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GenRad
GR 2220
Bug Hound
Form 2220-0100-A
Instruction Manual
Contents
Specifications
Introduction -- Section 1
Installation -- Section 2
Operation -- Section 3
Theory -- Section 4
Service and Maintenance -- Section 5
Parts Lists and Diagrams -- Section 6
WARNING - SHOCK HAZARD
SEE SECTION 5
GR 2220
Bug Hound
Form 22200100-A
Symbol ! on equipment signified that the instruction
manualcontajnsinformationaboutthe unit that personnel
shouldbeaware to prevent ossible
of p inadvertent
damage
to it. Refer to Section 5.
OGenRad 1977
Concord, Massachusetts, U.S.A. 01742
November, 1977
ID-OIOO
Specifications
Microvoltmeter Detector probe: sensitive to source if probe held within I /8" of
copper track
Accuracy: of full scale + IOVV)
Ranges: 50-0-50gv and 500-0-500gv Connectivity Test
Input impedance greater than 100 ohms Detects resistance less than 68 ohms ± 1096
Differential protection between probes 5 volts
Power Requirements
Dc Current Source 100 to 175 or 200 to 250V
Compliance: IV dc max., open circuit 50 or 60Hz
Output current: 10mA into short circuit Power consumption less than 5 watts
Current-Sensing Probe Mechanical
Source drive signal: 600kHz IV Peak max. into open circuit Dimensions: 8.5" w x 3.12" H x 12.25" D
20mA p-p square wave into short circuit 21.6cm W x 8.4cm H x 31. lcm D
Weight: 2.5 kg (5.6 lb) net.
Warranty
GenRad
WARRANTY
We warrant that this product is free from defects in material and workmanship and,
when properly used, will perform in accordance with applicable GenRad specifications.
If within one year after original shipment it is found not to meet this standard, it will
be repaired or, at the option Of GenRad, replaced at no charge when returned to a
GenRad service facility. Changes in the product not approved by GenRad shall void
this warranty. GenRad shall not be liable for any indirect, special, or consequential
damages, even if notice has been given of the possibility of such damages.
THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES,
EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED
TO, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE.
GenRad policy is to maintain product repair capability for a period of ten years after
original shipment and to make this capability available at the then prevailing schedule
of charges.
ii
Introduction--Section 1
1.1 INTRODUCTION 1-1
1.2 GENERAL DESCRIPTION . 1-1
1.1 INTRODUCTION. Even without the optional DRM/FINDS package,GR test
The GenRad2220 Bug Hound is a small bench-top in- systems generally provide more information than most
strument designed to aid in the determination of the physi- other testers. For example, along with the node definition
cal location of printed-circuit board short and open circuits, the comment NODE LOW BUT ACTIVE may be added
and certain types of component faults. Several techniques instead of just BAD NODE AT X XX. This comment indi-
canbeusedto achieve end,depending the nature
this on cates that the node is not shorted to ground, but rather
of the fault. The Bug Hound is most compatible with, and to another node.
a logical extensionto, digital logic-circuit test systems.
Frequently, a shorted nodecan beelectrically resolved
1.2 GENERAL DESCRIPTION.
usinga test systemdesignedfor that purpose,but the physi-
1.2.1 General.
cal location is not readily found (as with hair-line etch-
circuit board shorts or opens). Also, as is the case with The 2220hasthreebasicmodesof operation;a signal
many of the higher-density and multi-layer boards used trace mode, a connectivity mode, and a microvoltmeter
today, evenif the faulty node is known, it can bevery mode. The unit is self-containedand suppliedcomplete
difficult to locate the physical fault by visual inspection. as a bench-top instrument.
