Below is a compilation of original Philco service documents detailing the Mystery Control system with installation hints and recommendations to the serviceman.
There is some redundancy in the documents, but armed with these documents and the service schematics for your particular model receiver, they will give the you the best insight and information to sucessfully repair and restore a Mystery Control receiver to its former operation.
Reprinted from a PHILCO Letter to Dealers, September 15, 1938
TO ALL PHILCO DEALERS
Subject: Philco Mystery Control
Subject: Philco Mystery Control
SETTING UP STATIONS - Select eight of the most popular stations received in the locality and remove their call letters from the Call Letter Sheets supplied. Place the call letters in the windows on the set and also in the Mystery Control Box. When placing the tabs in the Mystery Box, the tab marked "LOUD" is placed in the window on the extreme right, the tab marked "SOFT" goes into the next window. The following window should contain the same station that appears in the window on the extreme left of the radio set, and the balance of the tabs should be placed in the same order on the Mystery Box as on the set.
Two adjustment screws for each station are located on the rear of the radio chassis. Each set of screws is numbered and covers a frequency range as follows:
1 and 2 - 540-1030 kc.
3 and 4 - 670-1160 kc.
5 and 6 - 900-1470 kc.
7 and 8 - 1100-1600 kc.
3 and 4 - 670-1160 kc.
5 and 6 - 900-1470 kc.
7 and 8 - 1100-1600 kc.
Looking at the front of the cabinet, the first station on the left is adjusted by set screws #1, the next station by set screws #2, and the remaining stations in the some order.
Connect the aerial and ground to the "ANT" and "GND" terminals of the receiver. Turn the receiver Tuning Range Selector to "Broadcast" and tune the receiver to the station whose tab is in the first window on the left. Connect your test oscillator to the "ANT" and "GND" posts of the receiver and tune it to the frequency of the station being received. Turn the Tuning Range Selector on the set to "Remote" and, using the Mystery Control Box, dial the station corresponding to the one which has just been tuned in. Adjust the two set screws until the oscillator signal is heard at maximum volume. Then disconnect the test oscillator from the set, and, with the volume of the receiver low, readjust the two set screws until the station is heard at maximum volume. After setting up the first station, the same procedure is used for the other stations.
AERIAL - Mystery Control sets are matched and tuned to the Philco Safety Aerial. The Safety Aerial should be installed whenever the Mystery Control receivers are permanently set up in homes. For dealer demonstrations and in your own showrooms, Mystery Control receivers should be connected to Part No. 40-6112 Philco High-Efficiency Aerial, with a Part No. 32-2763 Set Transformer in the line. This will assure freedom from radio interference which might affect both the control circuits and the radio receiving circuits. It is very important that the aerial being used be connected to the receiver at the aerial post and lead away from the cabinet without any excess wire at or near the cabinet. It is important that you avoid coiling up excess aerial wire and dropping it at the back of the cabinet. Run the aerial and ground directly to the aerial and ground terminals and remove all excess wire.
GROUND - The connection of the ground on Mystery Control receivers is of great importance. The terminal panel for aerial and ground has a link which is connected to the ground post only when a ground is not used. If a ground is available, we recommend connecting it directly to the ground terminal of the receiver. When this connection is made, the link should be swung around so that it does not touch the ground post. Mystery Control chassises are being shipped with the link open and in the correct position in case a ground is connected to the ground terminal. If no ground is used, it will be advisable to connect the link as suggested above.
CONTROL FREQUENCY - Mystery Control receivers are shipped with five (5) different control frequencies, which range from 350 to 400 K.C. These are identified by code numbers appearing on the serial number ticket and on the steel sub-base. Receivers bearing Code 5 are shipped with the control frequency adjusted to 355 K.C; Code 6 receivers have a control frequency of 367 K.C.; Code 7 receivers are adjusted to a control frequency of 375 K.C.; Code 8 receivers are adjusted to a control frequency of 383 K.C.; Code 9 receivers to 395 K.C.
YOU WILL RECEIVE ASSORTED CONTROL FREQUENCIES - We are shipping receivers with all of the above control frequencies. In shipments you may receive an assortment of any or all of the above control frequencies. It is possible that in some shipments many of the receivers will have identical control frequencies although it is also possible that they will have some of each of the control frequencies.
