HOW TO GET THE MOST OUT OF YOUR READY-MADE RECEIVER
No. 7: THE GREBE SYNCHROPHASE RECEIVER
This series of articles explains the theory, operation, equipment and care of standard receiving sets. This series does not indorse the product of any manufacturer or make comparisons between receivers. The sets already described include: No. 1, the Eagle Neutrodyne; No. 2. the Radiola Superheterodyne; No. 3, the Melco Supreme Receiver: No. 4, the Crosley Trirdyn; No. 5, the De Forest Reflex: No. 6, the Atwater Kent.
By S. GORDON TAYLOR
[--- Figure 1 will be posted as soon as it is completed --- 5/1/04]
WIRING DIAGRAM OF THE GREBE SYNCHROPHASE RECEIVER
Figure 1: The designating letters refer to parts described in the text and also shown in the other figures.
The Grebe Synchrophase receiver is of the tuned-radio-frequency type and makes use of the neutralization method for preventing oscillation in the radio-frequency amplifier portion.
An oscillating condition of the vacuum tubes in a radio-frequency amplifier usually prevents -- or at least distorts -- reception.
The feed-back in the circuits of a tube results from magnetic coupling between coils in the output and input circuits of the tube and also from the coupling effect of the capacity between the tube electrodes. The former type of coupling is known as "inductive coupling," while the latter is called "capacitative coupling."
In the Grebe receiver the effect of inductive coupling is practically eliminated by the use of a special type of coils, known as "binocular" coils.
This name was derived from the shape of the coil, a picture of which is shown in figure 3. With this arrangement any inductive coupling from another
part of the circuit will affect both halves of the coil but in opposite directions; therefore, the induced energy in one half will oppose the induced energy in the other half. And inasmuch as the induced energy is equal in the two halves, the resulting energy flow for the entire coil is zero. Thus the undesirable inductive coupling is eliminated.
In the wiring diagram, Figure 1. coils 2 and 3 of the coupler RFC represent the two halves of the binocular coil. Coil No. 1 of this coupler is the primary coil and is connected in the plate or output circuit of the first tube while coils 2 and 3 are in the grid or input circuit of the second tube. Coil No. 1 cannot be seen in Figure 3 as it is placed inside of coil 2.
The capacitative coupling is eliminated in this receiver by means of the neutralization method. As was explained before, a condenser inserted across the plate and grid circuits of a vacuum tube results in a tendency for some of the energy to flow from the plate circuit back to the grid circuit, in which case the tube becomes a miniature generator of radio-frequency currents; in other words, it reaches a condition of oscillation, thus interfering with its functioning as an amplifier.
In the radio-frequency amplifier a condenser is not actually inserted between these two circuits, but unfortunately the opposing surfaces of the grid and plate of the vacuum tube act as such a tiny condenser. lnasmuch as these two elements are indispensable to a vacuum tube used as an amplifier, some means must be found of combating the tendency to oscillate. To do this another circuit is added and so arranged that its current-flow tendency is opposed to that from plate to grid.
In Figure 1, the capacity which causes the trouble is shown by dotted lines at VT 1. The neutralizing circuit consists of the coil N-V and the condenser VC4.
How the Receiver Works
The signals from a broadcasting station are intercepted by the antenna and are brought to the receiver through terminal No. 1. From here they pass on through condenser C1, or switch S1 if the latter is closed. This condenser is provided to adapt the receiver to oversized aerials. By allowing the incoming signals to pass through this condenser the same effect is obtained as would result from using a smaller antenna. With a normal antenna the switch S1 is closed, thus short circuiting C1 and allowing the signal to pass on without going through the condenser.
The signal energy then flows into the coil of RFC1 at A and passes through a few turns to B which is connected to the ground.
The coil A-B is part of the entire coil of RFC1, and therefore a flow of energy through coil A-B induces a similar current in the entire coil by conductive coupling. This whole coil is the secondary of the coupler and is connected in the grid circuit of the first radio-frequency amplifier tube, VT1. It is tuned to resonance with the incoming signal by means of the variable condenser VC1. The connection to the grid of the tube may easily be followed down through the switch S2 (which must he connected to point 1 when an outdoor antenna is used) and from there to the grid.