Close visual tracing of a track is not a necessity with the
Bug Hound. The 2220 design is such that the electrical 1.2.2 Signal-Trace Mode.
path of a shorted track can be followed and "sniffed" out
The principal application for the signal-tracemode is in
by audio and visual means. A speaker emitting I of 2 audio
locating a short between 2 nodes when both nodes are
tones and 2 LED diodes indicate that the probe is in close
known. Whenthe Bug Hound is operating in this mode,
proximity to the track. If no indications are emitted, the
a 600 kHz trace-current is connected between the two
probe is not near the track of interest. In addition, no
shorted nodes. Two clip leadsare attached,at any point
power to the board under test is required.
on each node. The current-tracing probe is then utilized
The Bug Hound is intended for use after a board fault
to track down the physical fault location by visual (LED
has been diagnosed by a logic-circuit test system. The util-
indicators) and audio (speaker-tone) means.
ity of the messagedefining the fault depends on the so-
phistication of the system. GenRad manufactures high-
resolution logic-circuit test systems that can utilize the GR 1.2.3 Connectivity Mode.
Diagnostic Resolution Module (DRM) with the Fault Isola- The signal-trace mode can only be used when two or
tion Nodal Diagnostic Software (FINDS) package. With more shorted nodesare known. If a diagnostic message
DRM/FINDS, it is possible to resolve the fault from the identifies one node that is shorted to another unidentified
nodal level down to a specific fault type or a bad IC pin. node, the connectivity mode is used to physically locate
Havingfound the faulty node, an investigation usingDRM/ the secondnode. Two probes areused;one is placedon
FINDS is madeto resolvea fault to one of the following any point of the known node, while the other is swept
fault types: across the etch track. When an audible tone is heard, the
I. Shorts/opens between an IC pin and power or other node (or another point of the known node) is lo-
ground. cated. Probing is continued until the second unidentified
2. Shorts/opensbetweenadjacent IC pins. node is found. The actual short is then located by either
3. Faulty IC's. visual means, or by using the signal-trace mode.
4. Shorts between tracks on a board. This mode is also used to locate open circuits.
INTRODUCTION 1-1
1.2.4 Microvoltmeter. probe is visually followed to the point where the short is
The microvoltmeter has a zero-center meter with full- located.
scale sensitivities of either +/--50 or +/--500 uVdc. As with
The microvoltmeter is especially well-suited for finding
the signal-trace mode, the microvoltmeter mode also re-
shorts between power bus and ground tracks. In this situa-
quires that two shorted nodes be identified previously. A
tion, the signal-trace mode is ineffective because the power-
10 ma dc current source is then connected between the
bus bypass capacitors look like short circuits to the
nodes to provide a potential gradient along the tracks
600 kHz trace current.
carrying this current. This very small IR drop is sensed
by the microvoltmeter as an increasing or decreasing poten- The microvoltmeter can also be used in lieu of the signal-
tial as one probe is moved along the track. The moving trace mode as an alternate means of finding board shorts.
1 2 3 4 5
7
2220 Bug
OPERATING MODE
SOB V SOO"V tivitv
12
11 9 8 7 6
10
Figure 1-1. Front panel controls and indicators.
1-2 INTRODUCTION
Table 1-1
CONTROLS, INDICATORS, AND CONNECTORS
Figure 1-1
Ref. No. Name Description F unction
1 volume Knob control. Adjusts audio level of speaker tone in Signal-trace
Rotary cw adj. and Connectivity modes.
2 meter Microvoltmeter. Indicates reading in microvolts of either 50 or
Minus-zero-plus. 500 uv full scale (see Fig. 1-1 items 7 and 8).
3 signal clips Red and black In Signal-trace mode these clips provide a
push-type clips. 600 kHz trace current for the probe (Fig. 1-1,
item 10) to follow; in the microvoltmeter mode a
10 ma dc signal is clipped to two track points and
the test probes (Fig. I-I, item 4) are used to
detect the signal.
4 test probes Red and black In Connectivity mode these probes are used to
probes. find a point on an unknown node. In the micro-
voltmeter mode these probes are used as the
meter input probes.
5 signal -trace I of 4 Operating Sets instrument to Signal-trace mode. activating
Mode interlocking the current-trace probe (Fig. 1-1, item 10) and
pushbuttons producing a 600 kHz signal across the clips
(Fig. I-I item 3).
6 connectivity 1 Of 4 Operating Sets instrument to Connectivity mode. The
Mode interlocking probes (Fig. I-I item 4) are used to probe for
push bu ttons two points with a resistance ohms.
7 microvoltmeter I Of 4 Operating Sets instrument to Microvoltmeter mode with a
500 uv Mode interlocking full-scale reading of +/--500.
