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 Alignment of Modern Radio Receivers Part One

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Cliff Jones
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PostSubject: Alignment of Modern Radio Receivers Part One   2010-12-03, 11:54 pm

This is my first attempt to convert a graphics image to Text using OCR.
MALLORY-Yaxley
- RADIO SERVICE ENCYCLOPEDIA 1937-

Alignment of Modern Radio Receivers
----------------------------------

MANY VOLUMES have been written on this subject, both from a technical and a serviceman's viewpoint, and justly. because improper alignment is one of the most common ailments encountered in service work today. especially in receivers of the all wave type. Many servicemen are somewhat afraid to make adjustments on receivers. as long as the set plays at all, because they are not familiar with the various functions and workings of modern receivers. This article is intended to help clarify to the serviceman who wishes to learn the why of such adjustments, how the various radio frequency circuits of a radio receiver function, and how to make practical adjustments necessary in order to restore a set to its original factory performance and efficiency.

A vitally important part of a radio receiver is the small compensating condenser used to make adjustments of the various tuned circuits. These small adjustable condensers, usually called padders or trimmers, are constructed in various ways. They usually consist of two or more plates insulated from each other, one plate being made of spring material, so it will hold the adjustment or spacing given it by means of turning a screw or nut.

These condensers are used to obtain fine adjustment of the tuned circuits, so that they may be completely in resonance and perform at their highest efficiency. Since it is commercially impractical to construct coils or tuning condensers which would be accurate at every point on the dial, these trimmer condensers are placed across them so as to provide an accurate and easy means of making each circuit resonate at the proper dial position.

T. R. F. RECEIVERS
---------------------
In a tuned radio frequency type of receiver the adjustments of these padders are usually made at the high frequency end of the dial using a signal generator or a station as a signal source, and adjusting the trimmers until maximum output is obtained. In some of the older type receivers, where the tuning coils were not properly impregnated, they absorbed moisture through exposure, which causes considerable losses or reduces their Q, and appreciably reduces the already none too abundant selectivity.
Replacement or a baking and re-impregnating process is recommended for such cases before adjusting the trimmers.

In some of the older sets not using screen grid tubes, it is necessary to neutralize the circuits to prevent oscillations or howling, before the resonant circuit trimmer condensers are adjusted. Neutralization was usually accomplished by means of small trimmer type condensers, which served to compensate for the grid to plate energy transfer due to the grid to plate capacity of the triode tubes then used.

SUPERHETERODYNES
--------------------
The modern superheterodyne is considerably more complicated than these older type receivers and a brief review of the elementary theory involved in this type of receiver is necessary, so that the importance of making accurate adjustments on these receivers may be more fully appreciated. In this type of receiver circuit, the incoming R.F. signal is usually impressed across the primary of an antenna coil. The antenna coils' secondary is tuned over the desired frequency range by a variable condenser, which in turn is adjusted by means of the trimmer condenser connected across it. The signal usually goes from there into the grid of the first tube. In smaller sets the tube may be a detector oscillator or
in larger sets it may be the first R.F. tube. In other cases, the signal may be fed from the first coil into another coil which is also tuned over the range by a condenser across it. This is commonly called a band pass filter type circuit. The signal then goes to the first tube. In the circuits having a combination detector oscillator tube, the incoming signal is mixed with the local oscillator signal producing a
beat note, or the frequency difference between the incoming R.F. signal and the local oscillator signal!.

Some of the larger sets employ a separate oscillator tube and a separate first detector tube. In such cases, the oscillator tube generates the oscillator signal frequency which is combined with the R.F. signal in the first detector or modulator tube. In both cases the two frequencies are mixed in the first detector tube, so as to produce another frequency, which is the difference between the two. When the circuits are operating correctly, this frequency difference is equal to the intermediate frequency (I.F.) of the set. The local oscillator and the R.F. sections of the set are both tuned by means of variable condensers and the circuits are so adjusted that the beat note produced by the mixing of the two frequencies is always equal to the I.F. frequency of the set throughout their tuning range.

