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Because frequency modulated transmission is established as the system of modulation of transmissions on the 90 Mc/s VHF Band II, The Radio Constructor is featuring this article on a suitable tuner.
The construction and alignment of such a tuner is no more difficult than if it were a normal medium-wave tuner, and most of the coils are easier to make.
The same number of stages would be used, and in this case only three stages would be required Frequency Changer, IF Stage, and Ratio Detector. This tuner uses two more stages-RF and Limiter, the purpose of which will be described.
The medium waves are, of course, already overcrowded, and even with a limited audio range a whistle filter must be used to intercept the carrier of the adjacent stations. Also, the sidebands of the adjacent stations will be heard and the background can never be really quiet except under very favourable conditions. VHF was therefore the only choice, and this has the advantage that the range of transmissions is not much greater than line of sight, and so stations spaced 200 miles apart can use the same frequency with no danger of interference. This does not mean that the service area of the VHF station is less than that of a MW one, since the latter will not give a first class service area much greater than that covered by line of sight, although of course poorer signals can be obtained at much greater distances.
The decision to use FM was nearly unanimous by the committee set up to study the problem. One point made against FM was that it was more complex and difficult to align, and to ensure the life stability necessary to achieve the best results. This, surely, is a challenge which will be met by manufacturers, and is met in the tuner described in this article.
Pre-war life-stability of 465 kc/s IF's was something of a problem when almost universal use was made of compression condensers for tuning, but with the advent of dust iron cores this problem has nearly vanished. The simplest possible FM set is more complex than the equivalent AM set, but when the latter is fitted with the necessary impulse limiting circuits there is not much difference.
A simple listening test, though, at almost any position except, perhaps, within a couple of miles of the station, showed the superiority of the FM signal as regards freedom from Impulsive interference and far lower background noise. Complex limiters on the AM receiver could, of course, improve the rejection of the impulsive interference, but there is nothing that can be done about the background noise. This test could only be made when the BBC were radiating the same programme on both FM and AM.
It might be expected that the AM transmission would require a narrower band width receiver and that there could be more transmitters in the same band. Unfortunately, though, the complex limiters used relied, for optimum working, on having a wide bandwidth in the receiver in order to produce Interference pulses with a sharply rising wave shape. When this is taken into account, and also one remembers that the stability of VHF oscillators is not so good as lower frequency types, one realises that the spacing could not be less than 0.25 Mc/s between stations. This is, in fact, the spacing chosen for FM transmissions.
FM transmission can use any nominal band width; the choice of the BBC has been that +75 kc/s deviation of the VHF carrier represents full modulation, and this happens at maximum audio amplitude at the frequencies normally encountered.
As Band II transmissions have approximately the same service area as Band I TV transmissions, it is of interest to note that most of the new TV aerials are fitted with slot aerials, which are capable of radiating three FM programmes simultaneously.
The service to be expected from Wrotham is shown on Fig. I by the thin black line.
The receiver will now be described stage by stage.
An RF stage is included for two main reasons, one being to prevent oscillator voltage getting directly into the aerial circuit and being radiated from the aerial to the annoyance of all local TV users. Another important feature is that the aerial circuit can be isolated from the frequency changer input, and optimum design achieved in each. The aerial coil is designed for maximum gain rather than selectivity. Both the aerial and grid circuit of the valve damp this coil, the two together being equivalent to about 800kΩ across the coil. If the spacing of the aerial coil is set to minimise this damping, a loss in this coil of about 4 times would be found. In order to improve the signal-to-noise ratio, the maximum possible step-up is needed, since the valve the first "noisy" item that the signal meets. The coil has been wound directly on the dust iron slug, and the aerial coupling coil wound as tightly as possible in the middle of this coil. This achieves a gain of about 3, and the tuning is so flat that there is no need to adjust this circuit. There is, therefore, no second channel rejection here, but this coil is very effective against direct IF interference. The coil in the anode circuit is not so heavily damped, and a fair Q can be achieved. The fact that the grid circuit is heavily damped makes this point ideal for the injection of the AVC voltage, since changes in the input capacity of the valve will have very little effect.