8 microvoltmeter 1 of 4 Operating Sets instrument to Microvoltmeter mode with a
50 uv Mode interlocking full-scale reading of +/--50.
pushbuttons
9 earphone Earphone jack. Listen to Signal-trace tones or Connectivity mode
tone instead of speaker, in noisy environments
or without disturbing others.
10 current-trace Probe containing In Signal-trace mode this probe enables the
probe tuned circuit. 600 kHz stimulus to be tracked.
11 meter set Microvoltmeter Mechanical zero-adjust of microvoltmeter.
adjust screw.
12 off-on Toggl e-switch. Turns instrument power on and off.
Table 1-2
REAR PANEL CONTROLS, INDICATORS, AND CONNECTORS
Name Description Function
Power Connector Safety recessed 3-wire Ac power input. use with appropriate
(labeled 50-60 Hz). plug. power cord such as GR 4200-9625 or
equiv.
Fuse (labeled Fuse in extraction Short circuit protection. Use Bussman
1/32 A SLOW post holder. type 250V 1/32 A or equiv. rating.
BLOW)
Line Voltage Slide switch. Horiz Adjusts power supply for the appro-
Switch motion: left pos, priate input range.
100-125V; right
pos, 200-250V.
INTRODUCTION 1-3
Installation --Section 2
2.1 UNPACKING AND INSPECTION 2-1
2.2 DIMENSIONS. 2-1
2.3 TILTING
2-1
2.4 POWER-LINE CONNECTION 2-1
215.9 mm
311.15 mm
8.5 in
12.25 in
74.6 mm
0000
2.9 in
o o
63.5 mm
2.5 in
060%.
·
Figure 2-1. 2220 Dimensions.
2.1 UNPACKING AND INSPECTION.
front of theinstrument. o returnthebailto its storage
T
If the shipping carton is damaged, ask that the carrier's position, push it back andup againstthe bottom of the
agentbe presentwhen the instrument is unpacked. Inspect cabinet.
the instrument for damage(scratches, ents, broken parts,
d
etc.). If the instrument is damagedor fails to meetspeci-
2.4 POWER-LINE CONNECTION.
fications, notify the carrier and the nearest GenRad field
office. (See list at the back of this manual). Retain the Thepowertransformerprimarywindingscanbe
switched, by meansof the line-voltageswitch on the rear
shipping carton and the padding material for the carrier's
inspection. panel,to accommodate linevoltages eitherof two ranges,
in
aslabeled,at a frequencyof 50 to 60 Hz, ac. Usinga small
2.2 DIMENSIONS. screwdriver, etthis switchto matchthe normalvoltage
s
Figure 2-1.
of your power line.
The instrument is supplied in a cabinet with resilient
Connectthe 3-wire power cable (P/N 4200-9625) to the
feet for placement on a table. The overall dimensions are
line andto the powerconnectoron the rearpanel. The
given in the figure.
instrumentisfitted with a recessed-wirepowerreceptacle.
3
Thecontactsarerecessed eliminate
to fthe possibilityof
2.3 TILTING.
electricalshockwheneverhe powercord isbeingun-
t
The unit canbetilted backfor easier
panelviewing. plugged from the instrument. In addition, the center
Pull the central part of the bail, which is pivoted at the ground pin is longer, which meansthat it mates first and
front feet, down and forward. Let the bail support the disconnects last, ensuring user protection. The connector
INSTALLATION 2-1
is rated for 250 V at 15 A. It meets the requirements of with the jacket. The connector at the power-line end is a
Underwriter's Laboratories in the U.S. and the Canadian stackable hammerhead design that conforms to the
Standards Association. The receptacle accepts power cords "Standard for Grounding Type Attachment Plug Capsand
fitted with Belden type PH-386 connector. Receptacles",ANSI C73.11-1966. (Specifies125 V, 15 A.)
The associated power cord for use with that receptacle If the fuse must be replaced, be sure to use a "slow blow"
is GR part no. 4200-9625. It is a 210-cm (7-ft.), 3-wire, fuse of the current and voltage ratings shown on the rear
18-gauge cable with connector bodiesmolded integrally panel, regardless of the line voltage.
