Heathkit IT-12, Assembly Manual

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Noise Locator
A rather unique and definitely useful application of the Signal Tracer is its ability to locate noisy and intermittent
components in various circuits. Basically the noise locatmg feature of the signal tracer permits the application of a DC test
voltage to any component in the receiver circuit, and the action of the DC voltage in the component is picked up and
amplified in the signal tracer itself. This DC voltage of approximately 100 volts is available at the tip of the probe, and of
course the ground clip of the probe provides the necessary return circuit. Some precaution should be observed so that the
service technician will not encounter a relatively harmless shock hazard. The circuit arrangement is such that even under a
dead short circuit condition of ground clip to the test probe, the current drain will not exceed 1 mil and therefore, the
possibility of damage to a receiver component is eliminated. To use the Signal Tracer as a noise locator the following
conditions should be observed:
First of all it is important that the receiver to be checked should be disconnected entirely from the AC supply line as all
tests will be made on an inoperative receiver. The probe switch should be in the audio position for this test, and the panel
noise switch to ON. As previously mentioned, a DC voltage in the order of 100 volts is now present between the probe tip
and the ground clip.
WARNING: When testing in transistor circuits, it is suggested that one end of the component being tested be
disconnected from the circuit before applying the DC voltage. Failure to do so could result in transistor breakdown. Do
not apply the test voltage directly across transistor connections.
To illustrate an example of usage, the ground clip should be connected to a B+ supply point in the circuit. The test probe
is then applied directly to the plate terminal of an IF tube. The gain control of the signal tracer should be at near maximum
position. As the test probe is applied to the plate of the IF tube, a sharp click will be heard in the signal tracer's speaker. A
good, clean click will indicate proper circuit continuity and operation. If there is any frying or crackling sounds present, it
would be reasonable to assume that some difficulty is being experienced along the circuit, and then the test probe should
be moved step-by-step through each portion of the circuit in order that each component may be progressively checked. As
the test probe is moved along through coils, resistors and other elements toward the ground clip, it is quite likely that at
some point the noise will disappear. When this condition occurs, the preceding step should be repeated and further
investigation made of the particular circuit involved for a possible faulty component.
Noisy resistors will very definitely show up as a buzzing or crackling sound. The same condition holds true for a cold or
rosin soldered connection in the receiver wiring. Shorted turns in an IF transformer will also be evident by the noise
content. The important thing to remember is that a satisfactory component will permit a sharp click to be heard in the
speaker, whereas a faulty component will result in a characteristic of an objectionable noise.
Obviously the test voltage can be applied to any component in
the receiver. For example, if an audio coupling condenser is
suspected, the voltage can be applied across the condenser
terminal, and then, the condenser lightly jiggled or prodded to
determine whether or not a noisy or intermittent condition may
be present. Noisy volume controls can be detected, and of
course the same condition holds true for the rotor section of
variable condensers. Voltage can be applied to the condenser
while it is being rotated, and any shorting will be quickly
evident. Of course it will be necessary to disconnect any
external circuit wiring connected to the condenser terminals. Transformer windings that are faulty will also show up as
noise in the signal tracer's speaker. The continuity of the winding can be checked and any leakage from winding to frame
can quickly be determined.