|Home | Articles | Forum | Glossary | Books|
In this section we will be dealing with those sections of the receiver which handle the signal before it reaches the detector. It will be assumed that the audio stages are working.
This is verified by the results of TEST POINT 1--injection of audio at the volume control-which will produce normal sound from the speaker. When no sound results from this test, SERVICING CHART II at the end of this section refers to Section 3.
TEST POINT 2, CHART II MEASUREMENT OF OSCILLATOR GRID VOLTAGE
After it has been established at TEST POINT 1 that the audio section is working, the analysis shown on Chart II moves at once to a measurement of the oscillator grid voltage. This test isolates the trouble to either the RF or IF sections of the receiver. If a negative voltage is found at the oscillator grid, it is probable the oscillator section is operating, and further tests are confined to the mixer and IF stages. If no grid voltage is present, all further testing will be in the oscillator circuit.
The reason for the negative voltage on the grid of oscillators can be understood from a study of Fig. 4-1. Two common forms are shown in parts A and B. Part C is a drawing of an equivalent circuit which shows the action of the oscillator.
When a positive voltage is first applied to the plate, there is no voltage on the grid, and current starts to flow from the cathode to the plate. L1 and L2 are close to each other and are so wound that electrons passing through L1 induce a voltage across L2 with a plus polarity at the top. This is the same as a positive signal voltage applied to the grid of the tube and, therefore, the current increases. Increasing current through L1 increases the positive voltage applied to the grid, and the process continues.
This action of taking a small amount of the plate or cathode signal and feeding it back to the grid in such a way as to increase the current through the tube is called regeneration.
Every oscillator depends on regeneration to drive its grid. If the process were to continue indefinitely, the tube would be ruined because the cathode current would increase beyond the power capability of the tube.
CHARGE LEAKS OFF Cg THROUGH Rg MAKING THE GRID NEGATIVE
But the action does not reach this limit because of the drop across the plate resistor, Rv- In Fig. 4-1, if the plate current reaches 10 ma, the voltage drop across RP is 100 volts, and no voltage is left to appear between the plate and cathode. Thus, 10 ma is the limit of plate current before the action stops. In practice, the current levels off at some point below 10 ma where the remaining plate voltage is just large enough to keep it going.
During the time when plate current is rising, another action is taking place. The grid has been carrying a positive charge and has been attracting a small number of the electrons re leased from the cathode. These come out of the tube through the grid connection and are stored on Cg. By the time the plate current levels off, a charge of several volts will be present on Cg.
With no further change in current through L1, the induced voltage across L2 disappears, and this releases the electrons stored on Cg. These electrons leak down through Rg, creating a negative voltage on the grid of the tube. This voltage cuts off all current through the tube until the charge on Cg is reduced, and the cycle begins again.
As successive cycles repeat, a residual DC voltage is left across Rg, and it is this voltage which is measured to give proof of oscillator operation. Any failure in the oscillator will result in no DC grid-leak voltage across Rg Two precautions are necessary in taking this voltage measurement. (1) The internal resistance of the voltmeter connected between the grid and cathode of an oscillator can change the circuit sufficiently to stop oscillator action. (2) Some circuits use a small fixed DC voltage between the grid and cathode which could be mistaken for oscillator grid-leak voltage.
The first difficulty is overcome by isolating the meter with a 1-meg resistor fastened to the end of the probe. Some technicians insert a ¼-watt resistor into the probe and solder the test lead to it so that the resistor is in series between the end of the probe and the test-lead wire.
Experienced technicians use a simple trick to avoid the second difficulty. When a negative voltage is found on an oscillator grid, the finger tip is touched to the metal end of the probe. If the voltage is true oscillator grid-leak voltage, it will drop to almost zero when the probe is touched, because the circuit action is destroyed by the application of the finger. If the voltage does not drop considerably, it was not really grid leak voltage, and it is safe to assume that the oscillator is not running.
An exception occurs in rare cases. The oscillator may be operating, but it may still be the cause of the no-signal condition because it is running at the wrong frequency. When this happens, the technician should return to an examination of the oscillator coil and tuning capacitors after TEST POINT 4.
TEST POINT 3, CHART II Oscillator GRID VOLTAGE IS MISSING
After it has been established that the oscillator has failed, a new tube should be substituted. A substitution is recommended instead of testing the tube, because the tube tester does not check the tube in its true operating condition. Oscillator tubes can fail in ways which are not shown on the tube checker.
When the new tube does not restore operation, the volt age at the oscillator anode is checked. This is actually the screen grid of the tube and is composed of the two grids which are tied together within the tube. It is pin 6 on the popular 12BE6. This is a more important measurement than the plate voltage in the case of oscillator failure. This is because many circuits will continue to oscillate if the plate voltage is missing, but the screen voltage must be present in all circuits for the oscillator to operate. 60 to 100 volts is expected at this point.