2-2 INSTALLATION
Operation--Section 3
3.1 BASIC PROCEDURE. 3-1
3.2 SIGNAL TRACE 3-1
3.3 CONNECTIVITY 3-2
3.4 MICROVOLTMETER 3-2
3.1 BASIC PROCEDURE. c. Attach the signal clips to the identified nodes.
For initial familiarization perform the following pro- (Either signal clip can be clipped to either node.)
cedure. Then refer to the application information con- d. With the current-trace probe (largest black probe)
tained in this section for details on obtaining the maximum in hand, start from one of the clips with the probe nose
use from each mode. held within approximately 1/8 inch of the track.
a. Before connectingthe line cord, slidethe line-voltage e. Move the probe from one side of the clipped track
switch (rear panel) to the position that correspondsto your to the other using a zig-zag motion. Note that the LED
power-line voltage. Power must be nominally either 120 or indicators alternately flash on-and-off as the probe is moved
220 V (50 to 60 Hz). from one side to the other.
If the fuse must be replaced, be sure to use a "slow f. As the probe is being swept from one side of the
blow" fuse of the rating shown on the rear panel. track to the other, adjust the front-panel volume control
b. Pushthe interlocking OPERATING MODE push- cw for an audible tone of the desired level.
button switch to the desiredmode; presseither Signal g. Next, proceed along the track following it with the
Trace, Connectivity, or 50 uv (Microvoltmeter). probe. If, as the probe is moved down the track the LED
c. Set the power On/Off switch to On. indicators do not flash on and off, and the audio tones
d. Connect the earphone to the front panel earphone stop, back up the probe to the point where the indicators
jack if desired. are active again. Investigate the reason for the stoppage.
e., Proceed depending on the mode Selected to the Either the node branchesat this point, or the physical
appropriate paragraph as follows: fault has been encountered.
Signal Trace . para. 3.2
Connectivity . . para. 3.3 Example
Microvoltmeter . . . . . para. 3.4 Figure 3-1 illustrates the signal trace mode. This ex-
ample assumesthat a test system diagnosis has stated that
2 nodes U 1-13 and 1.13-12 shorted together. The red
are
3.2 SIGNAL TRACE.
and black signal clips are placed on these two IC pins as
The signal trace mode utilizes the current-trace probe shown (either clip to either point). The path of the
and two clip leads for injecting a 600 kHz trace signal. 600 kHz ac signal is illustrated by the dotted arrow. Note
Typically, the 600 kHz signal is clipped across two nodes that it passesthrough the short. The current-trace probe
that have been diagnosed by a test system as being shorted is then moved along the track starting from one of the
together. One of the nodes can be a ground bus. Perform identified pins (U 1-13). As the probe is moved to point A,
the following steps: the signal is lost. The probe is backed up and moved
a. Press OPERATING MODE Signal-tracepushbut-
the through a plated-through-hole. The signalis lost at point B
ton switch. on the track located on the other side of the board. As
b. Unclip the current-trace probe and red and black sig- the probe is zig-zaggedback toward the hole, the signal
nal clips from the probe tray. re-appears and the short is observed.
OPERATION 3-1
CURRENT-TRACE PROBE
SIGNAL CLIP
SIGNAL CLIP
1
02
SHORT CURRENT FLOW (600 KHZ)
Figure 3-1. Signal trace mode.
3.4 MICROVOLTMETER.
3.3 CONNECTIVITY.
3.4.1 General.
The connectivity mode is used in instances where a test
systemdiagnosisidentifies only one node. The other node The microvoltmeter uses a 10 mA dc current source as
must then be found manually. a stimulusalonga path between2 identified nodes.The
Perform the following steps: microvoltmeter indicates both the polarity and the ampli-
a. Push the OPERATING MODE Connectivity pushbut- tude of evena few microvolts drop alongthe path through
ton switch. which the current is forced. There are two ranges; 50 and
b. Unclipthe red andblackprobes(not the signalclips) 500 uV. Normally, the 50 uv rangeis usedasdescribed
from the probe tray. below. The 500 uv rangeis useful wheneverthe potential
c. Touch the 2 probe tips together and an audible tone gradient longatrackis large
a becausef anarrowtrack
o
is heard as the front-panel VOLUME control is adjusted. width.