Commercial design uses an oscillator frequency higher than the incoming R.F. signal, because it is more economical to build a set with less capacity and inductance (required to produce the higher osc. frequency) than when it is lower, requiring more capacity and inductance. Capacity and inductance values when higher oscillator frequencies are employed, are much lower than would be required if the oscillator frequency were lower than the incoming frequency (R.F.). \When a gang type tuning condenser is employed this requires that the capacity of the oscillator section be less than that of the R.F. sections. Commercially, this is done by either making all condenser sections alike and inserting a small padder type condenser in series with the oscillator section capacity across the oscillator coil, or by using a cut plate type oscillator section, which has the required reduced capacity. In either case, a small trimmer condenser is also connected across the oscillator condenser so as to correctly adjust the minimum capacity of the combination or adjust the highest frequency end of the oscillator range.
The signal resulting from the mixing of the incoming R.F. signal and the local oscillator is fed into the intermediate frequency amplifier. The first I.F. transformer serves to couple the output from the first detector into the grid circuit of the first I.F. amplifier tube. The signal is amplified by this tube and then passes through a second I.F. transformer, which may feed it into a second I.F. tube or the second detector tube, depending on the size of the set. I.F. transformers are designed so that their natural resonant frequency is approximately the required I.F. frequency. In order to obtain maximum selectivity and sensitivity, both their primaries and secondaries are tuned to the exact I.F. frequency of the set by means of trimmer condensers.
The advantages of this system are: that since it is easier to design an amplifier for lower frequencies, an I.F. amplifier can be designed to operate at one fixed frequency much more efficiently, resulting in far higher amplification and increased sensitivity and selectivity than an amplifier designed to operate at higher frequencies and over a wide frequency range.

ADJUSTING COMPENSATING CONDENSERS
-----------------

Adjustment of these condensers should be made when the set lacks selectivity or sensitivity after other possible sources of this trouble have been checked and eliminated; such as weak tubes, poor aerial, improper tube voltages, etc.

I. F. ALIGNMENT
-----------------
The I.F. trimmer condensers should be adjusted before the R.F. section is adjusted. This is best done by using a signal generator with an audio modulated signal tuned to the exact I.F. frequency of the set. The signal from the generator is fed into the grid of the first detector tube. In some cases it is desirable to "kill" the local set oscillator by placing a bypass condenser across the oscillator section of the
tuning condenser to eliminate any erroneous beats which may be produced. An output meter should be connected from the plate of the last audio tube to ground or from plate to plate in case of push pull output.
If one owns a meter of sufficient sensitivity. it may be connected across the voice coil of the set. Sets using automatic volume control should be adjusted either by reducing the signal output of the service oscillator to a point where the Ave does not function, and using the output meter, or by inserting a milliameter in series with the load resistor in the AVC network, or connecting a vacuum tube voltmeter across the AVC network, so as to read the AVC voltage developed.

If the set is provided with a resonance indicator such as the "Shadow Meter" or cathode ray "Magic Eye" this will provide an excellent indicator for adjustment purposes. After having made suitable provision for indicating resonance, the I.F. trimmer condensers should be adjusted for maximum output, or so as to tune the I.F. circuits to their exact resonant frequency.

A signal generator should always be used for aligning the I.F. transformers. If a station signal is used, one is apt to get the entire I.F. system "off" frequency, although it may be set for maximum output. thus causing poor tracking of the oscillator and R.F. circuits, producing dead spots on the dial and in many cases whistles and birdies. In high fidelity sets where the fidelity is variable, it is usually advisable to set the fidelity control to the low fidelity or sharp tuning position of the I.F. circuits, and adjust the I.F.
trimmers so as to produce an overall I.F. tuning curve with a "flat top." Possibly the most accurate method and easiest of adjusting such high fidelity sets is to use a cathode ray tube in conjunction with a frequency modulated test oscillator, so as to reproduce the entire tuning curve of the I.F. system on the screen of the tube. However, this is a subject requiring volumes for satisfactory explanation, and cannot be included in this article. Sets which are equipped with automatic frequency control should be adjusted with the A.F.C. control turned off. See article on A.F.C., page 161.

After these adjustments have been made, the I.F. system of the set will respond to a signal which is exactly equal to a frequency for which the circuit has been adjusted.

In many modern superheterodynes, a wave trap is provided in series with the antenna circuit which is tuned to the I.F. frequency of the set, so as to prevent any unwanted signals of this frequency from entering the set and getting to the first detector and coming on through the I.F. system. The proper adjustment of such a wave trap is to connect the signal generator to the antenna post of the set and then adjust it to the I.F. frequency. Then turn the generator to maximum output. The trimmer condenser
across the wave trap should be adjusted until minimum response is obtained in the output of the set.