The purpose of different control frequencies is to prevent interference between two Mystery Control receivers which may be on the same showroom floor or exceptionally close together in customers' homes. When several Mystery Control receivers are to be located close together it will be necessary to use receivers with different control frequencies to avoid interference between the sets. We recommend a difference of 20 K.C. between control frequencies of sets that are in the same room. If you have Code 5 and Code 7 receivers in stock, they could be placed along side each other without any interference of the control circuits. If all the receivers in your stock are the same code receivers, it would be necessary to change the control frequencies of the second set placed on the floor 20 K.C. from the first set. This would give you a difference in control frequency of the necessary 20 K.C. and assure freedom from interference between the two circuits.
If three receivers were to be operated at the same time it would be advisable to adjust the control frequencies of the first Set to 355, and the second to 375, and the third to 395 K.C. In homes or apartment houses the distance between receivers will determine the difference in frequencies that is necessary. When the control frequencies are 10 K.C. apart, receivers will not interfere with each other so long as the remote control cabinet is kept a minimum of 10 feet away from the second receiver. By having the control frequencies differ by 20 K.C., the second control cabinet can be placed anywhere, even on top of the first cabinet.
CONTROL FREQUENCY SENSITIVITY CONTROL - There is a knob on the rear of the chassis which is of tremendous importance to Mystery Control operation. The normal range of Mystery Control is within a circle of the receiver with 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 is 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 frequency 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.
SETTING OF SENSITIVITY CONTROL DETERMINES EFFECT OF INTERFERENCE - 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 chassis 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, you will very likely find 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.
LOCATING THE RECEIVER - In many homes the heater is located in the cellar beneath the livingroom. Wherever possible, avoid locating the cabinet directly above the heater. It should also be installed as far away as possible from radiators and should not be backed up against a wall which has a kitchen stove or refrigerator against it. By observing these precautions, you will extend the range of Mystery Control and assure satisfactory operation because it will then be possible to operate the set at normal sensitivity of the control frequency amplifiers.
ADJUSTING THE CONTROL FREQUENCY - When realigning or changing the control frequency of Mystery Control receivers, a Model 077 is used. A wire about 15 feet long is connected to the antenna terminal of the 077 and is then wound up into a roll of wire similar to a loop. The other end of this loop is then connected to the ground terminal of the 077 Signal Generator. The loop which has been made in this manner is placed in the center of the large secondary coil which is mounted in the cabinet between the speaker and the bottom of the cabinet. When realigning it is possible to determine the frequency to which the receiver is tuned by simply tuning back and forth between 350 and 400 K.C. with the 077. When a signal similar to that to which the control frequency amplifier is aligned is reached, the Thyratron tube in the receiver chassis will light and stay lit. When padding or when changing the control frequency simply set the Model 077 to the frequency desired (between 350 and 400 K.C.). Signal from the 077 will then be picked up by the secondary coil and fed into the control frequency amplifier. If you are shifting the frequency it may be necessary to turn the sensitivity control up towards the extreme position. As the different stages arc padded it will be advisable to turn the sensitivity control gradually down towards the "near" setting.
In the control frequency amplifier there is a type 2A4G tube. This tube is fired by the operation of the control frequency amplifiers. If the iron cores of the amplifier transformer are in alignment the, 2A4G tube flashes indicating resonance with the frequency fed into the amplifier. It is characteristic of the 2A4G tube to continue firing after the peak voltage has caused it to start operation. This accounts for its staying lit during a considerable arc of the padding stick. The correct setting of the padder may be determined by reducing the amount of signal coming into the circuit so that the 2A4G tube only lights when resonance is reached.
There are three transformers in the circuit of the control frequency amplifier which must be adjusted. With the Signal Generator connected as described above, the transformers in the amplifier circuit may be adjusted using the 2A4G tube as an output meter. If you wish to pad the amplifier to 375 K.C., the 077 should be set at 375 and the transformer nearest the 2A4G tube should be adjusted first. This is accomplished by turning the adjusting screw on top of the coil shield until the 2A4G tube lights to its brightest point. In order to see a bright spot it may be necessary to reduce the sensitivity control or the output of the 077. The 2A4G tube will light when the transformer is correctly tuned to the incoming 375 K.C. signal. The second transformer (the one in the center) is adjusted next, using the 2A4G tube as an output meter. Then the first control transformer is tuned, again using the 2A4G tube to indicate resonance.
There is also an air padding condenser connected across the secondary coil mounted in the bottom of the cabinet. This condenser is covered with a round paper box. The adjusting screw is reached through an opening in the box. It is necessary to carefully adjust this padding condenser in order to accurately tune the secondary coil to the induction energy coming from the Mystery Control Box. This padding is accomplished using the Model 077 as before, adjusting the padding condenser carefully so that the 2A4G tube lights. Extreme care should be used in setting the padding condenser to the point of exact resonance. This coil is extremely sharp and it may be necessary to turn down the attenuator of the oscillator as well as the sensitivity control of the control frequency amplifier several times so that the condenser is set at the exact point of resonance. This is indicated by having the 2A4G tube light and go out leaving the condenser adjusted so that the 2A4G is lit to its greatest brilliance.