Set for desired level. Perform the following generalstepsto usethe micro-
d. Place one of the probe tips on the identified node, voltmeter,and then referto the detailedtechniquesfor
then slowly and lightly sweepthe board etch with the other usedependingon the type of application.
probe until the tone in Step c. is heard. This indicates a a. Pressthe OPERATING MODE 50 uv pushbutton
path of lessthan 68 ohms. switch.
e. The tone indicates that either a point on the same b. Unclipthe blackand redsignalclipsandthe redand
node has been located, or an unidentified node hasjust black probes from the probe tray.
been found. With both nodes identified, the signal trace
mode is used to resolve the fault. NOTE
Figure 3-2 illustrates the useof the Connectivity mode. The following proceduresare intended for use
Note that if one probe is placed on U 1-3 the tone is heard with an unenergizedboard. To usethe micro-
if the other probe tip contacts either IJ2-3, IJ2-5, U2-2 or voltmeter on an energized board refer to
U3-2. Only the latter two points are on the other node. para. 3.4.3.
3-2 OPERATION
Ul.3
U2-3
NODE 1
U2-5
U2-2
NODE?
SHORT 03-2
Figure 3-2. Connectivity mode.
c. Place either signal clip onto one of the nodes; place the point-of-short is passed no further IR drop occurs
the other clip at a point on the other node. since there is no current flow through this path; the voltage
d. Place and hold either probe at one of the clips, and point is at the same potential as the short (ground poten-
with the other probe press lightly on the track and move tial). Thus the deflection remains constant past the point-
the second probe along the track. of-short.
e. A deflection should gradually be observed. The
polarity of the deflection is not important, and depends
on which clip and which probe is placed on which end of 3.4.3 Use of Microvoltmeter on Energized Board.
the track. If the deflection approaches full scale, move The microvoltmeter can be used on an energized sys-
the itationary probe to the vicinity of the other probe to tem (power applied to the board-under-test) to resolve
bring the meter reading back to zero, then continue the a short when system analysis pin-points a bad node. The
search for the short. following techniques illustrate the most common appli-
f. When an increase in the distance between the probes cations and can be used to determine, for example, if an
on the 50 uv range does not produce a corresponding IC driving a node is bad, or if one of the IC's receiving a
increase in the meter deflection, either the point of the signal from the identified node is bad (such ashaving an
short has been passed,or the short is on a node branch internal short to ground, Vcc, or another node.
that has just been passed. Back up the probe until a de- Figure 3-4 illustrates current flow and level for standard
crease in deflection occurs, then investigate. TTL logic O and 1 states under normal conditions. Under
fault conditions current is sourced to the short from all
other points on the node, regardless of whether or not
3.4.2 Vcc-To-Ground Shorts. the point is an output or input (though the levelwill vary).
Figure 3-3 illustrates a principal application for the Trouble-shooting using the microvoltmeter is easy if this
microvoltmeter. A shorted capacitor is not an uncommon concept is kept in mind. If current is flowing from an IC
fault and can be easily found as illustrated. With the signal pin to a short, it is sourcing current. It is easily determined
clips connected as shown, the dc current source flows if any given IC pin is sourcing or sinking current. Simply
through the shorted capacitor to ground. As the probe is place the positive (red) probe closer to the IC pin than the
moved along the power bus the deflection increases. As black (negative) probe. Place the black probe elsewhere
OPERATION 3-3
SIGNAL CLIP
(+) pROBE
(--PROBE) I--PROBE) (--PROBE)
VCC BUS
SHORTED CAPACITOR
10 MA DC
GND BUS
SIGNAL CLIP
Figure 3-3. Probing VCC-tO-ground shorts.
1.6 MA
40 gA
120 VA
4.8 MA
1.6 MA
1.6 MA
40 VA
NODE IN "ZERO" STATE NODE IN "ONE" STATE
Figure 3-4. Standard TTL loading.