THE PANEL VIEW OF THE RECEIVER
Figure 2: Tuning is accomplished with the three upper controls, and the three lower controls which permit extremely small variations for sharp tuning. The left-hand knob controls the volume and is usually set on point 4 or unless extreme volume is desired, when it is set on 6. The other knob controls the filament current
THE GREBE. BINOCULAR COIL
FIGURE 3: By winding the coil in two connected parts its magnetic field is limited and is little affected by stray currents from other parts of the circuit.
When a loop antenna is used it is connected to the terminals marked "Ground" and "Loop". In such a case, the coil RFC1 is not used. The loop acts not only as an antenna but also as a coupler coil. By connecting the loop as explained, and throwing, S2 to point 2, the signals picked up by the loop are impressed on the grid of the tube as before, and the loop is tuned by means of the condenser VC1.
The signals impressed on the grid of VT1 result in amplified signal energy flowing in the plate circuit of this tube. The additional energy required for the amplified signal is obtained from the high-voltage "B" battery which connects to the plate of the tube through point X of the primary of coupler RFC2.
By the process of electromagnetic induction the energy flowing through this primary sets up a similar flow of energy through the secondary coil which consists of coils 2 and 3. This secondary coil is also tuned to resonance with the incoming signal by means of condenser VC2 and the signal is impressed on the grid of the second radio-frequency amplifier tube VT2. Here it is further amplified in the same manner as before and passed on to VT3 through the coupler RFC3.
Up to this point: the signal has been twice amplified but is still in the form of an alternating current of too high a frequency to be audible to the human ear. The next step, therefore, is to change its form to make it audible, and this is the function performed by VT3 which is called the detector tube. The condenser C2 and the grid-leak R3 are provided to aid in this rectifying action.
The purpose of radio-frequency amplification is to increase the sensitivity of the receiver; therefore, if head-phones could be connected in the output circuit of VT3 signals would be audible, even from distant broadcasting stations.
Having provided ample sensitivity, the next requirement is to increase volume to make possible the use of a loudspeaker. It is for this purpose that the audio-frequency amplifier, consisting of VT4 and VT5 is used_. The coupler coils used are different from those used with the first and second tubes but their function and circuits are much the same. The energy in the detector plate circuit passes through the primary of the coupler (audio-frequency transformer) AFT1 and induces a current flow in the secondary which is impressed on the grid of VT4. The signal is then amplified through the action of vacuum tube VT4, is passed on to VT5 in the same way and is there again amplified. From VT5 it passes to the loudspeaker.
In the case of signals received from nearby broadcasting stations the volume in the loudspeaker would be too great for comfort in an ordinary room so the variable resistance R2 is provided to permit reduction of volume to a desired degree.
Construction of the Receiver
The general construction of the receiver is clearly shown in the accompanying illustrations. The set is intended for use with external "A," "B" and "C" batteries and with either an outdoor, an indoor or a loop antenna.
All of the parts are made by the Grebe Company, and are especially designed for this receiver.
One feature of the three large variable condensers used for tuning lies in the fact that they are of the so-called `'straightline-frequency type." With the ordinary type of condenser the lowwave broadcasting stations come so close together on the lower part of the condenser dials that it is sometimes difficult to separate them and the tuning of such stations is extremely critical. With the Grebe type of condensers, however, this difficulty is partially avoided. The spacing on the dials between low-wave stations is greatly increased while spacing between the higher-wave stations (where great spacing is not needed) is
THE REAR VIEW OF THE "WORKS"
FIGURE 4: The instrument layout is here clearly shown. The designating letters are the same as those shown in Figure 1 The balancing condensers VC4 and VC5 are in plain view here; the use of them is explained iii the text.
ANOTHER INSIDE VIEW
FIGURE 5: The instruments which are not mounted on the panel are all on a sub-base, so that the whole set may be removed from the cabinet for inspection or repair. Experimenters are not advised to attempt such removal unless it is absolutely necessary, however; even then it is better for the novice to leave such work to the dealer or the manufacturer.
decreased. In effect this spreads the entire broadcast waveband evenly over the whole scale on the dials, thus simplifying the tuning process.