Now turn off the 077, and remove the temporary coil of wire.
It is next necessary to pad the Mystery Control Box. This is accomplished by dialing any of the stations and pressing the volume change lever [the finger stop] on the control so that a continuous induction signal is sent out by the Mystery Control cabinet. Holding the lever down, bring the Mystery Control Box close to the receiver so that the 2A4G tube will light. Use a padding wrench on the adjusting screw located on the bottom of the Mystery Control box and turn the condenser until the 2A4G tube lights. Turn the sensitivity control down until the 2A4G just about goes out. Now set the padding condenser on the Mystery Control Box carefully, leaving it adjusted so that the 2A4G tube is lit to its greatest brilliance. The Mystery Control Box is now adjusted to the same frequency as the control frequency in the receiver. This procedure is used in realigning Mystery Control receivers and when changing the control frequency of a Mystery Control set.
Model 077 is used only in adjusting the control frequency amplifier and the secondary of the induction coil in the receiver. After that part of the circuit has been set up with the Model 077, the Signal Generator should be disconnected and the Mystery Control Box adjusted to the frequency already established at the control frequency amplifier.
COMPONENTS
1. Pulser - This assembly, located in the Mystery Control Box, includes all parts of the pulsing mechanism except the molded dial and the center of the dial which is a separate piece. This unscrews by turning it in a counter-clockwise direction. To remove a pulser assembly, which is Philco Part No. 38-9704, remove the base of the Mystery Control cabinet, disconnect the wiring plug between the pulser assembly and the oscillator tube. The pulser assembly is held in the cabinet by a mounting bracket located beneath the moulded dial. First remove the center of the dial by turning it in a counter-clockwise direction and then the dial mounting bolts. The pulser assembly mounting bolts can now be reached and should be removed. Slow dialing would make the Mystery Control circuit fail to operate and would necessitate the return of the pulser assembly. This would be indicated by incorrect operation of the relays located in the set. If the holding relay drops back before the stepping relay has climbed to the station dialed, it wi11 be necessary to return the pulser assembly for replacement.
2. Station Selecting Switch Assembly - Philco Part No. 42-1468. This assembly is located beneath the chassis but 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 onto 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 relocate 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.
3. The Stepper Assembly - Philco Part No. 39-9689. This assembly houses a holding and a stepping relay which are operated by the Thyratron tube. When the Thyratron tube lights the holding relay closes and the stepping relay pushes a ratchet as many times as the 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 8 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 Philadelphia for replacement. Dialing of an incorrect station, the skipping of stations or the galloping past of stations also indicates trouble in the stepper assembly.
VOLUME CONTROL MOTOR AND 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 volume and stops the gear train which drives the volume control immediately when the volume control lever [finger stop] 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. 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 continue running and might possibly cut 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.
Yours very truly,
PHILCO RADIO & TELEVISION CORPORATION OF NEW YORK
PHILCO RADIO & TELEVISION CORPORATION OF NEW YORK
Reprinted from Philco Serviceman, September, 1938
"Installation Hints for Mystery Control"
"Installation Hints for Mystery Control"
Thousands of questions have been asked by the public, by radio dealers, by servicemen about PHILCO Mystery Control. Information with regard to the correct installation of the Mystery Control Models is certainly all important.
Floor Location Important
When arranging the Mystery Control Model on the display floor or when making an installation in the home there are a number of points which should be carefully observed. If you find, for example, that the control box will not operate at any considerable distance from the set, you will probably find that the set or the position at which the control box is being operated is very near a large metal object of some kind. This might be re-enforced concrete in a wall or it might be a furnace in the home. Under ordinary circumstances the Mystery Control Models will operate by means of the Mystery Control unit at a distance equal to the distance the set can be heard clearly within the home.
