Alignment Methods

Author: J.B. Hoag

In order to operate several stages of tuned r.f. amplifiers, with a single control, the tuning condensers are mechanically coupled together. Often these "ganged" condensers are all mounted on a common shaft. It is necessary that, as the control knob is turned, the various stages shall all "track" or tune to the same radio frequency at every position on the knob's scale. When this is true, the circuits are said to be "tracking" each other. In order that this shall be possible, the coils must be as nearly identical as possible and the capacity of the individual condensers in the gang must all increase at the same rate as the knob is rotated. Even so, the circuits may not track because of differences at the zero setting of the condensers, where the small capacities are not all the same. These differences are adjusted by connecting trimmer condensers of comparatively small capacity c in parallel with the main tuning condenser C, as in Fig. 32 L.

Fig. 32 L. Tracking with a trimmer

A test oscillator, adjusted to the highest frequency to be received, is coupled to the last stage of the r.f. amplifier. The tuning condenser C is set at its minimum value, and the trimmer c is adjusted until maximum output is obtained from the receiver, i.e., until maximum hiss is heard, or, better, until an "output meter" reads maximum. The next to the last r.f. stage is added and its trimmer adjusted in the same manner. This is repeated stage by stage to the input end of the receiver. Next, the control knob of the ganged condensers CCC is turned to the other end of its scale, the test oscillator is adjusted to the lowest r.f. to be received and, stage by stage, the trimmers are adjusted for maximum output. If, during this test, a particular trimming needs to be changed from the previous hi. setting, the tuning coil should be altered. When larger trimmer capacitance is found necessary for the low-frequency than for the high-frequency end, increase the inductance until the original value of c is restored, and vice versa. When the tracking has been attained at the end frequencies, it is usually sufficiently accurate throughout the entire tuning range for all practical purposes.

In the case of a superhet, it is also necessary that the frequency of the local oscillator be changed simultaneously and by such an amount that the beat or intermediate frequency shall always be of a fixed amount regardless of the signal frequency.

Fig. 32 M. Tracking a converter

Figure 32 M shows a typical converter circuit with its padding condensers C₅. In this combination, C₅ consists of a small fixed condenser paralleled by a still smaller variable condenser. The trimmer condenser C₄ is first adjusted at the high-frequency end of the tuning range so as to deliver the correct i.f. (maximum output). The low-frequency end is then adjusted by means of C₅ to give the same i.f. (maximum output).

When a receiver has to cover a wide range of frequencies, the tuning coils are changed either by means of a switch or by a plug-in arrangement. For ease of tuning with a given coil, it is desirable that one rotation of the condenser should cover a definite band of frequencies. In order to spread these frequencies out over the entire dial of the condenser, small trimmer and/or padding condensers are used.

For the i.f. alignment:

1. A test oscillator is set at the i.f. and is connected, with one terminal to the grid of the last i.f. stage, the other to the chassis. The trimmers of the output i.f. and input second detector unit are adjusted for maximum audio output. The automatic-volume-control featured on some receivers should be disconnected during the i.f. alignment described here. If the receiver has an automatic tuning unit, this may be used instead of the output meter, in which case the a.v.c. is left on.

2. The test oscillator is similarly connected across the grid circuit of the next to last i.f. stage and the grid trimmers between the last and the next to last i.f. tubes are adjusted for maximum output. As one progresses towards the converter, the test oscillator's output should be decreased to prevent overloading the tubes and causing distortion.

3. When aligning the first i.f. coupling unit, the test oscillator is connected across the mixer grid; in which case it is often necessary to disconnect the mixer's grid-tuning circuit in order to be able to apply sufficient voltage to the tube.

4. If the i.f. amplifier has a crystal filter, set the test oscillator as closely as possible to its frequency, switch out the crystal and align as above. Switch in the crystal and vary the test oscillator's frequency to maximum output. Then repeat the complete alignment again.

The usual alignment sequence is: (1) i.f., (2) local oscillator, (3) mixer, (4) r.f. But differing sequences are used by different operators.