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Superheterodyne Alignment

By JARS Club Member Sheldon Kelley*

The alignment procedure of superheterodyne radio receivers has been a mystery to many technicians even though they normally have the required test equipment at hand. Some alignment procedures can be a little complicated but most are not. Some AM radios can be aligned satisfactorily with a simple signal generator and a volt meter. Sometimes simple radios are even aligned just by using broadcast radio stations. Alignment procedures for uncomplicated radios are presented below.

To be safe it is probably best to connect the radio receiver to an isolation power transformer before applying any power to the radio receiver and beginning the alignment procedure. From “Marcus and Levy’s Elements of Radio Servicing” book: In most receivers, alignment adjustments for tuned circuits are performed by varying small semi-variable capacitors in parallel or in series with the main tuning capacitors of the tuned circuits.  These capacitors are known as “trimmers” when in parallel and padders when in series.

Very often with 5-tube sets serviced, a signal generator is never used during the alignment procedure. The oscillator and RF trimmers and padders are adjusted for maximum audio volume with the receiver tuned at or near 1400 khz using broadcast stations while the IFs are peaked using broadcast stations at the low end of the dial. If the high and low dial ends track properly but the middle does not more often than not the dial itself is not accurate due to manufacturing tolerances of the variable capacitor or radio receiver dial. If the tuning capacitors have cut slots in their rotor plates, bending the sections of the oscillator plates that are meshed with the stator plates might do the trick. You can do this with the RF sections as well but it is seldom worth the effort since these circuits are relatively broad in tuning (not as selective).

 The final intermediate frequency (IF) used in the alignment process is sometimes not that critical, but the radio frequency (RF) used for alignment certainly is.  The average customer would never know that the IF stages which might have been designed for, say, 455kHz, were instead tuned to 460kHz, but they sure could tell if WBZ at 1030 kHz came in at 1030 or 1050 on their radio dial! The most commonly used intermediate frequencies used in design are 175, 260, 455, and 465khz. We chose an IF of 455 kHz for our examples given below. In general, during AM receiver alignment, IF alignment is accomplished first with oscillator and RF alignment accomplished last.

For IF alignment, a signal generator at the prescribed IF (i.e. 455 khz) and modulated near 30 % is connected to the receiver RF tuning capacitor terminal through a 0.1-mfd/600-volt capacitor with the receiver oscillator disabled to prevent broadcast stations from interfering with the alignment process or alternately connect the aforementioned signal generator through a 0.1-mfd/600-volt capacitor to the plate of the converter. In either case, disable the oscillator by shorting the stator terminal of the oscillator variable capacitor to the variable capacitor frame. The output voltmeter leads are connected between the plate pin of the last audio frequency tube and the chassis. The voltmeter plate lead is connected through a 0.1-mfd/600-volt capacitor. Each IF transformer stage is adjusted for a maximum audio output voltage meter reading. When doing this procedure, be careful to use the lowest RF signal generator signal level output that will give a reliable voltmeter indication otherwise, the receiver automatic volume control (AVC) circuit will interfere with the voltmeter level. Alternately the AVC voltage can be measured with a high impedance meter to monitor the receiver output while IF transformer alignment is being accomplished. 

RF and oscillator alignment is next. The radio receiver should be adjusted to 1400 on its dial. The signal generator should be adjusted to 1400 kHz, modulated near 30% and connected in series with a 0.00025-mfd/600 volt capacitor to the receiver antenna or with an antenna loop (see page 415 of Marcus and Levy’s Elements of Radio Servicing book). The receiver oscillator trimmer capacitor is adjusted to bring the 1400 kHz signal in at 1400 on the radio receiver dial, and then any other available receiver input RF sections are also adjusted to peak on the output voltage meter. The signal generator is now adjusted to 600 kHz, and hopefully the signal will come in at 600 on the radio receiver dial. If it doesn't, and the receiver has an oscillator coil padder adjustment the oscillator padder is adjusted to maximize the output meter signal at 600. Since this effects the frequency high end also, you must now go back and adjust the radio receiver oscillator trimmer capacitor with the receiver dial at 1400 and signal generator at 1400 khz for maximum on the voltmeter (don't touch the already-aligned RF input trimmer capacitors), and then go back to 600; repeating adjusting the oscillator coil padder as often as necessary to bring both ends of the dial into proper dial / actual input RF frequency tracking. For those sets with cut-plate oscillator sections having no oscillator padder adjustment, if the dial is off, the IF frequency must be adjusted instead. This takes a bit of experience but it's not really difficult. The easiest way is as follows (and we assume you have already aligned the IF at some frequency very near 455khz as described above). Let's assume the dial reads 590 when you are receiving a signal at 600kHz. Detune the dial towards 600 but not so far that you lose the signal. Adjust the IF trimmers to maximum, then adjust the oscillator trimmer capacitor at 1400 kHz as described above. Go back to the low end again and recheck, and repeat as necessary. If you find the error is greater, try adjusting the dial a bit lower than 590 (in this example) and repeat the adjustment of the IF trimmers for maximum output. Hopefully the above procedures will take care of most simple alignments.

"High Fidelity" receivers sometimes require a better method of IF alignment. On these sets the IF band-pass (or response curve) should not be a sharp peak, but rather a curve with a relatively flat top of, say, ten kHz width. One way to check the curve is to reset the signal generator a bit above and also below the IF frequency and noting the change on the output meter. Moving from, say, five kHz below to five kHz above the IF frequency should result in the meter reading a nearly constant voltage. If it does consider yourself fortunate. Usually one side of the curve will be higher than the other, or there may be a dip as the generator passes through the desired IF center frequency.
 

With a sweep generator (frequency modulated oscillator) and an oscilloscope it is easier to correct such curve deficiencies. Guessing at which IF adjustment screw to turn can result in perhaps reducing the level on one side of the curve but increasing the other side, reducing the overall curve, or moving the IF center frequency itself. With a sweep generator and scope it is easy to see what affects what.


Typical IF responses curve. Note that one side of the center frequency is higher than the other. Ideally both sides should be equal, with a minimum of dip in the center.

Another method of alignment to obtain a symmetrical IF response curve, is to use the signal generator set at the desired IF frequency and to use an audio modulation frequency of, say, ten kHz. Most signal generators use an internal audio generator of about 400 Hz, but also have provision for an external audio signal. Here you need an audio oscillator set at the desired frequency connected as the external source. Since the ten kHz modulation generates an IF signal with ten kHz sidebands, adjusting the IF transformers for maximum audio output should result in a somewhat flat-top response curve. Remember; again, keep the signal generator output down, and the AVC disabled, so as to not overload the circuits.

* Modified from an article Bruce McCalley including references from Marcus and Levy’s Elements of Radio Servicing book


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