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Friday, 24 May 2019

Alignment of Radio Receiver

Radio Receiver Alignment


The alignment of an amplitude modulated radio receiver consists of two types of adjustments of the different tuned circuits present in the receiver. The first part is to align the I.F. transformers to the correct intermediate frequency of the receiver. This is called the I.F. alignment. The second part is to align the input tuned circuits to match with the scale/dial markings such that the wanted station (frequency) is producing the correct value of I.F. when tuned on the dial to the frequency marked on the dial. This is called the R.F. alignment. The R.F. alignment is also called TRACKING.

There are certain preliminary arrangements to be done and special precautions to be taken either for I.F. alignment or for the R.F. alignment. These are explained here under. 

 I.F. Alignment  


The setup for the I.F. alignment of a radio receiver is shown in Figure. We use a power output meter in addition to the Standard Signal generator to measure the power output of the radio receiver.


 Preliminary Steps for I.F. Alignment  

The following arrangements are to be made before alignment :

1. The A.V.C/A.G.C. is to be made inoperative.
2. The volume and tone controls are to be kept in maximum resistance position.
3. The loud speaker is to be replaced by an A.F. power output meter in the proper impedance range equal to that of the impedance of the loud speaker.
4. The radio receiver is to be tuned to the medium wave band.
5. The radio receiver is to be tuned to the lowest frequency in the medium wave band.
6. A capacitor of 0.1 μF is to be connected in series with the R.F. lead, when the signal is injected.
7. The cores of I.F. transformers are to be tuned using only nonmagnetic screw drivers or with special core tuning alignment tools. 

Step-by-Step I.F. Alignment Procedure  

The following steps are to be followed.

1. The signal generator is to be tuned to the Intermediate frequency of the radio receiver.

2. It is to be kept in the internal modulation position. The depth of modulation is to be adjusted to 30% or above.

3. The attenuator is to be adjusted to minimum value.

4. The I.F. signal is to be applied to the collector of the second I.F. amplifier, through the 0.1 μF capacitor. The attenuator is to be adjusted to get a convenient reading in the power output meter. Now the core of the second I.F. transformer is to be adjusted to get a maximum reading in the power output meter.

5. The I.F. signal is to be shifted to the base of the second I.F. amplifier. There will be an improvement in the output as indicated by the power output meter. This is due to the gain of the amplifier. Again the core is to be adjusted to get the maximum output.

6. The same procedure is to be repeated at the collector and base of the first I.F. amplifier. The cores of the corresponding I.F. transformer are to be adjusted for maximum output in the power output meter.

7. The signal is to be shifted now to the base of the frequency converter transistor. In this case the attenuator is to be kept in the minimum position. Adjustment of core of the I.F. transformer in the collector of the frequency converter is to be done now. i It is to be adjusted to get maximum response.

8. Finally the R.F probe must be kept at a short distance from the ferrite antenna. The power output meter is to be disconnected and the loud speaker is to be connected. A reasonable 400 Hz note will be heard from the speaker showing the receivers sensitivity for I.F.

9. The alignment of the I.F. stages is to be repeated two to three times to finally arrive at correct setting.

10. When once arrived at final setting the cores are to be sealed to prevent tampering of the cores by the user. 

Step-by-Step R.F. Alignment or Tracking  

All the preliminary arrangements as listed for I.F. alignment are common for the R.F. alignment. The change is that we will select the different bands for tracking.

Further we will use a dummy antenna recommended for the particular frequency band under consideration. The following are the steps to be followed:

The Standard signal generator is to be tuned to the desired frequency as listed here under. The signal will not be directly connected to the input of the receiver, but the probe will be kept close to the antenna coils of the receiver. The frequencies to which the standard signal generator and the receiver are to be tuned and the adjustment to be made are given step by step here under in Table. 

Procedure for tracking the Radio Receiver


S.G. Frequency
Position of pointer
Adjustment to be made for getting maximum output
SWITCH THE RECEIVER TO THE MEDIUM WAVE BAND (550 kHz to 1600 kHz)
550 kHz
550 kHz
M.W. Oscillator coil
1600 kHz
1600 kHz
Trimmer of M.W. Osc. Coil
Repeat this adjustment twice then:
840 kHz
840 kHz
Slide the M.W. antenna coil on the ferrite rod.
Repeat the above two operations twice to get tuning as close to the dial marking as is possible.
SWITCH THE RECEIVER TO THE SHORT WAVE BAND (4.5 MHz to 16 MHz)
04.5 MHz
04.5 MHz
S.W. Oscillator Coil
16.5 MHz
16.5 MHz
Trimmer S.W. Osc. Coil
Repeat this adjustment twice then:
05.0 MHz
05.0 MHz
S.W. Antenna Coil
Repeat the above two operations twice to get tuning as close to the dial marking as is possible.


If the receiver has several short wave bands, the a different above process is to be repeated at all the different S.W. bands tuning to the lowest frequency, highest frequency, adjusting Oscillator Coil at the lowest frequency and adjusting the oscillator trimmer at the higher end. At the mid frequency in the band the antenna coil is to be tuned. This completes the process of tracking the receiver.

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