3-4 OPERATION
3
2
4
5 IJ2
SINKING
8
9
--50 +50
SOURCING
SOURCING
+50
4
IJ2
+50
SOURCING
8
3
--50 +50
Figure 3-5. Probing an energized board.
on the node. If the meter deflects upscale, current is Follow the track using the other probe. The deflection in-
being sourced by this IC point; a down-scale deflection creases and is at a maximum at the short. If the short
indicates it is sinking current. is passed the deflection Will stay at approximately the same
Figure 3-5 illustrates an example of the most common level or start moving down-scale as a result of the current
type of probing on an energized board. In this example being sourced from the node IC inputs (as from U2-4, and
assume that IJ2-I has an internal short to ground, and be- U2-8).
cause of this the node "fails to go high". The board-under-
test must be set-up by the test system to the failing condi- Open-Co//ector Logic. The technique just described
tion. Figure 3-5 shows the node both from physical and applies to open-collector logic with one difference. In the
electrical viewpoints. When U1-3 is probed, the positive logic high state a logic output of this type does not source
deflection indicates that the pin is sourcing current. Simi- current to the track. If, for example, a short to ground
larly, when 02-8 and U2-4 are probed, positive indications holds a node low, and a microvoltmeter measurement made
are indicated, though Of a much smaller magnitude. When at the pin driving the node indicates O uV, this output has
U2-I is probed as shown the deflection is negative indi- not failed. If it is determined that this output is good,
cating it is sinking current. This principle, carried one step probe the other points on the node using the techniques
further, is used if the short is somewhere in the track. Place previously described in para. 3.4.3 to trouble-shoot the
the positive probe on the driving output (as with U 1-3). short.
OPERATION 3-5
Theory--Section 4
4.1 GENERAL 4-1
4.2 CURRENT-TRACING PROBE (SIGNAL TRACE MODE) 4-1
4.3 CONNECTIVITY TEST . 4-3
4.4 MICROVOLTMETER 4-3
4.5 SYSTEM POWER 4-4
4.1 GENERAL. Figure 6-2. probe contains a coil that is tuned to detect the 600 kHz
This section describes the theory of the current-tracing stimulus. A small ac voltage developed in the probe coil is
probe, the connectivity tester and the microvoltmeter. The amplified and applied to a phasedetector. The phasede-
theory for the microvoltmeter also includes the 10 mA dc tector compares the phase of the signal induced into the
current source. An understanding of this theory is helpful probe with the 600 kHz stimulus signal. From the phase
in performing any trouble analysis on the unit. detector it is determined whether the probe coil lies inside
or outside of the current-carrying track. Two LED indica-
tors are associated with the phasedetector. Additionally,
4.2 CURRENT-TRACING PROBE (SIGNAL-TRACE
one of two audio tones indicates the phase.
MODE).
4.2. I Background.
42.2 1.2 MHz Oscillator and Clock Divider.
The current-tracing probe is designed to detect the
polarity (direction) of the magnetic field that is induced The 1.2 MHz oscillator (U3 and associated components)
around a wire (etch) when a current is applied through the is the heart of the "current-sniffing" probe. The 600 kHz
wire. Figure 4-1 illustrates the field relative to current flow source stimulus is derived from this oscillator through a flip-
and also shows that the induced voltage in the probe pro- flop frequency divider (U5). The divided output is also
duces a current flow in a direction depending on which side applied to the phasedetector circuitry to synchronize it as
of the track the probe lies on. described in para. 4.2.6.
The oscillator is tuned by L2, C15 and C16 to a fre-
quency of approximately 1.2 MHz. C17 provides a fine-
tune oscillator adjustment. C47 is a blocking capacitor
required to prevent L2 from shorting U3-7 (base) to the
PROBE gate (U3-3). C18 prevents L2 from shorting the gate (U3-3)
M·JDUCED
CURRENT
to the source. U3 buffers the oscillator output and converts
FROM the oscillator sinewave to a square wave.
PROBE The OSC OFF signal to the oscillator (U3-5) is tied to
INDUCED
CURRENT either +5 V or --5 V through the OPERATING MODE
FROM
switch section S5. When the Signal-Trace mode is selected,
+5 V is switched in to enable the oscillator. For all other
modes --5 V is switched in to disable the oscillator. This
prevents oscillator pick-up from affecting the other modes
of operation.
