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FIG. 14—Schematic diagram of series and parallel connected loudspeakers in a public address system.
How may modulation hum which is "given a ride" through the r.f. and i.f. circuits of a receiver on the back of any signal that may be passing through them be detected and cured?
A receiver which emanates hum only when it is tuned to a carrier wave gives the best example of trouble of this kind. This trouble is sometimes due to allowing main leads to run in close proximity to radio and intermediate frequency circuits, in which case the remedy is obvious.
More often, however, the interference enters by way of the main transformer, and nowadays a metal screen between primary and secondary is generally interposed as a barrier. In the absence of such a screen, the simple expedient of connecting a condenser of 0.001 mfd. between one of the main leads and ground is Usually effective. The condenser should be rated for working at full supply voltage, and a connection to each main lead should be tried.
What may cause "distortion of sound"?
A defect in the loud-speaker—the voice coil may have loosened, or is off center. Lowering the tone volume may show difference in sound; and this localizes the trouble. The power tube or stage may be defective. Tuned circuits may not be properly aligned; in other cases the detector circuit may be overloaded, i.e. the signals are too powerful.
This may be determined by shortening the outside antenn^, then noting the results; or the signal input may be lowered by reducing the volume by means of the control if the receiver is so wired that the volume control acts on the radio frequency tubes and not expressly on the audio tubes and stages.
What may cause "Fading" in a radio receiver?
Trouble of this sort is most likely to be caused by the following: Leakage within tubes; defective volume control; defective condenser resistor or other parts which changes in value with usage, or by extraneous conditions. The source of trouble is usually best found by an evaluation of circuit components; by means of the usual set analyzer or tube tester. In case replacement is required, care must be observed that correct components are used—otherwise, instead of eliminating the trouble it is likely to be exaggerated.
How may extensive squeals and pitch interference which changes with tuning, be eliminated in a receiver?
A type of interference referred to is often encountered in inexpensive receivers and usually appears as an annoying whistle on desired stations that changes in pitch as the receiver is tuned. This is known as image frequency interference and is due to lack of selectivity in the first detector circuit. (Constant pitch whistles are another matter and will be experienced even in high priced receivers.)
Using a shorter antenna will often help matters as it reduces pick up. This in itself has nothing to do with the interference ratio, as it also cuts down the strength of the desired signal. However, to compensate this loss in signal strength the volume control is turned up and this often sharpens the tuning of the first detector circuit.
That other type of interference—the pitch of which varies with tuning, is that of dot and dash code signals. This is due to code stations working close to the intermediate frequency of the receiver. A great many sets have a trap to reduce this type of interference, the adjustment for which appears as a small screw extending from the rear of the chassis. This screw should be adjusted until the interference is at a minimum. The adjustment can be made without much trouble.
However, in case of attack on the wrong screw, the original position of the slot should be noted, and the screw turned back to its original position, if no improvement is noted.
Will the all around performance of a receiver used for a year or more be improved by tightening grounds and other connections on the chassis base?
Yes, the operation of many receivers over one year old can be improved by this simple expedient—which is generally very simple to perform. In every receiver, dozens of by-pass condensers and resistors are grounded to the chassis by means of lugs and nut and bolt connections.
While the leads are soldered to the lugs, the connection between the lug and the chassis becomes oxidized in time, increasing the resistance of the connection to a considerable degree, in effect this places resistors in the receiving circuit where they do not belong with naturally detrimental results.
While the effect of such an added resistor in series with a resistor would ordinarily be negligible, it is another matter in the case of a condenser and the accumulative effect of many such resistors in series with a similar number of condensers may result in lowered sensitivity, instability, alignment shift, etc.
This effect can usually be eliminated by tightening all screws from the top of the chassis. Even when screws are already tight, the slight movement will be sufficient to erase oxidation. Most of the connections are made to the bolts that fasten the socket to the chassis, and it will generally be necessary to remove the tube shields to get at them.