The design of the neutralizing condensers VC4 and VC5 also facilitates obtaining maximum results from the receiver. Neutralizing condensers are usually adjusted and sealed before a manufactured receiver leaves the factory. Difficulty is sometimes encountered where this is the case because tubes, other than those used when the receiver was adjusted, may have different internal capacities, thus making the neutralizing circuit only partially effective. In the case of the Grebe receiver the owner of the receiver can readjust his own neutralizing capacities to take care of any variations in his own tubes. Instructions for this neutralizing adjustment will be given later.
.A condenser, VC6, is provided to permit synchronizing the dial settings of the condensers VC2 and VC3. by once adjusting VC6 these two variable condensers can be made to tune just alike for a given wavelength, therefore a given broadcasting station may be tuned in with both of these condensers set at exactly the same reading.
The type and layout of controls on the panel are such as to permit operation of the three-tuning controls at the same time. The controls for condenser VC2 and VC3 can be moved simultaneously with the thumb and first finger of the right hand, while the control of VC1 is being moved with the left hand. This eliminates some of the difficulty in tuning encountered in many receivers which have three controls.
The Antenna and Ground
Almost any antenna is suitable for use with the Synchrophase receiver. A wire strung around the picture moulding will give good results, especially if it runs through three or four rooms in nearly a straight line, rather than around the four sides of one or more rooms. Better results will, of course, he obtained with an outdoor antenna, which may be anywhere from 50 to 200 feet in length. Probably the best all around arrangement is a single wire about 100 feet in length including the lead-in. The layout of such an antenna is shown in Figure 6. A longer antenna will produce greater volume and distance but will have a tendency to broaden tuning thus resulting ill interference between stations operating on nearly the same wavelength. In any event the condenser C1 and switch S1 help to take care of this, as explained above.
A short antenna will not produce quite as much volume on distant stations nor will it permit reception from such great distances as will the 100 foot wire. However, a short outdoor antenna (or an indoor antenna) picks up less static and this is naturally an important consideration for even local reception during the summer.
The ground connection may he made to a cold water pipe or any other pipe system, which connects with the ground -with the exception of gas pipes. Hot water pipes or steam radiators usually provide suitable ground connections but gas pipes are usually insulated from the ground by rubber washers in the pipeline near the meter.
If no pipelines are available in the house a quantity of copper wire buried in moist ground or dropped into the water of a well or cistern will serve. In any case a good electrical connection should he made to the ground line
A SUGGESTION FOR THE ANTENNA LAYOUT
Figure 6: The design of the average antenna is necessarily governed by physical conditions such as yard space-limitations. Regardless of conditions many of the ideas incorporated in antenna illustrated in this figure may be used to advantage. The antenna for use with the Grebe receiver should preferably be about 100 feet in length, measured between insulators, although good results are obtainable with shorter spans.
TWO HANDED OPERATION OF THE THE THREE TUNING CONTROLS
Here the author demonstrates the method of tuning the three dials simultaneously, thus eliminating the necessity for a third hand.
selected. If it be a pipe any paint or other coating on the pipe should be filed off until the pipe is bright and shiny at the point where connection is to be made and connection made by means of a regular ground clamp.
If for any reason the use of an antenna is not practical, a small loop antenna may be used. This is connected to terminals 2 and 3 (Figure 7). A lightning arrester may be connected as shown in Figure 6.
What Vacuum Tubes to Use
This receiver is designed to use vacuum tubes of the UV-201-a type throughout. These may be the UV-201-a, the C-301 -a or the De Forest DV-2, These are all "hard" tubes which require the use of a storage "A" battery for filament-current supply, and consume approximately 1/4 ampere each. The use of a "soft" detector tube, such as the CV -200 is not recommended. A special model of this receiver is on the market for use with dry-cell tubes -- one which is especially designed for operation with these tubes. Satisfactory results will not be obtained if dry-cell tubes are used with the model described in this article.
What Batteries to Use
This receiver uses three separate types of batteries the "A," "B", and "C" batteries.
The "A" battery must be of the storage type. This storage battery may have a rated ampere-hour capacity of between 75 to 150. A battery charger is also desirable so that the battery may be charged at home, thus avoiding the inconvenience of carting the battery to a neighboring charging station; or the expense of having the battery called for and delivered by the charging station.