Sensitivity Control
The purpose of the sensitivity control located on the back of the chassis is to adjust the sensitivity of the Mystery Control amplifier so as to reduce to a minimum the possibility of interference from static. In the average installation the sensitivity control, when set at the "near" position, will operate up to thirty-three feet. When set at the "extreme" position the control will operate up to seventy-two feet. This distance will vary with local conditions and with the amount of large metal objects present near the radio and near the control box. In order to minimize any possible static interference, the sensitivity control should she set at the minimum position in which satisfactory operation for the particular installation can be obtained. For example: If it is known beforehand that there will never be occasion to operate the control box at a distance of more than twenty-five feet from the radio, the sensitivity control should be set at the "near" position. On the other hand, if the presence of metal objects has a shielding tendency for control operation, then the sensitivity control setting should be increased. It will usually be found that such large metal objects will shield static so that an increased sensitivity control setting will not necessarily bring in excessive interference.
Operating Two Sets
In some cases where two sets are being operated on the same display floor, there may be interference experienced between control boxes. The control box from one set might operate the second radio set instead of the first one. A very simple adjustment can be made to correct this condition, and your PHILCO distributor is in a position to furnish this information at any time it may be desired.
Good Aerial Essential
When demonstrating Mystery Control in the store every installation precaution should be taken to assure the finest possible demonstration under the best receiving conditions. It is recommended in all cases that the PHILCO noise-eliminating High-Efficiency Aerial, Part No. 40-6112, be used for the store demonstration. This aerial affords the maximum possible noise reduction and is thus the more desirable type of aerial to use in a store location which is usually found to be noisy. The transmission line of the aerial should be connected to the Mystery Control set through the Set Transformer, Part No. 32-2763. If more than one radio is to be operated from this aerial, the Multiple Aerial Switch, Part No. 45-2183, should be used at each radio set to connect the aerial automatically and to disconnect it from other sets not being demonstrated. More information will be coming next month on some of the technical features of Mystery Control. Be sure to read more about Mystery Control in the October issue of the PHILCO SERVICEMAN.
Reprinted from Philco Serviceman, October, 1938
"Understanding Of Mystery Control Frequencies Helps Demonstration"
"Understanding Of Mystery Control Frequencies Helps Demonstration"
Installation Hints Improve Performance
In the Mystery Control models a controlling energy is transferred from the Mystery Control box to the radio receiver. The number of pulses sent out when dialing determines the station to be tuned.
5 Different Frequencies
Mystery Control receivers are shipped with five different control frequencies, which range from 355 to 400 KC. These are identified by code numbers appearing on the serial number ticket and on the steel sub-base and also on a sticker on the underside of the Mystery Control box. Receivers bearing Code 5 are shipped with the control frequency adjusted to 355 KC.; Code 6 receivers have a control frequency of 307 KC.; Code 7 receivers are adjusted to a control frequency of 375 KC.; Code 8 receivers are adjusted to a control frequency of 383 KC.; Code 9 receivers to 395 KC. Note that the code number and the middle number of the frequency correspond in all cases.
Receivers are shipped with all of the above control frequencies. In shipments you may receive any or several of the above control frequencies. It is possible that in some shipments receivers will have identical control frequencies, although it is also possible that they will have some of each of the control frequencies.
Preventing Interference
The purpose of different control frequencies is to prevent interaction between two Mystery Control receivers which may be on the same show-room floor or exceptionally close together in customers' homes. When several Mystery Control receivers are to be located close together, it will be necessary to use receivers with different control frequencies to avoid interaction between the sets. It is recommended that a difference of 20 KC. be maintained between control frequencies of sets that are in the same room. If you have Code 5 and Code 7 receivers in stock, they could be placed alongside each other without any interaction of the control circuits. If two receivers on your floor are the same code receivers, it would be necessary to change the control frequency of the one set 20 KC. from the first set. This would give a difference in control frequency of the necessary 20 KC. and assure freedom from interaction between the two circuits.
If three receivers were to be operated at the same time, it would be advisable to adjust the control frequencies of the first set to 355 and the second to 375 and the third to 395 KC. In homes or apartment houses the distance between receivers will determine the difference in frequencies that is necessary. When the control frequencies are 10 KC. apart, receivers will not interfere with each other so long as the remote control cabinet is kept a minimum often feet a way from the second receiver. By having the control frequencies differ by 20 KC., the second control cabinet can be placed anywhere, even on top of the first cabinet.
Control Frequency Sensitivity Control
There is a knob on the rear of the chassis which is of tremendous importance to Mystery Control operation. The usual range of Mystery Control is within a circle of the receiver, with a radius of about twenty-five 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 is 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 frequency 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.
Setting of Sensitivity Control
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 interaction than an ordinary radio receiving system. It requires an extreme and unusual type of disturbance to interfere with the operation of Mystery Control. There is little possibility of interaction 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 chassis of the Mystery Control cabinet, it will bc 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, you will very likely find that interaction is not present and that a higher setting of the control is possible.
