This one is a little off the beaten path, but it is something that can save you some money in some circumstances.
If a radio, a stereo reciever or tuner is not pulling in many stations on AM or FM, and you do not have alignment tools (singal generator, sweep generator, etc), AM is fairly easily adjusted, and certain portions of the FM section can be tweeked without too much effort, and without affecting stereo separation in FM. While alignment gear is needed for a full FM alignment, what I describe deals with solely those FM recievers, radios and stereo tuners that have no problems with separation or with a single station appearing in multiple spots on the dial. In that circumstance of multiple station “hits” and lack of separation, or other issues with the demultiplexer; those are issues that really require a full alignment for correct adjustment that this outline of steps will not correct. This will correct weak reception issues, and these tweeks have no effect on the demultiplexing circuitry of FM recievers.
This procedure will only marginally apply to totally digitally tuned recievers. Tweek at your own risk, but the risks are minimized if you follow the steps correctly. Plus- Mark the starting points when at all possible so in a worst case scenarion you can “undo” the tweeks if you make a mistake. Most digital tuners only have trimmers for antenna matching and some have “wave traps” which may or may not be tunable- the traps are not discussed here, but the antenna trimmer(s) may be part of the digital tuning circuit.
Note* Some digital tuners are analog tuners with a digital display. The tweeks discussed here will work on those units, but the variations in how the station numbers are displayed vary and are not discussed here beyond the simple Oscillator trimmer- this “may” or may not affect the digital display. Only a service manual will give the definitive discussion of how to correct digital displays where the oscillator trimmer does not adjust the display to the recieved station. The trimmers should not impact a full digitally synthesized tuner beyond matching the reciever to the antenna.
AM reception. First check to see if your stations line up reasonably accurately on the dial- those stations you can pull in, especially those that are weak. If your stations line up correctly, You may only need to adjust the antenna trimmer capacitor on the “tuning gang”. Some have the FM tuning gangs as part of the same gang, and some have a seperate FM gang. Sometimes, as in the circa 1969 Sansui Model 2000, there is a seperateFM tuning section, and instead of trimmer capacitors, it uses slug tuned inductance trimmers. The process is essentially the same, and in this case, the slugs in with the FM tuning section are the only slugs being tweeked. A plastic alignment tool is a must, and it must fit the slug perfectly, or else you run the risk of splitting the slug and causing other issues.
The tuning gang will have sets of small blades and sets of larger blades- the AM sections are the larger blades. In the event you have a set of blades that have about the same number of blades as the larger blades, but is a little bit smaller- this smaller set is the oscillator section. adjusting that will move where the AM stations line up on the dial.
Getting back to the antenna trimmer- adjust this with the antenna attached, and on a station you can pull in weakly towards the upper end of the dial. If you have flourescent lighting noise on the power line, you may be limited to about 1200Khz or less of tunability until the antenna is matched to the reciever. If you do the adjsutments carefully, you should have no issues as these trimmers have zero impact on the stereo decocing circuits. However, once in a while you will encounter a reciever that responds a little bit slowly, or you have to rock the dial knob back and forth in order to “zero in” on the station. In that event, if it is an open gang where you can actually touch the plates, use some contact cleaner, such as LPS-1, or “Contact Cleaner” available under various manufacturer’s names- often solvent based. If you do not know if it will attack plastics or paint on the reciever or radio- check on the inside in an inconspicuous spot to be sure. The LPS-1 is non-reactive to most plastics (I have not encountered any that are attacked or fogged by it, but for everything there is first time, so just to be safe, direct the spray away from plastic parts. This would be applied at the ends where the shaft that supports the movable blades as well as the brass contacts that press against the shaft itself, or to a blade of each section to make constant electrical contact. The movable blades are at chassis potential, which is usually chassis ground.
If you have a schematic or a “map” of the component layout- often from the “alignment” section of a service manual if you can locate one, identify the AM and FM “cans” and do not adjust the FM cans- mark their starting positions jsut in case you turn one accidentally, so you can adjust them back.
Usually, the AM “cans” are going to be one oscillator coil, (adjust this only when the oscillator trimmer on the tuning gang is not adequate to correct issues of station location), sometimes you will only be able to adjust the trimmer.
There will be at least one “IF” (Intermediate Frequency”) can in the AM section, If ceramic oscillators are used, there is little adjustment you can do with these. And most of the time with older radios or recievers, there will be 2 IF cans for AM. In some cases in newer radios and recievers, one IF can may be replaced by a ceramic oscillator, and once in a great while both “IF” cans will have been replaced with ceramic resonators. These will usually be a small yellow block with “455” inked on the side or small ceramic rectangle inked on the side in most cases, and are only mentioned here if you can only find one IF, or none at all.
On some recievers, usually the expensive units, they are in a plastic case with mylar between the blades. These will have 2 or 4 trimmers on the back side of these. Typically AM or FM is going to be diagonal from each other. In these units, one is always the oscillator and you will spot it because the stations will change. Otherwise these are adjusted as above for peak signal.