Connections for a battery charger are shown in Figure 7, for use where the house is equipped with alternating current for lighting.
In the case of a direct-current house supply it is important that the polarity of the current first be determined. This may be done by holding the two ends of the leads (from a light socket) a half-inch apart and clipping them into a cup of water to which a half teaspoonful of salt has been added. Bubbles will rise from one of the wires when the current is turned on, and this wire is the negative ( ) side of the line. ln making this test there is no danger provided insulated wire is used and the hands are not allowed to come in contact with the bare ends of the wires. Connections for a bank of lamps (to be used for the charging resistance with direct-current supply) are shown in Figure 8. No other apparatus is needed for charging from direct-current mains.
It is essential that a hydrometer be obtained also, to determine when it is in need of recharging. When the hydrometer shows a reading of 1,185 it is an indication that the battery needs charging. A reading of approximately 1,285 indicates a fully-charged battery. Actually a fully discharged battery will give a reading of about 1,150 but it should never he allowed to run down to this point as the battery is quite likely to be injured.
The "B" battery consists of four 22-1/2-volt batteries, or two 45-volt units. In either case the total voltage required is 90. Large size batteries (either 22-1/2 or 45-volt units) are desirable because their life is approximately 16 times that of the small size of the same voltage; and the initial cost is only about twice the cost of the others. The new extra large "Heavy Duty" batteries are recommended, however, as they offer still greater economy. This is made clear in the table on page 126.
Storage "B" batteries may also be used. In fact the use of them will reduce the battery upkeep cost in the long run and the service obtained from them, provided they are given an occasional charge, will he as satisfactory as that obtained from batteries of the dry-cell type.
Charging equipment for such batteries may be purchased for a small sum
CONNECTIONS FOR THE COMPLETE EQUIPMENT
Figure 7: In addition to the connections for the batteries, loudspeaker, antenna and ground, connections to the battery charger are shown. A switch is incorporated in the "A" battery circuit. When thrown to the up position the battery is connected to the receiver and when thrown down connects to the charger: there is then no danger of starting the charger while the battery is still connected to the receiver.
CHARGING THE "A" BATTERY FROM DIRECT CURRENT HOUSE SUPPLY
Figure 8: In this case no changer is needed. Only resistance is required and this is furnished by the use of five ordinary 100 watt lamps connected as shown. If desired a special charring resistance may be purchased to us t, in place of the tamps.
and where the house supply is alternating current an attachment may be purchased to permit the regular "A" battery charger to he used for charging the storage "B" batteries also.
The "C" battery is of the dry-cell type and consists of a regular 4 1/2 volt unit sold for this use. There is practically no current consumed from this battery and its life is therefore long. It will he good until it just naturally dies of old age. It will very likely last a year.
The Operation of the Receiver
The connections for the batteries, antenna. ground and loudspeaker are shown in Figure 7. These connections are all self-explanatory, with the possible exception of the connections to the terminals marked 1, 2 and 3 in Figure 7. When an outdoor or indoor antenna is used, it is connected to terminal No. 1. When a loop antenna is used, however, its two ends arc connected to terminals 2 and 3 and terminal No. 1 is left unconnected. In either case the ground is connected to terminal No. 2. \\'hen an outdoor or indoor antenna is used the switch S2 is swung around so as to make contact with point No. 1 (the left-hand point looking down on the receiver from the top). When a loop antenna is used this switch arm is connected to point No. 2.
After the receiver has been connected up in accordance with the above, turn the "volume increase" knob to point 6 and the "filament increase" knob to zero. Next insert the five vacuum tubes in their sockets and turn the "filament increase" knob to 4. This will light the tubes and the receiver is then ready for operation.
