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"Weak, Noisy Signals" is the last of the general categories of symptoms to be discussed, and it is the least complicated.
The condition occurs when signals can be received weakly, but electrical interference and atmospheric noises seem to be almost as strong as the stations. This symptom should not be mistaken for the No-Signals condition of Sections 3 and 4. In the case of Weak, Noisy Signals we shall be concerned with problems of misalignment or weak amplification, and the tests used will be of no use in finding the cause of the No-Signals symptom.
TEST POINT 1, CHART IV INJECT SIGNAL AND MEASURE AVC
SERVICING CHART IV shows that the symptom di vides itself into two distinct conditions. Thus, the testing is confined to the right or left side of the chart, depending on whether the signals are found to be weak at only one position of the dial, or over the entire broadcast band.
When using radio stations as a source of signals, it may be impossible to tell if reception is weak over the entire band, because of the different strengths of the incoming carriers at various points on the dial. The local stations, of course, will normally have the strongest signals, regardless of the condition of the receiver. So the method recommended at TEST POINT 1 is to use a signal generator and to measure the sensitivity of the receiver in terms of the A VC voltage present when the tuning is adjusted to various points on the dial. The chart suggests taking samples of the A VC using 600- khz, 1000- khz, and 1500- khz signals from the signal generator.
If the alignment instructions are available, the signal should be fed to the receiver in the manner recommended. If there are no instructions, a good way to inject the signal is to form several turns of wire into a loop of approximately the same dimensions as the loop antenna used on the receiver being tested. This homemade loop is then connected to both of the leads from the signal generator, and when it is brought close to the receiver, sufficient signal will be injected to make the test at TEST POINT 1. A method used by some technicians is to remove the loop antenna from a discarded receiver and store it with the signal generator for use when it is necessary to inject signals simulating the incoming stations on the broadcast band.
When the receiver being checked does not use a loop antenna, then an actual antenna must be installed before any testing of the A VC response is done. Signals can be injected into this type of receiver just as easily as before and using the same method.
Set the signal generator at a low output, and maintain the same output strength while attempting to compare A VC readings at the various points on the dial. If all the readings are the same, then further testing will proceed on the right side of the chart. If the AVC is different on one portion of the dial than on other portions, then this indicates that the sensitivity of the receiver is not consistent throughout the broadcast band, and the remedy will be found on the left side of the chart.
Further Tests When the AVC Is the Same All Over the Dial
This indicates that a defect exists which is not affected by changing the tuning circuits. The trouble could be a single tube, or it may be a general run-down condition of the entire receiver which will require several new tubes and even some other parts. All the tubes should be checked first.
When tubes have been eliminated as a cause for the weak signals, the antenna is inspected for damage or incorrect connections. The ohmmeter is a handy instrument for this job, and it can be used further to check the connections to the mixer-grid tuning capacitor. In receivers using printed-circuit boards, continuity between parts should never be assumed it should always be checked with the ohmmeter.
With a weak signal at all points across the dial it is likely that the trouble is in the IF or detector stages, but there is a possibility of a deficiency in the audio amplification. One can usually decide which is the case by noting the sensitivity of the receiver to outside electrical noise and atmospheric static; the audio should be suspected when there seems to be good sensitivity.
TEST POINT 2, CHART IV INJECT PHONOGRAPH SIGNAL AT VOLUME CONTROL
In cases where it is difficult to place the symptom in the audio or in the IF detector section of the receiver, a signal from a phonograph player can be injected as described in Section 5-5. Normal volume resulting from this test proves that the fault' is in the IF or detector circuits, and the tests to be made are shown on the left of the chart under TEST POINT 2. Weak reproduction indicates that the audio sections are at fault, and reference is made on the chart to the analysis in Section 3.
Further Tests When Audio Is Normal
If a Sams PHOTOFACT is available for the receiver under test, the alignment instructions will be found on the front page of the folder. This should be consulted to determine the IF frequency used in the receiver. Older models vary greatly in the frequency of the IF, and there is no simple way to determine the original frequency experimentally. The most common IF frequency currently in use is 455 khz.
It is important while injecting the IF frequency to keep in mind that the signal strength should be held to the lowest value consistent with good voltmeter readings. With the generator tuned to the IF frequency, the hot lead is connected to the signal grid of the mixer stage through an isolation capacitor. The value of the capacitor is not critical, and is usually given in the alignment instructions as .001 mfd. On many receivers an easy way to inject the signal is to connect the generator lead to the solder terminal on the mixer tuning capacitor. The DC voltmeter is connected to the AVC line as shown in Fig. 6-1. Alternate connections for the voltmeter are shown in the figure, but in these positions the readings will be affected by the setting of the volume control.
After the generator and the voltmeter are adjusted for minimum signal input consistent with good readings on the lowest scale of the meter, adjustment of the IF transformers begins.
It is not necessary to follow any order in the adjustment, but all the transformers should be adjusted a little at a time, working back and forth between them so that all the tuned circuits are brought to peak at the same time.
As the stages are tuned, the AVC voltage will increase, and the signal input should be decreased, rather than changing the voltmeter to a higher scale. If necessary, the generator can be disconnected and the signal fed into the receiver through the loose coupling obtained when the lead from the generator is merely brought near the mixer tube.
