Tuned Circuit Tracking.
Tuned Circuit Tracking
By Bill Jones
When Tuned Radio Frequency (TRF) radios were originally mass marketed, the station selection means was by individual variable capacitors. Tracking of the capacitors was not a problem for the adept operator. Each capacitor could be optimally adjusted for the best reception of a station. I suspect that it was obvious to the designers that a mechanical connection between the variable capacitors would be desirable, but it was not available and it would be expensive to design and build, anyway radios were being sold without this feature. I suspect that there was also a form factor involved, and once there is an accepted form of a product by a producer, other manufacturers tend to follow that form. Usually a product is defined by the available components and this will have a strong influence on the form factor. The original components were large and using three separate capacitor/inductor stages made for a reasonably sensitive radio and the “three-dialer” became quite popular. Some years after the three dialer became the standard radio the “ganged capacitor” became available and these ganged capacitors were manufactured in two, three and four ganged versions (or even more, for example the (Majestic built a five gang pot metal capacitor) with the three gang probably the most used. These variable capacitors were originally put together with screws rather than permanent means such as rivets and eyelets. The older capacitors that used screws are the capacitors that lead to the tracking problems discussed here. As corporations became larger they began to make their own components and this allowed a marked change in the form factor of the radio and a great improvement in the variable capacitor.
This note is mainly concerned with the TRF tracking problems, but it should not be assumed that there are no problems with the superheterodyne. The superhet actually has a problem because it starts with a built in tracking error (padded capacitor version) of about five percent. The ganged variable may actually be built with a perfect match between the variable capacitor sections but there may still be tracking problems for both the TRF and the superhet. This problem may be caused by at least three things. One is the unwanted “in circuit” capacitance, Second is a mutual inductance factor when using inductive coupling between stages. Inductors are generally very well matched, but when in use there are errors because of the mutual inductance between the primary and the secondary of the unit. For example, the antenna circuit in these old radios usually had a large primary inductance and the mutual inductance of this primary plus the variation in the antenna impedances will cause this stage tuning to vary considerably. Tuned circuits are often coupled using a large primary inductor that couples to the secondary (by mutual inductance) and, this may lead to an undesired frequency error. The tuned circuit driving the detector will present a very different load on the tuned circuit as compared to the usual r.f. amplifier. The third and probably the worst problem for the old radio is the shifting of the stator and rotor parts with time.
The first ganged capacitors were constructed such that the rotor was fixed to the main shaft with setscrews. The stator fins are often not properly centered within the rotor fins but if it is off slightly; how could it matter? It only means that if one rotor plate is closer to one stator plate, it will be farther from the other stator plate. It is, however, shown in the “Variable Capacitor” article on this blog that it will cause a serious increase of capacitance. If any section of a ganged variable is not properly aligned it will have a larger capacitance than a properly aligned variable capacitor. Said another way, a properly aligned variable capacitor will have its least or smallest capacitance compared to an improperly aligned variable capacitor regardless of its rotational position. That is, proper alignment will provide the least capacitance that a variable capacitor can have - in any fixed position.
There can be a very large capacitance difference between the sections of the ganged variable if the stator plates are not properly aligned with rotor plates. A tracking problem then occurs and the sensitivity of the radio will vary in an unacceptable fashion with respect to the rotational position of the variable capacitor.
The rotors may also be slightly misaligned rotationally speaking; a slight difference here can easily make a large difference especially when the plates are closed, therefore it is best to check the alignment with the rotor in the closed position. The centering of the stator is generally a tedious and time consuming chore. The stator fins must be adjusted such that they are parallel to the rotor fins in horizontal and the vertically directions. It is desirable to have some type of shim to make this adjustment. This is discussed further in our remarks where a shim is suggested; the shim I like is called a tappet adjustment gauge that is used to adjust the tappets for some cars. These gages are variable in I mil steps and make an excellent tool for the capacitor adjustment, and are inexpensive.
It is not unusual to find loose screws holding the stators. It is also not unusual to find variable capacitors with large offsets between the rotor and stator fins. There is often an endplay adjustment that provides rotational resistance when turning the rotor. This adjustment will also provide an undesired offset between the rotor and stator fins for each capacitor in the gang.
It is difficult by casual examination to determine if a radio has tracking problems. The easy way to check for a tracking problem is to set the trimmers as usual, at the high (rotor open) end. Then tune to a lower frequency and check to see if any of the trimmers provide increased signal or volume-if not the tracking is likely good but it is necessary to continue to lower the frequency and each time to vary the trimmers to check for greater signal. You will be surprised to find how much the trimmers need adjustment.
If you are not satisfied with the sensitivity of your early receiver that uses the old ganged capacitor – check the alignment of the stator and rotors. This adjustment is quite important!
The analysis of the capacitance of a variable capacitor follows. It shows how the capacitance varies as the position of the center (rotor plates) plate’s move horizontally as compared to a rotating movement. If the plates are perfectly centered the variable capacitor will have its smallest possible value of capacitance. See the article “Variable Capacitance Analysis”.