Under TEST POINT 3 on SERVICING CHART II is the notation "Trace toward B+." The voltmeter is moved across each component, following the line toward B+, until a voltage is found. The defective component will be the preceding one in the line. Fig. 4-2 shows some possibilities and the readings which will be found with the voltmeter.
Further Tests If Oscillator Anode Voltage Is Present
The series of tests to make is shown on the right side of the line under TEST POINT 3. They are listed from left to right, starting with the most frequent cause. The chart indicates that first a check should be made for plate voltage on the tube. Absence of this potential will stop the oscillator and, in this case, the other tests will not be necessary.
When the plate voltage is present, the technician should make ohmmeter checks of the oscillator coil, and from the cathode pin to ground. The grid capacitor and resistor seldom fail, but they should not be overlooked when all other parts seem good.
Although it is not shown on the chart, a receiver occasion ally has oscillator failure because of shorted plates in the oscillator section of the main tuning capacitor. This is usually apparent upon examination of the unit, but it can be checked with an ohmmeter if the lead to the oscillator coil is first unsoldered from the stator plates.
The little screwdriver-adjusted trimmers on the side of the tuning capacitor have also been known to short. The movable plate on these capacitors is insulated from the body of the main unit by a thin piece of mica. Sometimes this insulation is missing and the two parts of the trimmer are touching, which shorts out the entire tuning capacitor. The location of this particular fault is difficult because it requires the use of all the logical and systematic tests we have been discussing, in addition to the practical know-how required in remembering to disconnect the leads to the coils before using the ohmmeter.
TEST POINT 4, CHART II OSCILLATOR GRID VOLTAGE IS PRESENT
The presence of the grid-leak voltage in the oscillator leads to the conclusion that the NO-SIGNAL symptom was due to a failure in either the mixer, IF, or detector circuit. A measurement of A VC voltage at the top of the volume control or at the A VC capacitor will confirm this. The tuning capacitor should be rotated through its range while observing the A VC voltage on the meter. There will often be a slight negative volt age on the A VC line even though no station carrier is present in the detector. If this voltage does not vary as the receiver is tuned through its range with the oscillator operating, trouble in either the mixer, IF, or detector circuit is a strong possibility.
Some technicians use a signal generator to supply a signal to the receiver while checking the AVC. The generator can be tuned to a local-station frequency and loosely coupled to the antenna. When the receiver is tuned to the generator frequency, there should be a sharp increase in negative AVC voltage if all the circuits are working.
In a few cases, the A VC voltage will respond normally even though no signal is heard from the speaker. The causes for this are associated with some unusual circuitry in the detector circuit, such as where the volume control is not the detector load resistor, or the AVC originates from a source other than the detector load resistor. Fig. 4-3 shows some examples.
The phono switch used in some models will disconnect the detector load resistor from the input of the audio stage but will not disable the detector or AVC. When the switch is in the phono position, the AVC will be found to respond properly with no signal present in the audio stages.
Procedure When AVC Does Not Respond Normally
A few possibilities to be checked before concluding that the defect is in either the mixer, IF, or detector are:
(A) Delayed AVC. (B) Detector load resistor separate from volume control. (C) Phone switch does not disable AVC.
1. The mixer, IF, and detector tubes.
2. The A VC filter resistor and capacitor.
3. The loop antenna.
4. The mixer section of the main tuning capacitor.
The same unusual possibility of a shorted trimmer capacitor exists here, as with the oscillator tuning capacitor explained before. When these four possibilities have been checked, testing of the mixer, IF, and detector begins with TEST POINT 5.
TEST POINT 5, CHART II VOLTAGE AT PLATES AND SCREENS OF MIXER AND IF
At this point several different approaches can be taken.
Some technicians use a signal generator tuned to 455 khz to inject a signal into each of the suspected stages, beginning with the detector. When the modulation fails to appear in the speaker, the defective stage has been found. But this method has a disadvantage for inexperienced technicians because it is often possible to inject a strong signal into a defective stage and have the signal pass through to appear in the audio in spite of the defect. This is because of the tendency for IF and mixer stages to couple a strong signal from the input to the output through internal capacity in the stage. Also, the signal generator may lead to confusion because of the different impedances at the input and output of the stages, which cause a mismatch between the generator output and the point of signal injection. In some cases, the mismatch may produce less audio at the speaker when the signal is injected into the grid of the IF than when it is injected into the plate, and this can lead to the erroneous conclusion that the IF stage has failed.
The voltmeter gives a more positive indication of failure since most failures cause a change in one of the plate or screen voltages. So, use of the voltmeter to measure all plate and screen voltages in the IF and mixer stages is recommended at TEST POINT 5.