The clock-divider flip-flop I_J5divides the oscillator
Figure 4-1. Magnetic field resulting from current flow
through a wire. frequency by 2, supplying a 600 kHz (approx.) signal from
U5-1 to driver 07. The output of U5-2 is applied to U4.
The other input (U4-2) is approximately 1.2 MHz. The
output of IJ4 provides the toggle for the phase detector.
Figure 4-2 is a simplified functional diagram of the The timing relationships for the above signals are illustrated
probe. The signal clips are used to connect the ac current in Figure 4-3. The toggle (U4-3) is thus delayed one-fourth
source (600 kHz) to two shorted nodes. The current-trace of a period from the probe excitation signal.
THEORY 4-1
LEDS
PROBE
AC
THRESHOLD
AMPL DETECTOR pHASE
DETECT
SPEAKER
1.2 MHZ
osc FREQ
SELECT
VOL.
600 KHZ ADJ.
+2 CLIPS
SOURCE
Figure 4-2. Current-trace probe simplified functional diagram.
and shutdown of IJ2. The comparator output (TP2) pro-
U4-2
duces a +/--5 V signal swing when sufficient excitation
(greater than 100 mV pk-pk) is applied to U I. The com-
05-2
parator output is applied to the phase-detector flip-flop.
U5-1
4.2.5 Threshold Detector.
The comparator output is also applied to a threshold
detector. The detector ensures that the tones and LED
04-3
indicators are extinguished when the probe is not near the
track being probed. CI I passes the comparator output to
the detector. The positive-half of each 600 kHz cycle is
Figure 4-3. Oscillator and divider timing. conducted through CR18. This charges C30 positive.
During the negative half-cycle CR23 conducts and C30
attempts to discharge through R10. The time constant is
such however, that the next positive half-cycle is received
4.2.3 600 kHz Source Drive. before C30 is discharged. This maintains 04-5,6 positive
The 600 kHz signal from U5-1 is applied to the Q7/Q1 which enables phase detector IJ5. When the comparator
driver stage through R13/C12. The current is limited output is not active, C30 discharges through RIO and
through R 13. C13 minimizes the effect of stray capacitance U4-5,6 are at a low logic level. The phase detector is then
both in the track and 07 from attenuating the 600 kHz disabled.
signal. 07 and QI provide the source driver (20 ma
nominal) required to force a current through a pc-board 42.6 Phase Detector.
track to produce the magnetic field (para. 4.2. I). The The phase information is received by U5-9 and this flip-
600 kHz output is protected from overload (accidental flop is clocked at U5-11 (para. 4.2.2). The logic level from
connection to a voltage source) to +/--15 V (max). CR20 the amplified induced signal from the probe is clocked to the
is back-biased for +15 V. the IJ5-13 output (PHASE I) on the leading edge of the
toggle pulse (1-15-11). The LEDs CR14 and CR15 alter-
4.2.4 Probe and Preamplifier. nately blink back and forth as long as a phase difference is
The probe nose contains the tuned circuit LI and C4. being detected. The current to the LEDs is limited by RI 2.
This circuit is resonant at 600 kHz. Ul is an ac-coupled 05 and Q6 are drivers for CR14 and CRI 5, respectively.
amplifier with a gain of approximately 80 at 600 kHz. The RI 6 and R40 limit the current that 05 is required to sink.
amplified phase information is then applied to comparator The complementary phase detector outputs PHASE 1
tJ2. The comparator has a switchable hysterisis. Initially, and PHASE 2 are applied to analog switches U15-5 and
the hysterisis is set at 100 mV pk-pk (R5/R11). When this U 15-6. PHASE 1, when high switches PULSE 2 through
threshold level is reached, the hysterisis is changed to speaker drivers 04/02 to the speaker. Similarly, PULSE 3
50 mV pk-pk by Q8/R48. The change of hysterisis ensures is switched through by PHASE 2. The PULSE 2 and
that the drive level is sufficient to produce positive startup PULSE 3 signals are produced as described in para. 4.4.2.
