Alignment Of Receivers

Modern radio receivers employ from two up to eight, ten or even more circuits to achieve the selectivity desired. These circuits, however, are of little benefit unless all of them are working at their proper frequencies simultaneously. Only someone acquainted with the alignment of receivers in a radio production department, or someone engaged in radio service work who has adjusted a receiver on which someone has tightened all of the adjusting screws, can realize how dead a receiver can sound when all of its tuned circuits are out of adjustment any considerable amount.

The purpose of "Aligning" a radio receiver is two-fold — to adjust it for maximum performance, and to make the dial indicate within two or three percent the frequency of the station being received.

Since a trimmer adjustment is more sensitive when the circuit *■' capacity is low, the trimmer adjustment is usually made near the high-frequency end of the tuning range. If the adjustment is made at the very end of the range, the maximum mis-tracking over the adjacent portion of the band will be greater than if an alignment point is chosen some small distance from the extreme high-frequency end of the tuning range. In the broadcast band, 1400 KC is the usual choice and is the frequency recommended as standard by the Institute of Radio Engineers. On shortwave bands on the same receiver, it is a good practice to align them at the same position of the gang condenser.

On a TRF receiver, all tuned circuits operate simultaneously at one frequency. When aligning a factory-built receiver, or a kit receiver having a dial calibrated to match the coils and condenser used, the dial is set to indicate the frequency of some signal of known frequency and the individual circuits adjusted to maximum performance on that signal at that setting of the condenser.

On a Superheterodyne receiver, circuits must operate at three different frequencies, properly related if satisfactory performance is to be obtained. Beginning with the circuits closest to the output tubes, the intermediate-frequency circuits must all operate at the same frequency in order to give satisfactory amplification. Actually they will work over a wide frequency range, but if they are operated very far from the intermediate frequency specified for the given dial, coils and tuning condenser, the dial indications will be in error more than the customary few percent and, in the case of receivers employing specially cut tracking plates in the Oscillator condenser, serious mistracking of the oscillator with other tuned circuits will result, producing a loss in sensitivity and reduction in image-ratio.

The first adjustment on a superheterodyne receiver is therefore to align the intermediate-frequency amplifier at the correct frequency. Fortunately for satisfactory receiver operation, but unfortunately for the home set builder, there are no steady signals on the air at intermediate frequenceis to be used for aligning IF transformers. The IF transformers furnished by Meissner are aligned in the factory to the frequency specified in the catalog. If no equipment is available to furnish the proper aligning frequency, the transformers will be closely enough in alignment to pass a signal from a local broadcasting station when the complete receiver is operating. The transformers should be adjusted to give the strongest signal by adjusting, in turn, each of the adjustments on all of the IF transformers. As the adjusting screw is turned continuously in one direction, the output of the receiver will ^continue to increase up to a certain point beyond which the signal begins to fall again. By reversing the direction of rotation of the adjusting screws, each can be set for maximum signal output. As alignment proceeds, and the receiver becomes progressively more sensitive, the input should be reduced by retarding the setting of the sensitivity control, if the receiver has one, or by using progressively shorter antennas or merely short lengths of wire, or by tuning in weaker stations. The last expedient is not recommended unless all others fail, because in tuning in a new station the receiver may not be accurately tuned and it may be necessary to slightly retune all IF circuits.

When the alignment of the IF amplifier is completed, alignment of the RF and oscillator circuits should be made. If there is a signal generator or service oscillator available, it should be used as the frequency standard for alignment only if it is known to have an accurate frequency calibration. A manufacturer's statement of accuracy should not be assumed to hold for long periods of time especially if tubes have been changed in the oscillator. The accuracy can be quickly checked by beating the signal from the service oscillator against stations of known frequency using an ordinary radio set to receive both signals.

If the generator has an accurate frequency calibration, set the frequency to an appropriate frequency for the band to be aligned (all aligning frequencies are specified in Meissner Kit instruction sheets) which is usually about 80% of the maximum frequency tunable on that band, set the receiver dial to the corresponding frequency, connect an appropriate "Dummy Antenna" (see following section, "Dummy Antenna") between the high side of the signal generator output and the antenna connection of the receiver, turn the volume and sensitivity controls of the receiver full on, turn the generator up to high output and adjust the Oscillator trimmer until a signal is heard. Reduce the signal from the service oscillator as alignment proceeds always using as little signal input as possible because weak signals permit a more accurate alignment than strong signals.

Next align the RF amplifier circuit. On the bands below 6 MC the, frequency of the RF amplifier circuit has very little effect upon the oscillator frequency, but at higher frequencies the adjustment of the RF circuit has a slight effect upon the frequency of the oscillator, and consequently it is necessary, when aligning a high-frequency RF amplifier, to rock the gang condenser very slightly as the alignment proceeds to be sure that a shift in oscillator frequency has not shifted the heterodyned signal out of the range of the IF amplifier. The antenna circuit is then aligned in the conventional manner.

Shifting the tuning dial to a point about 10% up from its low-frequency end, the oscillator circuit should be "padded" for best tracking with the antenna and RF circuits. If the radio set is sufficiently sensitive to produce a readily discernable hiss in the

/ix speaker, probably the easiest way to pad the oscillator circuit is to adjust the padding condenser for maximum hiss or maximum noise. If the receiver is not sufficiently sensitive to align by the noise method, a signal of constant amplitude should be tuned in, and then as the padding condenser is turned continuously but very slowly in one direction, the gang condenser should be rocked back and forth to keep Fiaura 17

the signal tuned in. If the sound output is plotted against time, Fig. 17 shows the result of the above described Operation. The padding condenser should be set as it was at point A, giving best sensitivity.

When this point is padded, it is well to return to the high-frequency end and realign that part of the band.

On coils operating in the frequency range 150 to 400 KC with an IF amplifier of 456 KC, the padding capacity is so important in the oscillator tuning scheme that the oscillator should be padded before the high-frequency alignment, and then the circuit aligned and padded at least twice.

Simple receivers can be aligned without instruments by tuning in stations of relatively constant volume, but it is a difficult problem to obtain optimum alignment on any kind of a signal except a constant tone. If the receiver to be aligned is complicated and no equipment is at hand for alignment, it would be well to take the receiver to a serviceman possessing adequate equipment and have him align the receiver.

Was this article helpful?

0 0


    How to carry out alignment on radio receiver?
    1 month ago

Post a comment