The circuit shown was found to be the most effective ever tried at these frequencies. It may seem rather strange to mix in the oscillator valve and put the IF in its anode circuit, but it is really only the old hexode circuit brought up-to-date.
Various other types of circuit were rejected for the following reasons:-
Normal triode hexode
The triode valve is usually on top of the hexode section and so the leads are rather long. The internal capacities are much higher due to the mixing electrodes, and these points all make for poor frequency stability. The gain is lower and the noise figure higher than the circuit chosen.
Twin triode frequency changer
This is a good type of circuit for these frequencies, and until recently was the most common type to be found in American TV receivers. Oscillator stability is good, and channel changing by switches is easy. Conversion conductance is poor due to the heavy damping on both the RE grid circuit and the IF anode circuit due to the Miller capacity between grid and anode.
Separate triode oscillator and pentode mixer
Approximately equivalent performance to the circuit chosen, but of course, the separate valve is a considerable extra cost.
Triode pentode frequency changer
Approximately equivalent performance to the circuit chosen, but the valve is more expensive. This type is best of all where channel switching is required.
The circuit, given here, is easy to align and is very stable. It is, in effect, a tuned-anode tuned-grid oscillator, and therefore no critical coupling is required between L2 and L3, and these windings should be spaced 1/4in apart. L3 has only 4 turns, and this coil is tuned by parting the turns to give a maximum limiter grid voltage reading. L2 is the main frequency controlling circuit and, as this has no grid capacity connected across it, the oscillator is inherently very stable. Since the valve is oscillating, a voltage of about 5 is built up at the grid, and this is highly suitable for mixing. A gain of about 15 is achieved in this stage instead of about 5 if a normal miniature triode-hexode were used.
The intermediate frequency is 10.7 Mc/s, as this is becoming a standard IF for FM receivers. Coil winding is reasonably easy, and the frequency high enough to achieve a good second channel rejection in the RE stages. IF's of between 5 and 21 Mc/s have been used in the past.
The coupling is chosen to be slightly over-coupled in order to achieve a band width of about 250 kc/s. This leaves ample margin for drift and ageing. Automatic gain control is not used here since the resulting change of valve input capacity would adversely affect the frequency response and cause distortion.
This tuner will perform quite well without this stage; indeed, many sets do not use one. But the use of this stage considerably improves the rejection of impulsive interference and also avoids the need for carefully balancing the ratio, detector component values. Even when these components are carefully balanced the AM rejecting properties rapidly deteriorate as the set is detuned, and therefore frequency stability becomes more important. The use of this stage makes the warming-up drift unnoticeable, and also makes redundant the usual balancing potentiometer required in the ratio detector circuit.
The Ratio Detector was chosen in preference to the Foster-Seeley discriminator because of the better AM rejecting properties of the former. Winding instructions for this coil should be carefully followed as achievement of good balance and low capacities improve the circuit performance.
The operation of the Ratio Detector is as follows:
Two voltages appear at the diode anodes which vary in relative phase as the signal frequency changes, and this results in the appearance of an audio signal proportional to the original modulation appearing across C18, which is, of course, made large enough to integrate it into a continuous wave, but not so large that the high frequencies are affected. This phase difference of the signals derives from the fact that the voltage-current phase relationship in a tuned circuit varies as the frequency is altered, and are only in phase at the resonant frequency. Such a voltage is impressed on the diode anodes by the inductive coupling between the primary and secondary. Also impressed on the diode anodes is a reference voltage derived from a tightly coupled coil wound directly on the anode coil, and this is impressed equally on both diodes by being fed into the centre tap of the secondary coil.
The aerial coil is wound in the screw thread of the core and the ends tied together with cotton. After winding one turn of PVC the whole coil is “distrened.” The core is 1/4in long and is a normal core cut in half. The diameter of the slug is *in, and it has a 2-BA thread moulded on it.