R. F. ALIGNMENT
-----------------------

After the I.F. section of the set has been aligned to the proper frequency, the next job is to align the R.F. and oscillator sections. Compensating or trimmer condensers are connected across the R.F. and oscillator coils in order to provide a means of accurately adjusting these circuits. The test oscillator should be connected to the antenna and ground terminals of the set and adjusted for a frequency close to the highest frequency portion of the range being adjusted. On the broadcast band. the adjustment is usually made at 1400 kc. Then adjust the R.F. trimmers for maximum output when the receiver dial is set at 1400 kc. The trimmer condenser provided across the oscillator condenser is f, the purpose of making the oscillator track at the hi frequency end of the dial. For instance, in a superheterodyne with an I.F, frequency of 465 kc., when the R.F. sections are tuned to 1400 kc., the oscillator must oscillate at 1400 kc. plus 465 kc. or 1865 kc. This frequency difference must be maintained between the oscillator and R.F sections throughout the tuning range of the set. I order to maintain this frequency difference at the low end, frequency of the dial, there is a compensation condenser placed in series with the oscillator gang.

On broadcast this adjustment is usually made at 600 kc. If the R.F. is set at 600 kc., a set with a 465 kc. I.F. would have the oscillator oscillating at 600 kc. plus 465 kc., or 1065 kc. Therefore, the high and low frequency padders provide the necessary tracking adjustments for two points on the dial.

Modern receivers are designed for three point tracking, for instance, on the broadcast band at 1400 kc. 900 kc., and 600 kc. The tracking at the third point or 900 kc., is determined by the oscillator coil Inductance. Although this is entirely a matter of set design, occasionally a serviceman gets a set in which oscillator inductance trouble is suspected and in which case he usually tries to obtain a new one. Unfortunately, however, they are sometimes unobtainable any price, so the only choice he has is to repair' one available. A few hints on how this can be done will be given.



TRACKING  OSCILLATOR COIL WITH R. F. COILS --> Next posting


Last edited by Cliff Jones on 2013-06-14, 12:37 pm; edited 5 times in total
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ve1arn
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PostSubject: Re: Alignment of Modern Radio Receivers Part One   2010-12-04, 6:06 am

Wish I could have had this much luck on my first attempt. 8c)

There are only a few words that didn't complete when converted. And those words, for me anway, were easy to figure out and change.

Thanks for posting it. I now have it saved in my radio folder.

Cheers, Bob
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Cliff Jones
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PostSubject: Re: Alignment of Modern Radio Receivers Part One   2010-12-04, 9:11 am

I tried using the online speller from google, but it evidently doesn't catch things well. Next time I will use my (OpenOffice.org) speller. Trying to convert it was a hassle, first time I tried to convert to text, I pasted it into notebook and it came up blank. So back to the drawing board. I'm glad I can post articles this way though. I think everyone will benefit. Thats the first time Iv'e used it. I may revert to MSOffice if I don't like OpenOffice.
------------------
I did notice in one of my readings, that the slots that are in the variable capacitors are set to specific points for dial adjustments, and/or dial (frequency) and oscillator points on the radio dial. Wink

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ve1arn
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PostSubject: Re: Alignment of Modern Radio Receivers Part One   2010-12-04, 9:26 am

Hi Cliff,

I've been using Textbridge OCR and then transferring it to Microsoft Word. I'll then use Word's spell check while reading the original scanned page.

In a long document, I tend to do one or two paragraphs at a time. It seems to work better for me that way. Then, I'll scan the images separately with my graphic software, and paste them in the proper location in the document. Image scanning from the OCR package just doesn't seem to have the quality I like when images are concerned. That's my feeling though. Others opinions will I am sure vary.

Cheers, Bob
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PostSubject: Alignment of Modern Radio Recievers part 2   2010-12-04, 10:51 pm

TRACKING OSCILLATOR COIL WITH R. F. COILS

First, the local set oscillator should be killed by shunting a bypass condenser across the oscillator gang section, and the set operated as a T.R.F. set and the dial calibration checked at several points across the band. The R.F. should be adjusted so that the dial corresponds as nearly as possible with the R.F. tuning. Also, the extreme ends of the range should be noted. Say that they are 1500 kc. and 550 kc.