Adjusting Control Frequency Of Mystery Control Models
When realigning or changing the control frequency of Mystery Control receivers a Model 077 [an RF signal generator] is used. A wire about fifteen feet long is connected to the antenna terminal of the 077 and is then wound up into a roll of wire similar to a loop. The other end of this loop is then connected to the ground terminal of the 077 signal generator. The loop which has been made in this manner is placed in the center of the large secondary coil which is mounted in the cabinet between the speaker and the bottom of the cabinet. When realigning it is possible to determine the frequency to which the receiver is tuned by simply tuning back and forth between 350 and 400 KC. with the 077. When a signal similar to that to which the control frequency amplifier is aligned is reached the 2A4G tube in the receiver chassis will light and stay lit. When padding or when changing the control frequency simply set the Model 077 to the frequency desired (between 350 and 400 KC.). Signal from the 077 will then be picked up by the secondary coil and fed into the control frequency amplifier. If you are shifting the frequency, it may be necessary to turn the sensitivity control up toward the extreme position. As the different stages are padded, it will be advisable to turn the sensitivity control gradually down toward the "near" setting.
Thyratron Operation
In the control frequency amplifier there is a Type 2A4G Thyratron tube. This tube is fired [or conducts] by the operation of the control frequency amplifiers. If the iron cores of the amplifier transformer are in alignment, the Thyratron tube flashes, indicating resonance with the frequency fed into the amplifier. It is characteristic of the Thyratron tube to continue firing after the peak voltage has caused it to start operation. This accounts for its staying lit during a considerable arc of the padding stick. The correct setting of the padder may be determined by reducing the amount of signal coming into the circuit so that the Thyratron tube only lights when resonance is reached.
There are three transformers in the circuit of the control frequency amplifier which must be adjusted. With the Signal Generator connected as described here, the transformers in the amplifier circuit may be adjusted, using the Thyratron tube as an output meter. If you wish to pad the amplifier to 375 KC., the 077 should be set at 375, and the transformer nearest the Thyratron tube should be adjusted first. This is accomplished by turning the adjusting screw on top of the coil shield until the Thyratron tube lights to its brightest point. In order to see a bright spot it may be necessary to reduce the sensitivity control or the output of the 077. The Thyratron tube will light when the transformer is correctly tuned to the incoming 375 KC. signal. The second transformer (the one in the center) is adjusted next, using the Thyratron tube as an output meter. Then the first control frequency transformer is tuned, again using the Thyratron tube to indicate resonance.
Padding Secondary Coil
There is also an air padding condenser connected across the secondary coil mounted in the bottom of the cabinet [or the radio]. This condenser is covered with a round paper box. The adjusting screw is reached through an opening in the box. It is necessary to carefully adjust this padding condenser in order to accurately tune the secondary coil to the induction energy coming from the Mystery Control Box. This padding is accomplished, using the Model 077 as before, adjusting the padding condenser carefully so that the Thyratron tube lights. Extreme care should be used in setting the padding condenser to the point of exact resonance. This coil is extremely sharp, and it may be necessary to turn down the attenuator of the oscillator, as well as the sensitivity control frequency amplifier, several times so that the condenser is set at the exact point of resonance. This is indicated by having the Thyratron tube light and go out, leaving the condenser adjusted so that the Thyratron is lit to its greatest brilliance. Now turn off the 077 and remove the temporary coil of wire.
Padding Mystery Control Box
It is next necessary to pad the Mystery Control Box. This is accomplished by dialing any of the stations and pressing the volume change lever [the finger stop] on the control so that a continuous induction signal is sent out by the Mystery Control cabinet. Holding the lever down, bring the Mystery Control Box close to the receiver so that the Thyratron tube will light. Use a padding wrench on the adjusting screw located on the bottom of the Mystery Control Box and turn the condenser until the Thyratron tube lights. Turn the sensitivity control down until the Thyratron just about goes out. Now set the padding condenser on the Mystery Control Box carefully, leaving it adjusted so that the Thyratron tube is lit to its greatest brilliance. The Mystery Control is now adjusted to the same frequency as the control frequency box in the receiver. This procedure is used in realigning Mystery Control receivers and when changing the control frequency of a Mystery Control set.
Model 077 is used only in adjusting the control frequency amplifier and the secondary of the induction coil in the receiver. After that part of the circuit has been set up with the Model 077, the Signal Generator should be disconnected and the Mystery Control Box adjusted to the frequency already established at the control frequency amplifier.
