Using Signal Tracers and Signal Generators for Trouble Shooting

Signal Tracers are used for trouble shooting misbehaving receivers. Signal Generators are used for adjusting receivers and for trouble shooting. The use of a signal generator for adjusting receivers deserves a presentation of its own. The author has written a document (available on the VRPS web site) on the use of the signal generator for adjusting receivers. This presentation covers trouble shooting only.

Whether using a signal tracer or a signal generator for trouble shooting, the goal is the same: isolation of a fault to a single stage of the receiver. In practice, this means isolating a malfunction to the components associated with a single tube or unit within a tube. Once a faulty stage is identified, the trouble shooting activity changes style to locating a single defective component, which usually requires different equipment.

Whatever equipment you choose to use, be sure to follow good safety practice. Usually, this means using line isolation transformers, DC blocking capacitors on test leads of signal generators (100 pF for RF, 0.1 μF for AF), and keeping one hand behind oneself when probing high voltage circuits. For more information about safety practices, see the "Introduction to Restoration" by this author on the VRPS web site.

A Signal Tracer is an amplified crystal receiver sans antenna and tuned circuits. That is, it has no tuning coil and no tuning condenser. It contains a detector, a high gain audio amplifier, output transformer, and speaker. The use of a Signal Tracer for trouble shooting is called Signal Tracing.

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Typical Signal Tracer

A Signal Generator is a low power AM transmitter tunable to any frequency in a wide range. The use of a signal generator for trouble shooting is called Signal Injection.

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Typical Signal Generator

In trouble shooting, signal tracers and signal generators are used to substitute for possibly defective stages in a receiver, for the purpose of locating the malfunctioning stage or stages in the receiver. In order to understand how this takes place, one must understand how receivers work. This presentation concentrates on superheterodyne receivers because they are both more common and more complicated than other receivers. If one understands how to trouble shoot superheterodynes, then he is able to handle other receivers as well.

In order to power a speaker from a radio signal, a receiver must have a power gain on the order of half a million. It is infeasible to achieve this much gain with one tube. Furthermore, in order to receive but one station at a time, one needs more selectivity than than a single tube can supply. Therefore, the signal processing in a superheterodyne proceeds in stages from the antenna to the speaker. The output of each stage provides the input to the succeeding stage. The input of a stage is typically a tube grid, though the detector has a diode plate as its input point.

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In vintage tube radios, Test Point 1 is the signal grid of the converter or mixer tube, usually accessible on the antenna section of the tuning condenser. Test Point 2 is the control grid of the IF amplifier. Test Point 3 is the second IF transformer output coil, usually connected to a diode plate, sometimes a triode grid. Test Point 4 is the "hot" end of the volume control. Test Point 5 is the control grid of the power amplifier.

In signal tracing, a signal is "traced" through the receiver in signal processing order. The signal tracer probe is placed on each numbered Test Point shown in the block diagram in order 1, 2, 3, 4, and then 5. The signal tracer substitutes for all stages to the right of the Test Point in the diagram. When the signal is no longer present, or fails to increase in intensity, one has passed over a malfunctioning stage.

A signal tracer is a complete amplified crystal radio, except for antenna and tuned circuits. The controls are two: Mode and Gain. The mode control is usually marked "RF/AF" or "RF/Audio", and simply bypasses (or not) the detector in the probe assembly. The gain control functions exactly as a volume control in a receiver.

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The signal tracing procedure is first to connect the ground lead from the shielded probe assembly to the ground of the receiver. For a transformer receiver this is usually the chassis. For an AC/DC receiver, it is the B-minus ground. Then the probe tip is placed on the input of each stage in order, looking for where the signal "disappears".

Tracing begins at Test Point 1, the input to the converter. Since we are before the detector, the probe's detector is used by putting the probe in "RF" mode. At this point in the receiver, the signal level is very low, so the gain control is turned up very high. The receiver's tuning control is tuned across the band, and if the antenna circuitry is working, several stations should be heard in various places. If not, there is a problem in the antenna circuitry.

Once the antenna circuitry is working, the receiver is left tuned to a station. The tracer's probe is moved to Test Point 2, the input of the IF amplifier. If the signal is still present, then the probe is moved to Test Point 3, the input to the detector. As the signal strength increases with each stage of processing, the gain control of the signal tracer is turned lower. If the signal strength is not increasing, then a problem is indicated. However, most detectors do not provide gain.

When tracing reaches Test Point 4, the volume control (input to the first audio amplifier) the signal is past the detector, and is no longer a radio frequency signal here and in succeeding stages. The probe mode switch is set to bypass its internal detector by setting it to AF (or AUDIO) at this point. Some prefer to use the grid of the first audio amplifier tube as the test point. In this case, be sure that the receiver's volume control is advanced so there is signal present.