This length should be carefully followed in order to make the coil tune in the correct part of the band.
The oscillator and RF anode coils are wound on a 5/16in mandrel in a clockwise direction. They should be mounted about 5/16in below chassis, and one lead left long enough to go through the chassis to the condenser. The spacing should be approximately 1/4in.
The IF transformer calls for no comment except that the coils should be wound in the same direction, and the opposite ends are made “live” to the signal. Care should be taken that the condensers do not short to the can.
The anode coil is wound normally, and then one turn of polythene tape or paper is wound over the coil. The coupling coil is wound at the earthy end of the anode winding. Normal clear office tape should not be used since the sticky side absorbs moisture, and will also attack the copper wire.
The closely coupled diode winding is produced by winding 4 wires 24in long together, 15 turns being wound. The winding is fixed with polystyrene dope. After about 10 minutes two alternate wires are removed leaving a spaced bifilar winding. Do not lose patience with this, as a spaced winding is necessary to reduce the capacities.
The Osram valve type Z77 has been found very suitable, together with crystal diodes type (3EX34. Low output may be caused by poor crystals. Some cheap crystals of a well-known make have been found quite effective. Others provide a heavy damping on the ratio detector coil, and spoil the linearity.
A diode in the ratio detector is not recommended because IF harmonic feedback occurs along the heater line:
R3 6.8kΩ 1/2W
R6 47kΩ 1/2W
R5 4.7kΩ 1/2W
R11 47kΩ 1/2W
Resistors all 1/4W unless otherwise stated.
C1 1,000pF ceramic
C2 1,000pF ceramic
C3 1,000pF ceramic
C4 47pF silver mica
C5 33pF silver mica
C6 33pF silver mica
C7 5,000pF ceramic
C8* l5pF silver mica
C9 l5pF silver mica
C10 5,000pF ceramic
C11 5,000pF ceramic
C12 5,000pF ceramic
C13* l5pF silver mica
C14* l5pF silver mica
C15 47pF silver mica
C16 5,000pE ceramic
C17* l5pF silver mica
C18* 47pF silver mica
C19 300pF mica or ceramic
C20 0.05μF paper
C21 500pF mica or ceramic
C22 300pF mica or ceramic
C23 300pF mica or ceramic
C24 8μF 150V electrolytic
C25 l,000pF ceramic
C26 1,000pF ceramic
VC1-2Split stator 25pF+25pF.
* Supplied with coils.
V1, Osram Z77
V2, Osram Z77
V3, Osram Z77
V4, Osram Z77
D1, D2, G.E.C. GEX34 germanium diodes.
Coils, complete with condensers;
Dial calibrated in Mc/s, The Jason Motor & Electronic Co.. 328 Cricklewood Lane, London, N.W.2.
Formers and Cans
|IF1||L4||38||38DSC||3/16in||C8 l5pF s/m|
|38||38 DSC||C9 l5pF s/m|
|1F2||L5||38||38 DSC||1/8in||C13 l5pF s/m|
|38||38DSC||C14 l5pF s/m|
|Discriminator||L6 and coupling||29||40 DSC||3/8in||C17 l5pF s/m|
|15 + 15||38DSC||Cl8 47pF s/m|
|Heater Choke||-||See text||25 enam.||-||
The Z719 may be used in the RF stage as in the fringe area’-version described. The aerial coil as described previously is suitable for this valve. If a 6AK5 is employed in the RF stage, this coil is not suitable because the grid capacity is lower. As the input impedance is higher, a looser coupling of the aerial coil may be used. The same former as for the IF coils may be used, but cut off to about 1/2in long. 5 turns of 26 swg DSC wire are required, with 1 turn on the aerial coil. This coil must then be tuned with the dust iron core.
Note that the numbers on the coil bases are not given, because the i.f. transformers are symmetrical and therefore it does not matter which coil is used as the primary winding.