How may noisy reception originating in the automatic motor driven tuning unit be eliminated?
The great multiplicity of parts necessarily associated with the tuning device is located immediately within the electric field surrounding the receiver; and as the mechanical connections loosen with usage, microphonic contacts are formed which cause noise.
A loud signal may be sufficient to induce a rasping accom-paniment, as well as from any slight movement of the receiver, as might be caused by steps across the floor. In aggravated cases, a hissing scratch, akin to static, will be present at all times.
This difficulty can be removed by bonding—making a good electrical connection between the chassis of the receiver and the floating parts. Fasten a short length of flexible wire securely under some convenient nut or bolt on the chassis, and touch the free end to various portions of the framework supporting the timing mechanism, at the same time endeavoring to produce the sound by tapping the receiver.
If the rectifier tube of a receiver has become defective and trouble still exists after changing the tube, what may be the cause?
A defective electrolytic condenser. The heat, especially in midget type receivers where space is limited and ventilation poor, causes a drying up of the condensers and thereby a lowering of capacity. Voltage measurement of the output of the rectifier tube may show a decreased output, and hence point to condenser trouble.
If tubes do not light in a series wired receiver, what may be the cause?
A burned out tube or a loose connection. In a series circuit an open circuit will affect all tubes. In either case the remedy is obvious.
What important precautions should be observed when replacing parts in a radio receiver?
It is of extreme importance that they be of the identical types or of the same characteristics throughout, as the parts removed. If components of different values be inserted, they may upset the values of the circuits or cause trouble in other ways. A list of replacement parts for various receivers is usually furnished by the manufacturers of the various sets, and it is these parts that should be used for replacement, to prevent continuation and even exaggeration of the trouble.
What are the two general classes of resistors used in radio receivers, and where are they used?
The two types are classified as the wire-wound and the carbon type. Wire wound resistors generally are used where a comparatively large current is required to flow, such as in voltage dividers, and as power tube grid biasing resistors, etc. whereas the carbon resistor is utilized for small currents, i.e. the carbon resistor as a rule has a high resistance, and will usually handle power requirements up to 1 watt. The resistance value of a carbon resistor may be identified by means of special coloir code markings (see page 182) and the resistance of wire wound resistors are usually plainly marked or tagged on it.
Is it well to change or remove part of the wiring to eliminate trouble in a radio receiver?
Only where substitution of wires is absolutely necessary, but the circuit should not be changed. It is evident that if the circuit was incorrect the receiver would never have functioned in the first place. It may generally be assumed that before a receiver leaves the manufacturers test room, the circuit as well as its components are correct. Hence, servicemen should not change circuits on the assumption that they are wrong after the set has been*operating properly for some time, but should seek to find the trouble that has occurred and rectify it.
How many short wave coils be assembled so as to cover a range of short waves as well as medium waves?
In order to be able to successfully cope with a problem of this sort, the following precaution should be taken: The several coil sections must be reasonably separated from each other, a minimum spacing of one-half in. is recommended. Opinion is divided as to whether to short out the unused sections or to use a change over, but actually neither method is satisfactory by itself. The main difficulty arises from absorption of any particular waveband from the coil serving the band immediately above it.
Thus in fig. 15 (a) consider the circuit switched from the reception on coils C and D in series, while coils A and B are shorted out. Absorption will be found however, at the wavelength corresponding to coil B, tuned with its own self capacity since it is actually not short-circuited.
As a rule when it happens to fall within the band covered by coil C a dead spot will result. In (b) is shown an arrangement with a change over switch and it will be clear that this trouble is still present—in fact in an aggravated form. The remedies are to arrange the sections of the coil so that they do not couple to one another, which is not always convenient, or else to arrange to short out the section immediately next to the coil in use, in conjunction with a normal shorting or change over scheme.
Figs, (c) and (d), illustrated the methods likely to be most suitable when critical reaction is being employed.
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