When one finds a stage which has signal at the input, but the succeeding stage does not, then that stage is malfunctioning, and trouble shooting concentrates on locating a defect in that stage.

Some signal tracers provide access to their output transformers and speakers, for use as temporary substitutes. This can be helpful if the speaker is not available due to being sent off for repair, or if it is simply inconvenient to use, as in a console radio.

Some signal tracers provide a "NOISE" mode, which applies a current limited voltage (about 50 VDC) to the probe. The author has found this useful for repairing shorted tuning condensers. One removes the tuning condenser from the chassis, and applies the voltage from the signal tracer to it, in a darkened room. As the tuning condenser control shaft is rotated, sparks show up points of contact between vanes.

Signal injection is exactly the inverse procedure. The signal generator is adjusted to produce a signal simulating the output of the earlier stages, which is injected into the Test Points in order 5, 4, 3, 2, and then 1. The signal generator substitutes for all the stages to the left of the Test Point in the diagram. When the receiver no longer reproduces the simulated signal, one has passed over a malfunctioning stage.

The Signal Generator is a low power AM transmitter, less antenna, tunable over a wide frequency range. It has three kinds of control: Frequency, Amplitude (or Level or Attenuator), and Modulation. The frequency controls are two, one coarse and the other fine. The coarse frequency control is the "BAND" switch, which selects a broad range of frequencies. The fine frequency control is the "TUNING" control, which allows one to select a particular frequency within the selected band.

The Amplitude (or Level or Attenuator) controls are also two, again one coarse and the other fine. The coarse level control is usually a switch, marked "HIGH/LOW". The fine control is a volume control type adjustment.

The last control is the Modulation control, which enables or disables the internal modulation. It is usually marked something like "INT/EXT", with "INT" selecting the internal sine wave oscillator as the modulation source. Usually in the "INT" position, the internal modulation source is also available for use as an audio source or oscilloscope trigger source. The "EXT" position allows one to supply an external modulation source, or none at all for an unmodulated signal.

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Signal Generator Block Diagram

First the ground of the signal generator is connected to the receiver's ground through a 0.1 uF Y2 rated capacitor. The capacitor is used for DC blocking in order not to disturb the tube bias in the receiver, and also as some shock protection with vintage signal generators. The receiver's ground is found as described in the signal tracing section.

Signal Injection begins at Test Point 5, the input (grid) of the power amplifier, and proceeds toward the antenna, stage by stage. The audio modulation signal is applied to the input of the power amplifier. If a tone is not heard, then the power amplifier stage has a malfunction. Otherwise the signal is then applied to Test Point 4, the input to the first audio amplifier, the volume control. The sound is expected to be louder, since there is another stage of amplification.

When we move to Test Point 3, the input to the detector, the expected signal is no longer audio, but radio frequency. The signal generator is tuned to the IF, and the output is taken from the RF output of the signal generator. The Level control may need to be advanced. At Test Point 2, the IF amplifier grid, the Level control may need to be turned to reduce the signal strength due to more gain being available. If more gain is not noticed, then a problem is indicated in the IF amplifier.

When Test Point 1 is reached, the signal generator must be re tuned to a frequency in the AM band which the receiver is capable of tuning, and the receiver tuned near that frequency. The tuning of the signal generator should be adjusted around near the nominal tuning of the receiver in order to "find" it. If this isn't possible, then the converter (or local oscillator if separate) is suspect. If all still seems well, then the antenna circuitry is suspect.

The procedure to test the antenna circuitry depends upon its type. If the receiver has an external antenna terminal, then the signal generator output is attached to it. If the receiver has an internal loop antenna, then the signal generator output is connected to a few turns of wire, which used as an antenna and positioned near the receiver's loop.

A refinement to either procedure is to go to the midpoint of the receiver and characterize the problem as "front end" (radio frequency portion of the circuit) or "back end" (audio frequency portion). One starts at Test Point 4, the volume control. The signal tracer is set to audio mode, and the radio is tuned around. If stations are heard, then the RF sections (converter, IF amplifier, and detector) are presumed not to have a problem, and further investigation proceeds with the audio stages, otherwise the RF sections are looked at more closely.

Alternatively, the signal generator's audio output is injected into the volume control, and if the speaker reproduces the tone, the audio stages are exonerated, and further investigation proceeds with the RF portions of the circuit only, otherwise the the audio stages are pursued. This technique usually saves some time.

Neither technique is inherently superior to the other. Which to use is mostly a matter of personal preference. Signal generators are more complicated to adjust, and more expensive to purchase. However, one is needed for alignment work, anyway. So one could consider a signal tracer to be an unnecessary expense. Still, signal tracers have simpler controls, and can often be substituted for a bulky or missing speaker.