Second, connect the test oscillator so that it will beat with the local set oscillator into the first detector. The R.F. tubes should be taken out so as to prevent any unwanted signal coming through. A pair of bypassed ear phones in the oscillator plate or first detector plate circuit, will allow one to detect the beat between the two oscillators. Then set the dial exactly to the previously noted high position, say 1500 kc., and set the test oscillator to 1965 kc. (I.F. frequency equals 465 kc. plus R.F. frequency). Then adjust the high frequency padder so that the local oscillator comes to zero beat with the test oscillator. Repeat the same performance at the low frequency end, setting the dial at exactly 550 kc., and the test oscillator to 1015 kc. Then adjust the series padder so that the local oscillator comes to zero beat with the test oscillator.

Third set the dial to 900 kc. and determine what frequency the set oscillator now has by beating it against the test oscillator. It should be 1365 kc. If it is above this, the inductance of the oscillator coil is too low. If it is below this value, the inductance is too high. Turns are added to increase the inductance and taken off to decrease it, one turn being a large amount unless the frequency discrepancy is extremely large. The easiest way of increasing the coil inductance is to arrange a piece of radio frequency iron on a screw, so as to move it up into the coil, or if the inductance is already too high arrange a copper penny in similar fashion. In this manner, the coil inductance may be adjusted “on the nose” so as to make the oscillator's frequency 1365 kc. when the dial is set at 900 kc.
Fourth, after the midpoint has been made to coincide, the set should again be operated as a super and the padding and trimming of the local oscillator completed or redone at 600 and 1400 kc.
This method is not perfect, since changing the inductance to make it coincide at 900 kc. changes the other two points, but by working back and forth a fairly good job can be done of tracking the oscillator with the R.F. and the dial and a great improvement can be made in the sensitivity and reduction of squeals over the entire band.

ADJUSTMENTS OF R.F. CIRCUITS IN ALLWAVE RECEIVERS

Adjustment of allwave receivers is somewhat more involved than a broadcast band receiver due to the multiplicity of circuits involved and consequent increase' in number of necessary adjustments. Before adjusting any allwave set, it is good practice to allow the set to warm up for approximately 30 minutes to allow for thermal expansion of the parts.
Each band of an allwave receiver must be adjusted separately in the same fashion as an ordinary broadcast receiver is adjusted; that is, connect the signal generator to the set's antenna and ground (usually doublet antenna equipped sets should have the two antenna posts connected together), and adjust the generator to some frequency near the high frequency portion of the band. NOTE: In some cases it is extremely important that a dummy antenna be provided as recommended by the manufacturer. This is usually a carbon resistor and a small condenser.
Then the oscillator trimmer (shunt) condenser should be adjusted until the generator's signal comes in at the desired point on the dial. Incidentally, it is extremely important that the oscillator trimmer be adjusted to the fundamental and not the image frequency. This can be assured by backing the trimmer screw entirely out, then slowly turning it in, until a maximum peak occurs. Turning the condenser slightly beyond this point will bring in another peak somewhat weaker than the first which is the image frequency.

Another check is to set the trimmer on the fundamental and leaving the generator at the same frequency rotate the gang condenser to a lower frequency position until the image signal is heard. This signal should be lower in frequency than the generator frequency by twice the value of the set's I.F. frequency.

Double responses or image interference is due to a lack of R.F. selectivity before the first detector tube, and is especially noticeable in the higher frequency bands. Very few manufacturers use more than one tuned R.F. stage ahead of the first detector on their high frequency bands, and some do not use any.

After the oscillator has been adjusted, the R.F. trimmers for the band being adjusted should be adjusted for maximum output. Then if there is a low frequency oscillator padder for the band concerned, it should be adjusted so as to make some known signal generator frequency near the low frequency end of the band come in at the correct dial setting.

Each band is adjusted in the same manner until the R.F. section of the set is completely adjusted. In some allwave sets, the circuit arrangement is such that there is an interlocking of adjustments between bands. In this case, the highest frequency band must be adjusted first, the next highest frequency band, and so on, until all bands have been adjusted, unless otherwise recommended by the set manufacturer. It is very helpful when adjusting allwave receivers to obtain a chassis layout showing location of all trimmer condensers before the alignment process is attempted.