Questions or Comments? Please e-mail meat whnj
By Bill Jones
When Tuned Radio Frequency (TRF) radios were originally mass marketed, the station selection means was by individual variable capacitors. Tracking of the capacitors was not a problem for the adept operator. Each capacitor could be optimally adjusted for the best reception of a station. I suspect that it was obvious to the designers that a mechanical connection between the variable capacitors would be desirable, but it was not available and it would be expensive to design and build, anyway radios were being sold without this feature. I suspect that there was also a form factor involved, and once there is an accepted form of a product by a producer, other manufacturers tend to follow that form. Usually a product is defined by the available components and this will have a strong influence on the form factor. The original components were large and using three separate capacitor/inductor stages made for a reasonably sensitive radio and the “three-dialer” became quite popular. Some years after the three dialer became the standard radio the “ganged capacitor” became available and these ganged capacitors were manufactured in two, three and four ganged versions (or even more, for example the (Majestic built a five gang pot metal capacitor) with the three gang probably the most used. These variable capacitors were originally put together with screws rather than permanent means such as rivets and eyelets. The older capacitors that used screws are the capacitors that lead to the tracking problems discussed here. As corporations became larger they began to make their own components and this allowed a marked change in the form factor of the radio and a great improvement in the variable capacitor.
This note is mainly concerned with the TRF tracking problems, but it should not be assumed that there are no problems with the superheterodyne. The superhet actually has a problem because it starts with a built in tracking error (padded capacitor version) of about five percent. The ganged variable may actually be built with a perfect match between the variable capacitor sections but there may still be tracking problems for both the TRF and the superhet. This problem may be caused by at least three things. One is the unwanted “in circuit” capacitance, Second is a mutual inductance factor when using inductive coupling between stages. Inductors are generally very well matched, but when in use there are errors because of the mutual inductance between the primary and the secondary of the unit. For example, the antenna circuit in these old radios usually had a large primary inductance and the mutual inductance of this primary plus the variation in the antenna impedances will cause this stage tuning to vary considerably. Tuned circuits are often coupled using a large primary inductor that couples to the secondary (by mutual inductance) and, this may lead to an undesired frequency error. The tuned circuit driving the detector will present a very different load on the tuned circuit as compared to the usual r.f. amplifier. The third and probably the worst problem for the old radio is the shifting of the stator and rotor parts with time.
The first ganged capacitors were constructed such that the rotor was fixed to the main shaft with setscrews. The stator fins are often not properly centered within the rotor fins but if it is off slightly; how could it matter? It only means that if one rotor plate is closer to one stator plate, it will be farther from the other stator plate. It is, however, shown in the “Variable Capacitor” article on this blog that it will cause a serious increase of capacitance. If any section of a ganged variable is not properly aligned it will have a larger capacitance than a properly aligned variable capacitor. Said another way, a properly aligned variable capacitor will have its least or smallest capacitance compared to an improperly aligned variable capacitor regardless of its rotational position. That is, proper alignment will provide the least capacitance that a variable capacitor can have - in any fixed position.
There can be a very large capacitance difference between the sections of the ganged variable if the stator plates are not properly aligned with rotor plates. A tracking problem then occurs and the sensitivity of the radio will vary in an unacceptable fashion with respect to the rotational position of the variable capacitor.
The rotors may also be slightly misaligned rotationally speaking; a slight difference here can easily make a large difference especially when the plates are closed, therefore it is best to check the alignment with the rotor in the closed position. The centering of the stator is generally a tedious and time consuming chore. The stator fins must be adjusted such that they are parallel to the rotor fins in horizontal and the vertically directions. It is desirable to have some type of shim to make this adjustment. This is discussed further in our remarks where a shim is suggested; the shim I like is called a tappet adjustment gauge that is used to adjust the tappets for some cars. These gages are variable in I mil steps and make an excellent tool for the capacitor adjustment, and are inexpensive.
It is not unusual to find loose screws holding the stators. It is also not unusual to find variable capacitors with large offsets between the rotor and stator fins. There is often an endplay adjustment that provides rotational resistance when turning the rotor. This adjustment will also provide an undesired offset between the rotor and stator fins for each capacitor in the gang.
It is difficult by casual examination to determine if a radio has tracking problems. The easy way to check for a tracking problem is to set the trimmers as usual, at the high (rotor open) end. Then tune to a lower frequency and check to see if any of the trimmers provide increased signal or volume-if not the tracking is likely good but it is necessary to continue to lower the frequency and each time to vary the trimmers to check for greater signal. You will be surprised to find how much the trimmers need adjustment.
If you are not satisfied with the sensitivity of your early receiver that uses the old ganged capacitor – check the alignment of the stator and rotors. This adjustment is quite important!
The analysis of the capacitance of a variable capacitor follows. It shows how the capacitance varies as the position of the center (rotor plates) plate’s move horizontally as compared to a rotating movement. If the plates are perfectly centered the variable capacitor will have its smallest possible value of capacitance. See the article “Variable Capacitance Analysis”.
Questions or Comments? Please e-mail meat whnj
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