It is necessary to tune the remote oscillator to the frequency of

the secondary coil and amplifier. As the range of the remote control device may be 75 feet under normal conditions, a variable frequency in both the oscillator and amplifier is provided so that no interference will be produced on neighboring mystery control sets. A choice of 5 frequencies from 355 to 395 k.c. is provided to eliminate the possibility of two or more sets interfering with one another.

The signals picked up by the loop are coupled to a tuned grid coil by a low-impedance link circuit. From the grid coil, the signals go through two stages of amplification to the grid of a 2A4G thyratron tube. The output from the thyratron tube is fed into the relays which in turn control the stepper unit and station selecting switch.

The Sensitivity Control.—In the control (pulse) amplifier circuit there is a sensitivity control which is employed for the purpose of adapting the set to the particular location where it is used. This control is in the cathode of the type 78 first amplifier tube.

The setting of this sensitivity control is of tremendous importance to the mystery control operation. The normal range of mystery control is within a circle of the receiver ^ith a radius of about 25 feet. It is important to remember that mystery control operates in a circle around the receiver cabinet. To get the most from mystery control it is therefore advisable to place the cabinet as close to the center of the "operating circle" as possible.

If the receiver be located against the front wall of a home only half of the effective operating area is within the house. The remainder is outside the walls. There is a distinct advantage in operating the control amplifier sensitivity control at the lowest possible setting.

Extra sensitivity in the control frequency amplifier is provided so as to permit operation in the presence of inductive shields such as steel girders, metal lath construction and large bodies of metal, furnaces, boilers, stoves, refrigerators, chandeliers, or any similar metallic objects.

The sensitivity of the control frequency amplifier is variable to fit a large range of operating conditions. Normally, sufficient precautions are taken in the amplifier and remote control circuits to greatly reduce the possibility of electrical interference. The control amplifiers are very much less subject to interference than an ordinary radio receiving system. It requires an extreme and unusual type of interference to interfere with the operation of mystery control. There is no possibility of interference affecting mystery control receivers if the sensitivity control is kept down to the first half of its total movement. This illustrates the importance of setting the sensitivity control to the minimum position possible.

In some installations, however, owing to the presence of" large metal objects around or near the receiver chasses of the mystery control cabinet, it will be necessary to increase the sensitivity of the control frequency amplifiers owing to the absorption of the metal surfaces.

When this occurs, it will very likely be found that the same metal objects are shielding the receiver from excess static which would normally interfere with the mystery control circuits in a high setting of the sensitivity control. Therefore, when it is necessary to increase the setting of the sensitivity control in order to get operation of mystery control, it will likely be found that interference is not present and that a higher setting of the control is possible.

In all. installations be careful to set the sensitivity control at the lowest possible position and to locate the receiver away from metal objects which would absorb the induction field of mystery control.

The 6ZY5G and 6J5G tubes act as a noise gate to exclude unwanted interference which might control the stepper assembly. This noise gate makes the amplifier respond only to pulses having a time interval equal to that of the pulser mechanism. Thus pulses of random timing do not operate the set.

The operation of the thyratron tube is entirely different from any tube so far encountered by the radio serviceman. It is a gas-filled tube which can handle large plate currents—in other words, large amounts of power. Before getting into the operation of the stepper relay unit for station selection, the r.f. circuits of the receiver should be examined. The wave-switch selects any one of three wave-bands or automatic tuning (mystery control operation).

The Tuning Circuits.—To illustrate the automatic operation, the wave switch has been drawn in that position. The wave switch sections disconnect the r.f. amplifier from the circuit and transfer the antenna coil to the grid of the converter tube. Also, the antenna coil is connected to the station selector switch which selects the proper trimmer condenser for any one station. The gange condenser is cut out of the circuit for remote operation.

The oscillator coil system is completely cut out of the circuit and trimmer type inductances with iron-core tuning are connected by the station selector switch.

A third rotary switch turns on the proper station indicator lamp. The assembly for the station selecting circuits is located beneath the chassis and is driven by the stepper assembly.

There are three groups of contacts operated by the switch. One group switches in the oscillator coils, the second group switches in the antenna padding condensers and the third group of switches, lights the pilot lamps indicating the station dialed.

Excessive friction in this switch would cause improper action of the stepper assembly. It should be adjusted so that when the relays have selected the station dialed, the contact arm is squarely on the contact. The tension of the contact arm is regulated by the setting of the hub on the switch shaft. The long wiper contacts exert a firm pressure on the contacts which may be increased or decreased by adjusting the location of the hub.

The position of the contact arm is determined by the set screws which hold the driver arm on its shaft. This is located above the chassis but beneath the stepper assembly. If the contact arms do not come to rest on the contacts it may be necessary to loosen the set screws on the switch shaft and re-locate the position of the driver arm so that the contacts are made correctly.

Excessive tension in the switch would act as a load on the relays and might result in chattering on one of the stations, part way up, and then failing to reach the station dialed.

The Stepper Assembly.—The stepper assembly which operates the station selecting switch is operated by the thyratron tube referred to previously. The coils which operates this assembly as shown as the plate load of the thyratron in fig. 12.

When the thyratron tube lights, the holding relay closes and the stepping relay pushes a ratchet as many times as there are pulses sent out by the pulser in the mystery control box. There is a primary and a secondary ratchet. The stepper relay operates the primary ratchet which is connected to the primary switch. This switch controls the volume control motor and shorts the voice coil to ground in the station selecting positions.

A muting switch, which connects the plates of the output tubes together, is closed during the station selecting operation. The set, of course, is playing during changes in volume but it ' is muted as the secondary ratchet returns to its home position, and climbs to the station dialed.

This means that whenever any of the eight stations are dialed the set is muted as the secondary ratchet switch turns the "station tuning" switch contacts.

