Radio servicemen are frequency called upon to replace Antenna, RF or Oscillator coils that have failed either through corrosion, or because of the failure of some other component in the receiver, or because damaged by some outside agency such as lightning.
Usually the damage is confined to the primary of the coil, in which case very frequently a new primary can be »installed in place of the old one.
If the primary is replaceable, the winding direction of the old primary should be noted before removing it so that the new one may be installed with its winding direction the same.
If the damaged coil is beyond salvaging by installing a new primary, or if the secondary has been damaged, it will be necessary to install a new coil and check its tracking with the remainder of the tuning circuits.
In order to permit replacement coils to be tracked rapidly and to eliminate the possibility of having removed too much inductance and thereby ruined the replacement coil, to say nothing of the hours of labor installing, checking, removing and altering the coil, etc., Meissner has developed "Universal Adjustable" replacement antenna, RF and oscillator coils which are provided with a screw-driver adjustment of inductance by means of a movable core of finest quality powdered iron. By means of this adjustment, it is as easy to add inductance as to remove it, and to quickly obtain the optimum value of inductance. A coil of this type is shown in Fig. 16.
When a replacement antenna or RF coil is installed in a TRF receiver,, the process of aligning is very simple. The dial is set to 600 KC, a dummy antenna of 200 mmfd. connected between the
high side of the service oscillator and the antenna connection of the receiver, an output indicator of some type is connected to the output of the receiver, the service oscillator tuned to the receiver and the screw adjustor in the top of the can rotated until maximum sensitivity is obtained. The receiver and signal generator are next tyned to 1400 KC and the circuits aligned in the usual manner by adjusting the trimmers on the gang condenser. The process should then be repeated in order to obtain the best possible alignment at both checking points. It is best to seal the inductance adjustment on the coil by the application of a satisfactory cement, such as Duco Household Cement or equivalent.
When replacing an antenna or RF coil on a superheterodyne, essentially the same practice is .followed as above with the exception that, since the oscillator determines the dial calibration, if the adjustments thereon have been disturbed, it is necessary to readjust the oscillator circuit to agree with the dial calibration at the checking points before adjusting the inductance of the new coil or aligning it.
If a new oscillator coil is being installed, the greatest aid to rapid adjustment of the new coil to proper inductance is an undisturbed padding condenser adjustment. There are innumerable combinations of oscillator inductance, padding capacity, and trimmer capacity that will track an oscillator circuit at two places in the broadcast band, but these various combinations give varying degrees of mis-tracking throughout the remainder of the band. If the padding condenser has not been disturbed, one of these variables is eliminated, and, with only inductance to adjust for proper alignment at the low-frequency end of the band, and capacity to adjust at the high-frequency end of the band, the adjustment is practically as easy and rapid as installing and adjusting an antenna or RF coil.
If the oscillator padding condenser has been disturbed, it will be necessary to track the oscillator with the remainder of the receiver in the same experimental manner as used in the determination of the original design values. To arrive at a satisfactory alignment, the following experiment should be conducted systematically, writing do<wn the answers obtained, so that the data does not become confused in your mind.
1. Align the IF amplifier at the frequency specified by the manufacturer.
2. Adjust the padding condenser to some value known to be much lower in capacity than its normal adjustment.
3. Set the dial and signal generator (of known accuracy) to 600 KC and adjust the oscillator inductance by means of the screw in the top of the can until a signal is heard. If no signal is heard within the range of the oscillator inductance adjustment, screw the adjustment as far in as possible and increase the padding capacity until a signal is heard.
4. Attempt to align the oscillator trimmer condenser to agree with the dial at 1400 KC. If the adjustment cannot be made, again increase the capacity of the padding condenser and reduce the inductance (by turning the screw out) of the oscillator coil to obtain a new setting at 600 KC. This process should continue until both 600 and 1400 KC are correctly indicated.
5. When both 600 and 1400 KC are correctly indicated, tune the receiver to the generator set at 1000 KC and make a sensitivity measurement which should be recorded.
6. Now increase the padding condenser capacity slightly, decrease the inductance to give a 600 KC signal, align at 1400 KC and again measure sensitivity at 1000 KC. If the sensitivity at that point is better than it was before, repeat this operation until the sensitivity measurements show greatest sensitivity and then start falling off again. If the steps in the process have been written down, recording the number of revolutions and fractions thereof on the adjusting screw of the inductance, it should be easy to return to the adjustment giving maximum 1000 KC sensitivity. When this adjustment is set, seal it with some satisfactory cement such as Duco Household Cement or equivalent and then give the receiver a complete alignment.
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