I will not go into a discussion of the details of alignment of the IF”S, but I will discuss the principle of their purpose of the IF at this time, those are adjusted towards their designed peak frequency, this does a couple of things. 1) it increases the sensitivity of the reciever (sometime notably). 2) it sharpens the tuning point of a station, sometimes to the point where it is difficult to get it tuned in from the dial because it’s spot on the dial now is so small. If this happens and the tuning range from high to low is crowded to one part of the dial, you need to detune the IF slightly so you spread the tuning out over the dial. This is one area if you have an RF generator it can come in very handy, but with an average stereo reciever, they should not be that far out of tune unless someone tweeked it incorrectly previously or other circuit fault developed. Once you get used to using the radio station signals as guidepoints on the dial, the RF generator is only needed when major rebuilds are undertaken where IF’s are replaced to get them close. 3) it also deflects the tuning meter needle, which you should also view to make sure it deflects to “tuned” at the right point on the dial.
FM decoding traps/oscillators and FM IF’s should be left alone because they do require some accuracy in the alignment, and while the IF’s determine where things line up and how many times on the dial a station occurs, they require a great deal of patience and luck if you try to tune these with the tuning meter- in general, these should not be touched, although sometimes you can improve reception by tweeking the first IF only slightly. This may seem contradictory, but it is not. If you have a very good ear, and pay attention to the tuning needle and scrupulously check tuning linearity across the dial, this would be an adjustment of last resort. Not for the faint of heart. If you get the first IF too far out of peak tune, a full alignment is what you have to do, and that is not the scope of this post.
However with all the caveats and cautions, under normal circumstances, you should not need to make much of an adjustment, usually less than a small fraction of turn to get the first IF stage peaked.
In the end what your goal is: 1) To tweek the trimmers to achieve maximum sensitivity by matching the reciever to the antenna you have in use. As the first step and as you make other adjustments, you want to peak the signal. 2) Have linearity of tuning across the dial face. 3) Have the stations line up where they are supposed to on the dial.
The first few times may be agravating. There are times when only slight tweeks need to be made, and once you get used to making the adjustments, if you do it correctly, you can tune up the AM section very quickly.
On less expensive radios, they have a small plastic case which contains the tuning gangs, On these- mark your starting points, and once you have identified which two are for the AM/MW section- mark them in some way so you do not disturb them when you move to the FM section. In most cases, one will be directly acting on the oscillator circuit, the antenna trimmer will be diagonal to it. If there is also a Short Wave and/or a Longwave band, these will their trimmers near the tuning gangs as well, but will usually be on the chassis. These tweek the same was as the AM/MW trimmers- if you peaked the IF’s on AM and have everything correct, the IF’s are left alone at this point because the Long Wave and Short Wave bands use the same IF’s as MM/AM.
This is largely the same basic steps as the AM section. The Multiplex section rarely changes and as long as it is not disturbed, the need for a sweep generator is minimized. I transplanted a Demultiplexer removed from an inexpensive solid state console reciever into a Heathkit FM Stereo tuner that dated from about 1964. The unit had a broken coil I could not locate a replacement for. It had easily adjustable separation on the front of the unit originally to accomodate other decoding schemes if a different decoding method had been chosen by the FCC.
The primary adjustments on a typical Stereo reciever are going to be simply the antenna trimmer, the oscillator trimmer and the first IF can only (and rarely). Once in a while, if the reciever has trouble tuning in stations when it is in “FM Mute” mode, you will find the antenna coil roughly even with the section of the tuning gang corresponding to the antenna trimmer. You can really only tweek these with a very high accuracy by paying attention to linearity across the dial, narrowness/accuracy of tuning- using the tuning indicator to know when on a station as well as relative signal strength. There is also a threshold adjustment on the muting circuit, but often tweeking the antenna trimmer is all that is needed to get correct operation back.
On some tuning gangs there are three sets of FM tuning gang sets, with the one furthest from the “front” of the tuning gang being the oscillator (usually, but not always, Sansui has the oscilaltor next to the tuning wheel of the tuning gang.). In some cases, you will have two trimmer caps for the FM section. In this event of 2 trimmer caps on a 3 gang tuning section, the furthest one will directly interact with the position a station will tune in on the dial. If not, there will be an FM Oscillator coil with a tuning slug, So you can locate the station first, if the frequency is known- verify where it is in relation to where it needs to be. If the ocscillator close does not get you close enough, after the antenna trimmer, or coil has been peaked, you need to adjust the oscillator coil as the end step until things do line up, and the antenna trimmer should be adjusted for peak signal as well as the accuracy of the station locations because it also affects the sensitivity of the unit to some degree. The antenna coil will be right in the vicinity of the oscillator section of the tuning gang.
On those with PLL IC’s, there will be a small variable resistor that adjusts threshold of the “Stereo” indicator. Once the antenna has been matched to the reciever, this adjustment can be tweeked on a distant station that you should be able to recieve a stereo signal from- usually 40 to 50 miles is a good benchmark. It should only be adjusted if the antenna trimmer has not improved a PLL “Lock” on those stations, and it should only be adjusted to a point where it does just lock on those distant stations.
When tuning, it is best use a non-magnetic screw driver, but if you have no choice- you end up spending a bit more time to “dial in” because the magnetic screw driver will interact with the tuning components, and one needs patience if using one.
Also-if tweeking any tuning slug- you must match the slug drive exactly or you will split the core. Nylon tools are best and reduce the chance of splitting a core.
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