Failure of the primary switch to return home or the secondary ratchet arm to return home, failure of the receiver to mute during dialing would indicate trouble in the stepper assembly, and would make it necessary to return it to the manufacturer for replacement. Dialing of an incorrect station, the skipping of stations or the galloping past of stations also indicates trouble in the stepper assembly.

The Volume Control Assembly.—The volume control and the on-off switch are motor driven. The motor has an automatic clutch which releases and drops back as soon as the volume control is released by the stepper primary switch. This prevents "over-shooting" when changing vol time and immediately stops the gear train which drives the volume control when the volume control lever is released on the mystery control box. There is also a clutch in the volume control itself, so that the mechanism will not jam if the volume control lever is held down after the set is shut off.

The primary switch is a single pole, double throw switch which connects the desired winding in the volume control motor to increase or decrease volume, as shown in fig. 13. In parallel with this switch there is a single pole, double throw switch connected to the manual volume control. This switch is mounted directly beneath the receiver dial bezel.

The pilot lamp cable is close to this switch. If any of the pilot lamp wires become tangled with the switch they might cause the motor to continué running and might possibly tut through the insulation of the pilot lamp lead, causing the lamp to stay lit. It is important when the chassis has been removed, to check the location of the pilot lamp wiring cable to make certain that it is entirely clear of the volume control motor switch.

Method of Inter-station Noise Elimination in Automatic Control Systems.—In modern super-heterodyne receivers the potential amplification is very high, hence the tuning problem would be very difficult if an automatic volume control were not included in the receiver.

It is however a well known fact that all a.v.c. systems are designed to regulate the gain of the receiver only while a signal is being received; therefore between stations the sensitivity rises to a maximum.

This means, of course, a great increase in the background noise between stations and unless there be a noise suppression auxiliary provided in the receiver to limit this audible noise it often becomes objectionable, especially in locations where there is a large amount of man-made static'.

Several schemes have been advanced to solve the interstation noise problem in the a.v.c. equipped receiver. Perhaps the simplest one is to provide an adjustable bias on the i.f. tube (in addition to the a.v.c.) so that the receiver's maximum sensitivity may be manually decreased below the noise level. This undoubtedly settles the noise problem, but it may, through excessive adjustment, reduce the receiver's sensitivity to such an extent that weak stations, which might otherwise be received fairly well, will be skipped by unnoticed. Then, too, if this manual sensitivity control has to be continually retarded and advanced in an effort to locate weak stations, it loses much of its effectiveness as far as noise is concerned.

Another idea'for checking inter-station noise and one which has found greater favor among set designers and experimenters than that outlined previously, is the utilization of a vacuum tube as a carrier controlled relay to block the audio amplifier when no signal is being received. This system is very efficient as a noise suppressor.

It is fully automatic in action once the circuit has been properly adjusted. However, while some radio men have successfully installed it in existing receivers, it is generally most effective when included in the original design of the set since it is quite critical in its voltage requirements.

In analyzing the nature of this between-station noise, it has been found that most of it occurs in the high audio frequency spectrum; thus, if the high frequency response of the receiver is checked by a tone control, the intensity of the noise will be greatly reduced. However, the degree of high note suppression needed to limit inter-station noise is much greater than can be tolerated where good fidelity of tone is desired from a local station.

For this reason on the usual radio which is equipped with a manual tone control, it is necessary to adjust the control frequently to meet existing conditions. By adding a tube to the diode detector circuit as shown in fig. 14 this tone control action may be effected automatically in the a.v.c. equipped receiver. It is an idea that has been successfully used for noise suppression purposes in several of the larger super-heterodynes, and due to its simplicity it can be easily adapted to any receiver using a.v.c. A worthy feature of the system is that it will decrease noise without reducing the overall sensitivity. ■ This automatic tone control must operate in conjunction with a diode type detector. The left half of the accompanying diagram shows the fundamental diode second detector and a.v.c. rectifier circuit found in the majority of modern superheterodynes. Although the tube shown is a 6H6, it may also be the diode portion of a diode-triode or diode-pentode tube; and in some older model receivers, it may even be a triode connected as a diode.

If the associated parts of the detector circuit consisting of resistors Ri, R2 and i?3 and condensers Ci be arranged as shown,

they need not be disturbed when adding the tone control tube to the receiver. However, if Rs is a volume control potentiometer, it must be removed and used instead in the grid circuit of the first audio tube to control the input to the grid of this tube. The original fixed resistor in the audio grid circuit may then be shifted to the R3 position if it be .25 to .5 megohms in value.

In some sets, R2 may be replaced by an r.f. choke or it may be omitted altogether without affecting the performance of the circuit. The experimenter may also find that some receivers divide the functions of a.v.c. and detection, using separate diode sections or tubes for each purpose. In this case, connect the tone control tube to the detector diode circuit and disregard the separate a.v.c. system.

The circuit that is to be added to the receiver is shown in the right half of the cjiagram. The tube may be any sharp cut-off type, either tetrode or pentode, such as the 24, 36, 57, 77, 6C6 or 6J7. Experiment has shown that all of these types work equally well. The choice, therefore, will depend mainly upon the filament voltage available. The tone control tube and associated parts should be mounted as close to the diode detector as possible. Resistor is non-critical in value, a good compromise being 2,500 ohms. Rb should not exceed 100,000 ohms regardless of the plate supply voltage.

The audio coupling condenser C3 is probably already in the receiver and need not be changed. The rating of condenser C will have to be determined by experiment and values from .0001 to .001 (mica dielectric) should be tried. The final choice will depend upon the maximum degree of high note suppression that can be tolerated when the set is tuned to an extremely weak station. If distortion be encountered on some of the medium powered stations, the screen voltage should be slightly lowered.

CHAPTER 17

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