ADJUSTING THE SIGNAL GENERATOR

It is highly important that portable oscillators or signal generators used in radio service work be checked frequently for correct calibration.
Most generators have compensating condensers which can be adjusted to correct for any shifting of Calibration that may have occurred. Their adjustment is simple, since it only involves:,
First, tuning in a broadcast station near the high frequency portion of the generator's band.
Second, connecting the generator to the antenna of the set and beating it against the station.
Third, adjusting the generator's compensating condenser until the dial reading obtained on the generator at zero beat corresponds with the known station frequency.
Fourth, repeating the same process for some known station frequency near the low frequency end of the generator's dial and adjusting the series padder condenser until zero beat is obtained between the station and the generator, until the calibration of the generator corresponds to the known station frequency .
Some signal generators are not provided with a means of making their generated frequency track with the dial at the low frequency ends of the various bands. In such cases nearly perfect tracking can be effected by arranging a means of varying the inductance of the oscillator coil. If the inductance is too high, it can be lowered by moving a copper penny on a screw into the coil, or if it is too low by moving a piece of R.F. iron on a screw into the coil. There is probably nothing more time wasting in adjusting a set, than attempting to correctly adjust it with a generator whose dial calibration' is incorrect

CHANGING THE I. F. FREQUENCY

In some parts of the country interference is, experienced from nearby powerful code stations operating on a frequency near that of the I.F. peak of the
receiver. The first remedy for this trouble is to install a wave trap in the antenna circuit tuned to the frequency of the unwanted signal. If this does not reduce the interference to a negligible amount it may be necessary to shift the I.F. frequency slightly until the unwanted interference is eliminated .. Another cause of interference in the form of whistles and birdies, sometimes on every station received, is two powerful local stations whose frequencies are such that the beat between the two is nearly equal to the I.F. frequency of the set. For example, powerful locals broadcasting on ,1400 kc. and 1230 kc, would produce a beat frequency of 170 kc., which is very close to 175 kc., a common I.F. frequency. Unless the set has an unusual amount of R.F. selectivity it will be full of unwanted whistles. This can usually be remedied by shifting the I.F. frequency up to approximately 200 kc., and re-tracking the oscillator to match the RF. for maximum response over the band. It is also helpful in some extreme cases to install a wave trap in the antenna circuit tuned to one of the interfering signal frequencies.



AUTOMATIC FREQUENCY CONTROL
Automatic frequency control is probably the the most important circuit development of the year. Many manufacturers are employing it in their new sets. The circuits employed by various manufacturers may differ in detail, but basically they are the same.

The purpose of AFC is to vary the frequency of the set oscillator (over a predetermined range) so that the frequency difference or beat between the set oscillator and signal frequencies is always equal to the I.F. frequency of the set. This then takes care of improper tuning adjustment, oscillator drift, tracking inaccuracies, and makes commercially possible mechanical stop dial type tuning. In AFC circuits, two separate functions must be performed.

First, there is the discriminator. The purpose of the discriminator is to convert I.F. frequency changes into voltage variations. If the incoming I.F. frequency is higher than the resonant I.F. frequency of the set, the discriminator must produce a voltage varying in one direction. If the incoming I.F. frequency is lower than the resonant I.F. frequency of the set, the discriminator must produce a voltage varying in the opposite direction.

The second function is that of varying the frequency of the set oscillator. A separate tube known as the
"Frequency Control" performs this task. The frequency control must be arranged so that voltage variations in one direction tend to increase the frequency of the set oscillator, and voltage variations in the other direction tend to decrease the frequency of the set oscillator.

DISCRIMINATOR

As previously mentioned, the purpose of the discriminator is to change frequency variations from the resonant I.F. frequency into D.C. voltage variations.

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Cliff Jones
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PostSubject: Re: Alignment of Modern Radio Receivers Part One   2015-04-30, 9:49 am

Here's a well done video to put things in perspective a little more. I watched Mr Carlson's Lab  from 2am till almost 5:30am this morning and I was very Impressed to say the least.



 https://www.youtube.com/watch?v=z8k-h-g_8V8
Well Done Paul!

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ve1arn
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PostSubject: Re: Alignment of Modern Radio Receivers Part One   2015-05-18, 5:32 pm

Thanks for the link Cliff. I'm downloading it to watch later.
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PostSubject: Re: Alignment of Modern Radio Receivers Part One   2015-05-19, 12:28 pm

I will see if I can do a follow up on this publication, I just need to get in gear.

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