A missing voltage at one of these points leads to the familiar direction, "Trace toward B+," as shown on the chart. The IF-transformer primaries in the plate circuits are good suspects when plate voltage is missing. An open winding is identified with an ohmmeter check or by the presence of full B+ at one end with no voltage on the plate. The connections from the plate coil to the terminals on the bottom of the trans former are easily broken when the tuning slug sticks slightly during alignment. Even a slight torque exerted on the slug when it is binding inside the coil form turns the entire form and breaks the connections. When the tuning slug binds, it is always best to replace the entire unit because it is likely to stick again if freed.
TEST POINT 6, CHART II ALL PLATE AND SCREEN VOLTAGES ARE PRESENT
With plate and screen voltages present, the technician moves on to measure resistance of the cathode circuits in the IF and mixer stages. An open cathode causes very high plate and screen voltages and, frequently, the faulty stage can be located in this manner. But, when there is no large resistance in series with these tube elements, the loss of plate or screen current does not always cause an appreciable increase in the voltages. The cathode of the IF stage can be tested with a clip lead in the manner suggested in Section 3. This procedure is not recommended in the mixer stage, however, since the oscillator might be part of the cathode circuit and would be shorted out.
With the oscillator running and plate and screen voltages present, an open cathode in the IF amplifier is very likely. The tube should be checked before replacing the resistor because it is the only component which could cause excessive current through the resistor to burn it out. Where printed circuitry is used, breaks in the circuit board are a common cause of open cathodes.
TEST POINT 7, CHART II CHECK OF IF TRANSFORMER SECONDARIES
The technician reaches this point after finding the following series of results from his tests:
1. Oscillator running.
2. A VC voltage missing.
3. No failure in the AVC, loop antenna, or tuning circuits.
4. All plate and screen voltages present.
5. All cathodes connected to B-.
By process of elimination, this leaves only the secondaries of the IF transformers. The testing of these units is done last.
The partial schematic of Fig. 4-4 shows one way this can be done with an ohmmeter. One lead is clipped to the detector diode (pins 5 and 6 on a 12AV6), and 'the other lead is moved through the three positions shown. The resistance to be expected at each point is shown in the figure. An open or infinite reading at Position 1 means that one of the secondaries is open or that the A VC filter resistor R is open. Position 2 eliminates the input IF transformer as a possibility if the circuit is still open. Position 3 eliminates the filter resistor if the circuit is still open.
In making the foregoing analysis on a printed board it is well to consider the possibility of breaks in the wiring as a cause of the open reading before the IF transformer is replaced.
Replacing an IF transformer can be a problem even for experienced technicians because the replacement units are not consistent in their terminal markings. Reference should be made to the data supplied with the transformer before installation--make no assumptions. Also be sure to sketch the positions of leads as they are removed, because an error in wiring here can lead to much tiresome and wasteful rechecking of the entire analysis, or it may lead to destruction of the new transformer.
Some output IF transformers have a built-in filter network intended for circuits in which the detector RF filter capacitor is included in the transformer shield can. These units are a bit confusing because they have six terminals instead of four (see Fig. 4-5). If the original unit has only four terminals, then only terminals 1, 2, 3, and 4 and are used. If the original unit contains the filter, then either 5 or 6, or both terminals, will be used in addition to the four regular terminals.
The RF filter capacitor in the detector, labeled C in Fig. 4-6, was described in Section 3 because, when it is shorted, the symptom presents a very obvious clue making it possible to skip the entire analysis and go directly to checking the capacitor. When C is shorted, the receiver will produce weak signals when the volume control is set in the middle of its range, but no signals can be heard when the control is rotated to either end.
Most replacement transformers are already factory aligned, but a slight touch-up of the tuning slugs will usually improve performance. If the circuit tends to break into oscillation, try detuning the transformer slightly. Oscillation also responds to rearrangement of the leads in the IF stage. Keep the grid lead well away from the plate lead, and at right angles to it, if possible.
1. What precautions are necessary when making tests at TEST POINT 2?
2. What are the components which should be checked immediately after finding no A VC voltage and before proceeding to the next test point?
3. A receiver with NO SIGNALS was found to have no grid leak bias on the oscillator grid, but the screen-grid voltage was normal. What components would you suspect?
4. List all the steps and the results of each that would be found before concluding that the secondary of the output IF transformer was open.
5. If you are replacing an output IF transformer and find that the only replacement available has 4 terminals while the original had 6, what would you do to complete the repairs? Draw a schematic and label it with values.
6. Redraw the left side of SERVICING CHART II, beginning after TEST POINT 2, but using only a signal generator and an ohmmeter as test instruments.
7. The following series of tests and results were found in a case of no signals :
What parts would you suspect next, and how would you proceed with the testing?
8. List the results of each test point which would occur if the ground connection to the volume control were open in Fig. 4-6.
9. Look up the schematic of a receiver which uses an AVC circuit separate from the detector load resistor. How would the procedure described in this section be modified to take this into account?
10. What kind of defect could produce a positive A VC voltage?
--------- Servicing Chart 11: No Signals-Mixer, IF, or Detector Failure.