OPERATION
4-5
• If the warning bell sounds and the 950 returns to STANDBY, check that all the Plane
Stimulus leads are connected to the same plane. To protect components the
maximum voltage appearing on the stimulus outputs is 500mV, and disconnecting
any one of them will automatically turn the stimulus off and return to STANDBY.
Sensitivity Adjustment
The Drive Source control adjusts the sensitivity of the system. It is usually possible to use maximum
sensitivity (i.e. with the control set fully clockwise) but if the plane's internal resistance is high it will be
necessary to reduce the sensitivity to avoid over-ranging near the edges of the board (which would
prevent a meter reading or Direction Arrow from being displayed):
• Set the Drive Source control fully clockwise (maximum sensitivity).
• Connect the Plane Probe to a point on the stimulated plane near the board centre,
e.g. an IC ground pin if the ground plane is being stimulated.
• If no tone is produced, turn the Drive Source control anti-clockwise (to reduce the
sensitivity) until a tone is heard.
Once the general area of the short has been determined, the resolution of the system can be increased (if
necessary) in one of the following ways:
• If the sensitivity is not already set to maximum, it is possible to increase the sensitivity
once the Plane Probe has been moved closer to the short (by turning the Drive
Source control clockwise).
• Moving the Plane Stimulus leads in from the edges of the board will stimulate a
smaller area of the plane and hence increase the resolution within this area.
Note that increasing the resolution is usually only required when locating a high resistance short.
Operation (Single Short)
Use the Plane Probe to probe points on the same plane as the Plane Stimulus. These points will usually
be accessible at power supply connections to ICs or decoupling capacitors.
Do not touch the probe tip when probing, as this will interfere with the measurement being made.
The four Direction Arrows under the display indicate the direction in which to move the Probe towards the
short. This is accompanied by a tone which rises in frequency and a meter reading which decreases as
the fault is approached. The Direction Arrows can be used to determine the approximate location of the
short (within 40-50mm). Close to the short all four arrows will light, and the tone or meter readings must
be used to perform the final location.
The highest tone and therefore the lowest meter reading indicate that the probe is within a few millimetres
of the short.
As it is not possible to probe all of a plane from the board surface, it is possible that the fault may be on or
near an adjacent component or via (see Example #2 below). In these cases visual inspection of the
immediate area where the highest tone occurs may be necessary. If there is no apparent fault, refer to
"Multiple Shorts" later in this section.
Note that probing any point on the side of the short connected to the Plane Clip (i.e. the un-stimulated
plane, if the short is between two planes) will result in a high tone, low meter reading and all arrows lit.
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