When mounting the IF coils, it is unimportant whether the anode coil is at the top Or bottom. What is more important is that the coils and valveholders should be mounted as shown in the drawings and photographs so that the lead from the coil goes direct to the appropriate contact of the valvebolder. If the coil connections are made long by wrong mounting, instability may result because these leads radiate the IF signal and this will be picked up in the early stages of the tuner.
Before switching on, check that the h.t. line is not short circuiting to the chassis. Check h.t. voltages: H.T. rail-220 volts; junction of R2 and R3-180 volts; g2 of V2-100 volts; anode of V12 -200 volts; anode of V3 -180 volts; g2 of V4 -50 volts.
Do not attempt the alignment unless some results are achieved at the outset. The coils supplied by Jason are approximately aligned and it should be possible to obtain reception from the local transmitter. An indicator of some sort will be required; an oscilloscope may be used, or a 10 volt meter with a 10,000 ohms per volt movement. Alternatively, a valve voltmeter may be used.
Connect the indicator to the junction of C15 and R10 using a l00kΩ resistor in series with the indicator. The resistor should be mounted close to the above junction, the object being to prevent detuning of the IF transformer by the capacity of the leads from the indicator.
A signal generator should be connected to the grid of V3, and L5 aligned to produce a symmetrical bandwidth of approximately ±200 kc/s around 10.7 Mc/s. This will not be achieved by stagger tuning, but by virtue of the fact that the IF transformers are overcoupled.
Transfer the generator to V2 stage across the oscillator coil; if the generator is connected directly to the grid, instability will result. Set L4primary and secondary to produce an overall bandwidth of ± 150 kc/s. If an asymmetrical curve is found, it may usually be cured by bypassing the heater of V4. Connect a I ,000pF condenser between pins 4 and 6 of V4.
(See Heater Supply Notes.)
To adjust the ratio detector, connect the generator to V3 grid and set to 100 millivolts output approximately. Connect the meter between chassis and one end of C24 and adjust both cores for maximum reading, taking care of the anode coil. Be carefu1 also that the cores are tuning at the farthest point from the centre of the coil, i.e. that one of the cores is not tuning between the coils. This can give rise to very peculiar results.
Transfer the meter across the junction of R12 and C20 and finally set the secondary of L6 to give zero voltage output. The output positive and negative as he core tunes through the correct position.
Alignment by means of a wobbulator is by far the best method of achieving the ideal result, since the effect of each adjustment is very much clearer. Faults are also more readily observed, particularly instability, which is immediately seen by distortion of the curve just prior to the stage under adjustment bursting into oscillation. Experience soon shows just how fast the curve shown on the screen should rise, and more rapid increases of the rise in effect predict instability prior to any actual trouble being experienced.
Connect the scope to the junction of C15 and R10, using a l00kΩ resistor in series to prevent detuning effects. Set the wobbulator to 10.7 Mc/s and the sweep width to approximately 1 Mc/s. Feed either into the wobbulator the output lead a 10.7 Mc/s signal, then adjust the amplitude of this signal to give a small mark on the cathode ray tube trace, around which the curve should be balanced.
Connect the wobbulator to V2 grid and align the IF’s to give a balanced curve around the marker. To align the ratio detector, connect the scope to the junction of R12 and C20, increase the marker amplitude, and adjust L6 to give an S-shaped detector curve. The marker amplitude should now be increased as far as possible without reducing the amplitude of this curve. Readjust L6 secondary.
Connect an amplifier and the aerial. If all is well, a rushing noise will be heard. If not, re-check connections and the foregoing tests, as it is unlikely that stations will be received satisfactorily. When performing this check, disconnect the noise reducing resistor mentioned later, if this is fitted into the circuit.
Stations should now be found at approximately the correct settings as shown on the dial. If this is not so, part or close the turns of the oscillator coil. Read the voltage developed at the limiter grid with the indicator, and set this to a maximum by parting the turns of the R.F. coil. The final spacing of the turns will normally be approximately one-sixteenth of an inch m the case Of L3, and one-thirtysecond of an inch spacing in the case of L2.