4-2 THEORY
METER
PROBES RANGE
BUFFER
OFFSET SELECT
COMPENSATION
INPUT AMP. AND
OVERLOAD PROT
10 MA DC +10 MA
CHOPPER STABILIZATION
CURRENT
--10 MA OSC. AND COUNTER
SOURCE
Figure 4-4. Microvoltmeter simplified diagram.
4.3 CONNECTIVITY TEST.
voltage overload to +/--15 V. Lamp DSI provides this pro-
I-JIOand associatedcircuitry form the connectivity tection because its resistance increases and limits the current
tester. The test probe leads are connected to a resistance as the voltage across the lamp increases. Diodes CR16 and
divider consisting of R31, R32, and R42 on the U 10-3 CR17 clamp the voltage to +1--1V max. C41 along with
input. U 10 is a comparator, and because of the divider,on DSI form a low-pass filter designed to attenuate computer-
the input, the comparator output UlO-7 (TP6) goeshigh clock frequencies and prevent an erroneous meter offset
when the test probes are connected to a resistance of less when the microvoltmeter is used on an energized board
than 68 ohms. Whenthis occursanalogswitch U15-13goes (para.3.4.3). The filter cutoff is approximately 10 MHz.
high and closes. The PULSE 2 signal on U 15-1 is then DSI and R21 alsoform a voltage divider. Approximately a
applied through the 04 and Q2 pulse amplifier stage to the sixth of the input voltage is dropped by this network. This
speaker and earphone jack. The PULSE 2 tone (385 Hz) is loss is compensated for by the gain of U7. C43 across the
generatedby the Ull oscillator and 1312divide-by-eight input of IJ7 shuntshigh frequenciesto minimize switching
counter (refer to para. 4.4.2) and is also used in the micro- transients produced by IJ6. C44 reduces 60 Hz pick-up
voltmeter mode. from floating ground to case ground. U 7 is configured as
an ac-coupled amplifier. A gain of approximately 600 is
4.4 MICROVOLTMETER. developed by feedback resistors R23 and R24.
4.4.1 General. Analog switches 1.16-5, nd -13 in the input stage are
a
Figure4-4 is a simplified block diagram of the micro- used for chopper stabilization, described in the nexi
voltmeter. The probed voltage is applied to overload pro- paragraph.
tection circuits and then to the input amplifier. The
amplifier is chopper-stabilized and provides gain. Chopper 4.4.3 Chopper Stabilization - Oscillator and Counter.
stabilization is a technique that minimizes the effects of U7 The timing pulses for chopper-stabilization control are
offset drift resulting from temperature variations. With developed through oscillator IJI I, and divide-by-8 counter
this method the microvoltmeter input and ground are U12. The oscillator frequency of 3080 Hz is controlled by
alternately sampled. Any offset voltage in U7 is stored in C29, R41, and R43 (adjustable). R43 is variable to adjust
C23 during the time that ground is being sampled. The the chopper frequency away from any 50 or 60 Hz
input voltage is then sampled and algebraically summed harmonics to avoid undesirable meter pulsation. R46
with the stored offset voltage. The two offset voltages minimizes frequency drift resulting from any power insta-
cancel, leaving only the amplified input voltage which is bility. The oscillator output is applied to a divide-by-8
stored on C25. When this voltage reaches+/--25 and counter IJ12 to produce the PULSE 1,2,3 and PULSE 4
+/--250 mV the microvoltmeter will be reading+/-- full timing outputs as illustrated on the schematic (Figure 6-2).
scale on the 50 and 500 uv ranges, respectively. The timing pulses are applied to the IJ6 analog switches. A
high pulse level closes a switch. As shown in the schematic,
4.4.2 Input Stage.
the input is opened (U6-5) when PULSE I goes low. At the
The input stage hasa low-input impedance of 100 ohms. same time PULSE 3 goes high to ground the microvolt-
This low impedance stabilizes the chopÖer switches at the meter input (U6-13). This references IJ7 to floating
input of U7 so the meter will remain at zero when the micro- ground. After the switching transients have decayed away
voltmeter is not being used. The input is protected against and the operational amplifier hassettled, the output voltage
THEORY 4-3
of IJ7 will be in error by its output offset voltage. This set-