Besides giving assurance that the stages are aligned properly, the preceding alignment serves as a test procedure, checking out several different components and circuits at once.
If any of the IF transformers fails to show a peak adjustment, it is likely that the trouble which produced the symptom of weak signals will be found to be associated with that trans former. If there is no AVC voltage, or it does not vary with the adjustments, the defect may be in the AVC circuit itself.
Two more places which should not be overlooked when alignment does not seem to cure the symptom are the cathode circuit of the IF stage and the RF filter capacitor in the detector circuit. A routine check of all plate and screen voltages in the mixer and IF stages may reveal the stage which has failed.
Procedure When TEST POINT 1 Produces Weak Signals at One Part of the Dial Only
The mixer/ osc tube should be substituted at once, but since certain signals are amplified normally, there is probably no component failure of any kind; instead, the tracking of the oscillator and mixer tuning should be suspected. As explained in Section 2, the circuits are tuned so that the difference be tween their frequencies will always be the same, regardless of the setting on the tuning dial. In this manner, the IF frequency will be produced constantly as the oscillator and in coming carrier signal circuits are tuned simultaneously through their respective ranges.
For this explanation we shall assume that the IF frequency used is 455 khz, but the same principles will apply for receivers using any other IF frequency. Fig. 6-2 shows a typical tuning capacitor with the screwdriver-adjusted trimmer capacitors visible on the side of the stator frame. The purpose of these capacitors is to maintain the same difference frequency be tween the oscillator and mixer frequencies. It is these capacitors which must be adjusted to correct the "tracking." Suppose the mixer is tuned to 600 khz:
Osc freq. = RF + IF = 600 + 455 = 1055 khz
If the receiver is now tuned to a new station at 900 khz, the new oscillator frequency will be: 900 + 455 = 1355 khz
When the receiver is tuned through a range of 300 khz, both circuits must change their resonant frequencies by 300 khz.
The graph in Fig. 6-3 shows that a change of 300 khz in the mixer tuning is a 50% change, while a change of 300 khz in the oscillator is only about 28% . With the tuning capacitors coupled together on the same shaft, it is necessary to make one circuit change its resonant frequency by 50 %, while the other changes only 28% with exactly the same amount of shaft rotation. This is accomplished by using capacitors with each having a different total capacity and differently shaped rotor plates.
The trimmer capacitors aid in the adjustment of the tracking to keep the two circuits tuned exactly 455 khz apart at all points throughout the tuning range. The trimmers are adjusted as follows:
1. Tune in a station at the high end of the broadcast band.
2. Adjust the trimmer on the mixer tuning capacitor for maximum.
3. Tune to a station on the low end of the band.
4. Adjust the trimmer on the oscillator capacitor for maximum.
5. Readjust the main tuning and the oscillator trimmer until the best combination can be found.
6. Recheck the mixer trimmer at the high end of the dial for a maximum signal.
Step 5 is called the "rocking" process because it is accomplished by rocking the tuning dial across the station frequency and readjusting the trimmer until the point of best tracking is found.
Some circuits include an adjustment of the oscillator-coil tuning slug, or another small capacitor connected in series rather than in parallel with the main tuning capacitor (see Fig. 6-4). These are intended to adjust the tracking at the low end of the dial. When they are used, the oscillator trimmer on the main tuning capacitor is adjusted at the high end of the dial along with the mixer trimmer.
A signal generator can be used to create the alignment signal in the above procedure but it is not necessary since the tracking should be adjusted at the frequency of some local station which is desirable to receive.
Further Tests When Tracking Does Not Bring in Stations at the Proper Place on the Dial If the antenna loop or the tuning capacitor has been re placed, it is likely that the new part does not exactly match the circuit. This will spoil the original tracking. When this has happened, the performance of the receiver may be improved by changing the frequency of the IF channel.
By using a signal generator, the IF transformers can be peaked about 10 or 20 khz higher than normal, and the tracking adjustment repeated. If there is no improvement, the IF's can be peaked lower than normal, and the tracking adjustment repeated. It is often possible to make a great improvement in the tracking by altering the IF frequency in small receivers which do not perform well at one end of the dial. This method is not recommended, however, until all other possibilities have been investigated.
1. If the value of the cathode resistor in the IF stage in creases, would this cause weak signals all over the dial, or at one end only?
2. Explain two failures which could produce Weak, Noisy Signals in the circuit of Fig. 6-1 and which are not mentioned in the section.
3. In what ways is the convertor circuit of Fig. 6-1 different from most?
4. What components associated with the different circuitry described in Question 3 could cause poor tracking that would not be improved by adjustment of A5 and A6?
5. List in order and describe the results of all the tests necessary to isolate L4 in Fig. 6-1 as the cause of Weak, Noisy Signals.
6. Name two parts in the encapsulated circuit, K1, in Fig. 6-1, which could cause Weak, Noisy Signals.
7. Look up the alignment instructions for a recent-model broadcast receiver, and explain any differences compared to the instructions given in the section.
8. The following tests were made on a receiver having Weak, Noisy Signals:
What tests would you make next?
9. Under what conditions would you change the IF frequency of a receiver?
10. In Fig. 6-1, A VC voltage is found at the top end of R3, but it is not present at the bottom end where the meter is shown attached. What would you do next?