If the Jason chassis has a longer scale than that shown in the illustrations with the lowest frequency on the right-hand side of the scale, then a 10pF trimmer is required across the oscillator section L2 and a 5pF ceramic is needed across L3. These are not shown an the photographs or the layout diagrams and are a recent addition to the tuner. These changes enable a tuning condenser with wider plate spacing to be used, and the condenser is, therefore, less susceptible to microphony. Adjust the trimmer across L2 to give correct scale coverage. When set, tune to the Home Service and adjust the turns of L3 to give a maximum reading at the limiter grid.
The coils are approximately pre-tuned when received and, therefore, the final tuning of the core may entail withdrawing this somewhat. It is, however, possible to obtain a false peak by adjusting so that one of the cores lies midway between the windings of each coil. Both cores appear to peak tune, but the resultant gain will be not only of a low order but the bandwidth will be very wide. If in doubt, measure the depth of the core within the former.
The Jason Motor & Electronic Co. will align the tuner providing that the layout diagram has been followed and that the instructions generally, particularly lead lengths, have been faithfully followed as outlined in this series.
It should also be emphasised that the component parts should preferably be obtained from a supplier retailing either Jason, or other manufacturers’ equivalents, which are “Designer Approved.”
Many possible faults may be circumvented by adopting the correct layout as specified herewith, and it is assumed that this will be done by intending constructors. As an example of the possible outcome of alteration in layout, is the following-the layout drawing clearly shows Rt mounted near L5 placing this resistor near to V1 will inevitably produce IF instability. This circuit is normally very stable except for this point, and one other; the decoupling condensers C11 and C12 must make a total value of 10,000pF, and one condenser may be used. Either a ceramic or a non-inductive metallised paper condenser of small physical size is suitable. A mica condenser is usually too large, and normal paper condensers are inductive. The use of either of these two latter types will certainly produce instability.
It is very important to earth the centres of each holder. Doing so reduces the anode-to-grid capacity, and this is a very important factor in the final stability of the tuner. The cathode connection of V2 should be exactly as shown. If the audio output is low, check carefully that C24 is connected the right way round. The positive of the condenser should be connected to the red end of the crystal.
If this tuner is to be used with an amplifier having a centre-tapped 6.3V heater supply, then the heaters must be wired as shown in the inset in the circuit diagram, using heater chokes L7 and bypass condensers C25 and
C26. In some cases, an additional bypass condenser may be needed at the heater of V4 connected between pins 4 and 6.
This heater filter circuit is to prevent harmonic feedback along the heater line; this fault can easily be recognised by the fact that dead spots occur at harmonics of the IF, i.e. at 85.6, 96.3 and 107 Mc/s.
The specification of the chokes is as follows:
Using 1/8in diam. former, wind on as many turns as possible of 26 swg enamelled wire to make a winding length of 1 7/8in.
If one side of the heater is earthed, then no heater cokes are necessary and only one condenser, C25, is required. This is connected from the heater of V2 to chassis. In thin case, also, pin 4 of each valve should be earthed.
Up to a distance of some 20 miles from the station, the dipole aerial may be formed from flex parted to give arms 2ft 6in long.
Between 30 and 60 miles, an outdoor dipole may be needed. At distances greater than 60 miles, a 3-element as described in the fringe area article on page 22 will be required.
The outdoor dipole may be made from 1/4in duralumin rod, each arm being 2ft 6in long with a centre spacing of 1/4in. The aerial should be mounted broadside on to the station in a horizontal position, as the waves are horizontally polarised.
If this should be experienced a 5,000pF ceramic condenser should be added between H.T.+ line and chassis.
This may be effected by connecting a 500kΩ resistance between the H.T. rail and the negative end of C24.
In the absence of a signal, this causes the diode to conduct and, therefore, suppress the inter-station noise. On tune, this additional bias is negligible and does not affect the performance of the tuner.
I guess that you may have a few problems finding "The Jason Motor & Electronic Co.. 328 Cricklewood Lane, London, N.W.2.", though I could be proven wrong ?!