. On the left hand side of the front panel is a zero control for calibrating the frequency, labelled ZERO SET. At the top left hand side is a magic eye tuning indicator which shows zero beat during calibration. Before each use, the scale must be set to zero frequency. To do this, rotate the main frequency pointer to the zero reading, and adjust the ZERO SET knob till zero beat shows on the magic eye. As zero beat is approached, the edges of the green area begin to flutter, and at zero beat, the flutter is very slow or stops.
DIFFERENT MODELS
The manual I have, is labelled as a 3R but the circuit below is labelled 2R. This BFO is a 14R which indicates a later model. It is not known what improvements or what changes were made. The 2R circuit does show that 5 changes were in made in 1943 which were the adding of resistors, to make the screens of the oscillators and buffers into voltage dividers, rather than just a dropping resistor. This would improve the stability. Other changes mentioned are a different coupling method to the mixer, and the polarisation of a capacitor.
A.W.A. made other test instruments in these matching robust cases, including a high frequency oscillator and a frequency meter.
DESIGN
The BFO has an interesting method of developing the sine wave. Instead of using a straight oscillator or Wien bridge oscillator, it uses two oscillators. They both oscillate at 100 kcs. One of them is fixed and the other one can be varied in frequency from 100 kcs to 86.5 kcs by the main knob. The difference in frequency produces a heterodyne or "beat" and this is amplified and used as the output. Since both oscillators are the same, with similar layouts and components, any drift cancels out, and the output is stable. The output is filtered and amplified, then reaches the output terminals via a balancing transformer.
The main oscillators are 6J8G triode hexodes, with the triode as the oscillator and the hexode as a buffer. The coupling to the buffer uses an internal shared grid, and the top cap of grid 1 of the hexode is grounded, unusual but effective. The two oscillators are mixed in another 6J8G, and then filtered through a 2 stage Low Pass Filter, to remove any 100 kcs frequencies. A 6V6G amplifier with negative feedback, drives a 600 ohm output transformer for balancing and isolation. The transformer has a centre tap which may be connected to the ground terminal if required. The output level can be adjusted with a potentiometer.
The power supply is a 5Y3GT with 2 chokes, arranged as a choke input filter, producing 300 VDC. All the transformers are in metal cases, and filled with pitch.
PERFORMANCE
I tried to measure the output frequency on a counter, but the counter was giving false readings. When I looked at the wave form with an oscilloscope, it showed a sine wave without distortion, but it was fuzzy, indicating noise. So I measured the output on a Noise and Distortion meter and compared it to a modern audio oscillator.
FREQUENCY BFO TRIO
10 cps
3% 0.6%
100 cps
7% 0.1%
1 kcs
5% 0.05%
10 kcs
4% 0.07%
The figures show that the modern oscillator has a much cleaner wave form. This would reflect the modern use as well, since modern amplifiers have lower noise figures and thus would require a lower noise instrument with which to conduct tests.
The attenuator control on the BFO is a level potentiometer that gives out +19 dBm at maximum and -27 dBm at the minimum. The modern oscillator has a level potentiometer and a switched attenuator in 10 dB steps. It gives out +14 dBm maximum and -84 dBm minimum. This again reflects the modern lower noise amplifiers and the need for more precision.
The frequency response of the BFO is from 10 cps to 13 kcs whereas the modern oscillator is 10 hz to 1 mhz. I have used cps (cycles per second) when referring to the older instrument and hz (hertz) when referring to the modern instrument, even though they measure the same thing, which is the output frequency. Modern amplifiers have a better high frequency response and thus require a greater range.
The BFO audio oscillator is an instrument which has disappeared because it does not have the specifications to meet modern requirements. It was suitable for its time, when amplifiers didn't have as high a frequency response, did not as have as good a noise figure, and had less critical distortion levels. However, it is fine for use on older valve equipment and speakers.
RESTORATION
The main knob was missing its reduction knob and centre nut. This is a thin bakelite disc which can be easily lost if the centre nut works loose. The front panel has a deep scratch, visible in the photograph. It may have been dropped on its face and this may have broken the reduction knob and centre nut. One of the coil slugs was rolling around inside the case! I screwed it back into the coil it came from, and aligned it.
It was very clean and new looking inside, with some oil deposits from leaking capacitors. I replaced 3 capacitors and noticed that several others had already been replaced. I left the electrolytics alone and slowly powered the BFO up to full mains voltage. After a while, one electrolytic started to get hot. So I left the BFO off for a day. Next time it ran well and the capacitor stayed cool.
The power cord is a rubber coated flex with a rubber mains plug. The plug was a hard and had cracks, so I replaced this with a bakelite plug. The rubber flex was in good condition except where it entered the case, and here there was no grommet, and the outer rubber was breaking off. I put a grommet in the case, and cut off the bad part at the end and reattached it to the fuses inside.
When the BFO was powered up, it worked, but required calibration. I set both oscillators to 100 kcs using a counter (probably more accurately than necessary) by adjusting their tuning coil slugs. I adjusted the other tuned circuit for maximum output. I zeroed the frequency with the front panel controls, and checked the dial accuracy. It was within tolerance, even after 50 years! I tested the BFO by listening to it on a speaker first, and then driving an amplifier.
CONCLUSION
The BFO is a perfect audio oscillator for testing valve radios. It is easy to use, has a single frequency control, can be adjusted for low level amplifier inputs, or can even drive a speaker. This makes it simple to determine if any distortion is from the speaker or the amplifier, if there is any hum at full output, and if there is any abnormal frequency response. The BFO transformer output prevents the load from affecting the frequency, and protects the oscillator output circuits. It's obsolete name and appearance, looks good sitting on the test bench next to older test equipment. The BFO is very useful for fixing valve radios and would be a valuable addition to radio test gear.
REFERENCES and ACKNOWLEDGEMENTS
Instruction Book No. 3-7077R, A.W.A.
A Portable Modulated Oscillator, Downing H.L. Vol 5 No.4 1941 P165-169
A Portable Beat-Frequency Oscillator, Rudd J.B. Vol 5 No.5 1941, P171-179
Using the AWA BFO
Circuit 2R
Circuit 16R
Thanks to Bob Philips, and Ian O'Toole.
Copyright
Ray Robinson