To tune in a station the dial settings shown in the tuning chart below will be approximately correct for all receivers of this type and model. The best plan is to select a powerful nearby station which is known to be in operation at the
Table 1: This table shows the effect of the "C" battery on the life of the 'B" batteries. Greatest economy inn operation is obtained when using -11 ., Bolts of "C" battery. The table also shows the much longer life of the "Heavy Duty" type 'B" batteries.
time. Then consult the chart to ascertain the dial settings for that station. If that particular station is not shown on the chart, set the dials for the station shown, with a wavelength nearest to that of the station which you desire to tune in. If the receiver is in operation in Los Angeles, for instance, and KHJ of that city operating on 404 meters is the station which you wish to tune in, set the dials as shown on the chart for station WOR which operates on 405 meters. These dial settings will he nearly correct for KHJ; only a slight variation will bring in the desired station.
After one station has been tuned in in this manner, it will he a simple matter to tune in others. The three dials should he moved in unison, manipulating the left-hand dial with the left hand, the other two with the thumb and forefinger of the right hand. This movement should be slow, as it is an easy matter to slip over stations, especially if they be distantly located. When another station is heard readjust each of the three dials slightly to bring it in with maximum volume.
After this tuning process has been practiced a short time it will be well to investigate some of the refinements of tuning. First of all, try different settings of the "volume increase" knob while a station is tuned iii. Points 3. 4 or 5 usually provide plenty of volume on all but distant stations. In a large hall, or in receiving distant stations point 6 is useful, otherwise it will give too much volume for comfort in an ordinary room.
Next try interchanging the tubes from one socket to another. One tube will sometimes work better in the third socket than in the fourth, etc. The best combination can be determined in a few minutes and the tubes may be left permanently in the positions in which they give the best results.
These receivers are carefully balanced before they leave the factory, but when the purchaser of the receiver puts it into operation he may find that it oscillates or whistles when he tries to tune in stations in the lower or higher wavelength.
Where this condition is found it will be necessary for the owner to rebalance the receiver himself, or take it to his dealer to have it done. It is not a difficult task, however, and it is to facilitate this process that the adjusting screws on condensers VC4 and VC5 are provided.
If it is found that the receiver oscillates only when high-wave stations are tuned in the screw on VC4 should be turned slightly in the clockwise direction, keeping the signal tuned in in the meantime. A small screw-driver with a wood handle is recommended for this purpose. This screw should he adjusted in this direction a little at a time until the oscillation ceases.
If oscillation takes place only on the low-wave stations, an adjustment should he made on condenser VC5, in the same manner described for VC4.
In some cases it may even be found that the receiver oscillates to a certain extent at all wavelengths. In such a case the balancing process is somewhat different. The receiver should he left all connected in readiness for reception, with the tubes turned on. The second and third dials from the left are then set at 40, and the left-hand dial at 100. Dial No. 2 is then rotated between 30 and 50. .At some point a click will probably be heard. If not, turn the screw of VC5 in an anticlockwise direction until a click is heard as dial No. 2 is rotated back and forth from 30 to 50. Then turn the screw in a clockwise direction until the click disappears, and approximately a quarter turn beyond this point. This finishes the balancing of VC5.
Next set all dials at 40 and adjust balancing condenser VC4 in the same manner just described, but this time rotating the left-hand dial back and forth between 30 and 50.
There may still he slight signs of oscillation on the high waves, or possibly the low waves. If so a small adjustment of the balancing condensers as described first will remedy the trouble.
Finally it is well to know that the "C" battery voltage used has a strong influence over the life of the "B" batteries and therefore over the upkeep cost of the receiver. The tabulation above demonstrates that the life of the "B" batteries is much greater when the "C"' battery voltage is 4 1/2 than when it is 1 1/2 or 3. On the other hand, it is sometimes found that the tone quality and reproduction of the receiver improve somewhat with a decrease of "C" battery voltage from 4 1/2 to 3. This evidently is not always the case. but where it is found to be so, it is up to the operator to decide whether tone quality or maximum economy is the more important consideration.
Charting the Receiver
Tuning will be simple if a record sheet is kept, on which are recorded the dial settings for each station as it is tuned in for the first time.
A record of a few stations is shown below, in the form of a convenient chart. The antenna used when this chart was made up was a short one and there was no occasion to make use of the condenser C1. If a long antenna is used it will be necessary at times to connect C1 into the circuit no doubt. For that reason it will be well to add another column to the chart to note whether or not S1 is open or closed when the dial settings for each station are recorded. This is done because the setting of the dial of VC1 varies slightly according to whether S1 is open or closed.