Example Superhet Circuits PDF Print E-mail
Written by Bryce Ringwood   
Wednesday, 19 September 2012 21:39

This article provides a description of some typical radios you will never encounter - but at some point you may think you have! The first is from the 1936 RSGB Radio Amateur's Handbook. You can click on the images to download a higher-quality .pdf image.

An Early Amateur Communications Receiver

This circuit is typical of the mid 1930s to late 1940s period.  The power supply circuit is not shown, but might have used a 5Y3 or 80 rectifier.

Representative late 1930's superhet

The set  is very similar to (but not the same as) many radios of that period. For example, the National NC100 is very similar, the main difference being that the NC100 uses an "anode bend detector" instead of the conventional diode detector depicted here. To simplify matters, the coil switching arrangement is not shown - plug-in coils could be used.

The set would have AF Gain (Volume), RF Gain,Sensitivity (Manual IF Gain), Crystal Phasing, Selectivity and the normal tuning controls. A BFO (Beat Frequency Oscillator) can be switched in for CW reception. Single-Sideband (SSB) was yet to be invented, so the set would not work well in this mode.

The set has a single RF amplifier stage (6K7 or KTW63). The gain of this stage can be set manually, which would provide some defence against strong signals. The sensitivity control effectively provides increasing negative grid bias, and since the RF valve is  vari-mu type, the gain is gradually reduced. The RF stage would be effective in overcoming noise from the next (frequency changer) stage. The RF valves suggested are not very effective beyond about 20MHz, but are adequate for broadcast reception (e.g. BBC on 21.470 MHz as was the case until recently).

The local oscillator uses a 6J7 (The non vari-mu equivalent of the 6K7). HT is fed from the normal HT line. Many sets would use a VR150/30 voltage stabiliser for the HT in order to provide oscillator stability. (There's probably a name for this oscillator - which I will have to research). its the same circuit used in the two valve radio's detector stage.

There are two IF stages and you will notice a variable resistor in the cathode circuit of the two valves. The idea here would be to set the IF gain to a reasonable level - as high as possible without the IF stage breaking into oscillation! One of my home-made efforts suffered from this very annoying problem. If you are considering building this (or any other) superhet, be very careful to keep input and output leads to the valve short  and well away from each other. You might have to do what Eddystone does and put 12 Ohm stopper resistors in series with the grid. Note the AGC (Automatic Gain Control) . This is controlled by a separate AGC diode. The AGC is not "delayed" as far as I can see so will begin to operate even on weak signals.

The 'S' meter is in a bridge arrangement and will be quite annoying, as it will respond to changes in the HT voltage. Still, its a very standard arrangement used by many sets.

The crystal filter is the same circuit used on many  radios since that date. The phasing control provides a "notch" which is very useful for removing unwanted stations when receiving CW (it will also remove heterodynes from interfering AM stations.) The other control should, I think, read "selectivity", rather than "sensitivity". There seems to be no provision for switching out the crystal filter, so this design might not suit broadcast listeners. 

The BFO uses the same oscillator circuit as the local oscillator, and is coupled to the detector stage via a small capacitor (often omitted.) Altering the BFO tuning will change the pitch of the morse CW signal received.

The audio stage is a very standard circuit.

Similar designs to this were to carry on well into the 1960s. The 6K7s became 6BA6s, the mixer oscillator became the ECH81, and so on. The Eddystone 940 used the same crystal filter, and used a rather fancy ECC189 valve ahead of the first RF stage. 

A 1950's Tranny

The next example is a transistor set that never was. It is taken from Mullard's "reference Manual of Transistor Circuits. Unlike the early valve radio above, you might encounter some difficulties if you wanted to make this today.

1950's transistor radio design

By this time, the "Ferrite Rod" antenna had been invented and went under a number of names. I think Zenith's "Wavemagnet" was a form of ferrite rod antenna. Indeed, this circuit has much in common with that used in Zenith's "Royal 300" transistor radio, except, of course, the Zenith uses American transistors. Early transistors were very noisy and the OC44 probably struggled a bit to work at the higher medium-wave frequencies.  

The OC45 IF amplifiers are straightforward enough, but require neutralisation - the RC network at the top of the diagram. The first OC45 gets an increasingly positive base voltage depending on the strength of the received signal.  This is developed by the OA70 detector diode. The OC45 is by no means a "vari-mu" device and will cut-off quite suddenly, but in the 1950s transistors were a novelty and nobody minded a bit of terrible AGC.

The amplifier uses two OC72 transistors biased into "Class B" in a kind of totem-pole arrangement. (I have seen Philips do this with valves). I used a similar circuit in my home-made transistor communications receiver, but blew quite a few output transistors before I got it right. If you are going to try to build something like this - you may prefer to use a transformerless design, or cheat completely and use an audio IC. (TBA820M from Mantech, for example.) You may be able to get OC45s, OC71s and OC72s from RSE electronics. They also have suitable IF transfomers, and may even have ferrite rods. I think they also have suitable variable capacitors. But really, you shouldn't try to build this - just use it as an example.

Final warning - the pocket radios of today have almost nothing in common with the above circuit. Open up a Sangean Pro-Travel or a Chinese radio and you will find - one chip. Maybe a YD1131, or something similar. Not as much fun, I don't think. 

AM/FM Tuner

Finally - an AM/FM superhet from GGJ King's FM Radio Receiver Handbook:

Representative AM/FM tuner

This circuit is quite reminiscent of some Grundig radiograms. The key similarity being the one and only AM IF amplifier stage. In the Grundigs I have recently bought to life, there is always space for an extra IF amplifier. Note that the FM section uses the AM mixer as an IF amplifier.

In these sets, there are dual IF transformers. These operate at a 470kHz (often in European sets) intermediate frequency on AM and 10.7 MHz for VHF / FM. The FM radio frequency circuits often consist of a VHF double-triode, such as an ECC84, although a Sansui came to me with a 6C9 ten-pin double triode. The triode valve provides low-noise amplification at VHF, and the second triode mixer/oscillator also provides stability. Usually, this section is in a little compartment of its own. Tuning is not done with a capacitor, but usually with a permeability tuning arrangement, in which the inductance of the coils is varied.

The FM signal is amplified by the valves V2 and V3 (ECH81 and EF89 ?) and the FM signal is demodulated with a ratio detector circuit. (I think S15 and S16 should have one or the other closed? - both open will stop the set working, surely). At any rate, it looks as if AVC is applied to V3. This is fine for the ratio detector, but would not be too good if a Foster-Seeley discriminator were to be used. Somewhere, there is a de-emphasis circuit - the 47k resistor and 0.001u capacitor.

The AM radio section is simply V2, V3 and V4a as the AM detector. V4 would have been an EABC80 - triple diode triode. The output stage would probably be an EL84.

Note V5. This might have been am EM84 bar type tuning indicator, or maybe an EM71 magic fan. Some sets still stick to the EM34 magic-eye (different circuit). The AGC voltage is used to cause the indication to change.

In South Africa, the favourite radiogram seems to have been the "Pilot Dual". Alas, this is nothing like the above circuit. The Pilot Dual is really two separate radios with a common (or not) audio stage. The "or not" is because you could have AM on one stereo channel with FM on the other.

Finally,

I hope you like that brief tour of some typical circuits. You can use them to help you understand your own radios, or you can use them for your home-built projects.

Last Updated on Thursday, 08 August 2013 15:02
 
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