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TROUBLESHOOTING
MEDICAL INSTRUMENTS
1.0 Introduction
Troubleshooting medical instruments is a major task for biomedical engineering
technicians and clinical engineers. Historically, troubleshooting instruments has been a
self-taught art form slowly mastered by relatively few individuals. This document is
intended to be a brief guide to a basic, uniform troubleshooting technique which has been
successfully used with medical instrumentation, devices and systems for three decades. It
must be remembered that an individual can master the art of troubleshooting only
through constant practice over a period of time.
1.1 Concepts of troubleshooting
As with all creative activities such as writing, design or art, each act of
troubleshooting has a beginning, a logical procedure which is followed, and a
termination. If the practitioner follows each step of the process in its appropriate
order, it will lead to an unequivocal solution. With troubleshooting, there are three
basic steps:
 Analysis
 Setting initial limits
 Reduction of limits
The analysis is a search for clues which allows initial limits to be set within which
the malfunction must be located. Further analysis allows you to continually
reduce these limits until you have isolated the problem to its precise location.
When troubleshooting, it is recommended that a block diagram of the device be
used rather than detailed schematics because they are less complicated which
results in reduced error.
The analysis process involves the selection and use of one or more of six
techniques. These are:






Front panel analysis
Half split analysis
Converging branch analysis
Diverging branch analysis
Switching analysis
Feedback analysis
Using these analytical tools individually or in combinations, initial limits can be
set. These limits are to be continually reduced to the point where the problem is
isolated to a component or circuit in the system.
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Prior to discussing these troubleshooting techniques, we need to understand that
troubleshooting will take time and patience. We need to develop the skills needed
and not be afraid to make mistakes and learn from those mistakes. The
inexperienced troubleshooter will invariably approach the device with a mindset
that says lets take it apart and fix it. I would suggest a list of possible causes of the
problem before the inexperienced troubleshooter even begins to troubleshoot.
This will create a habit of thinking about the problem before action is taken that
may be embarrassing when done in front of the experienced staff. There is also a
time issue that needs to be considered if a process is not followed and strictly
developed in the novice troubleshooter. If the process is not developed and
followed the novice will not know where he/she is going or where he/she has
been. This creates confusion and circuit redundancy, (going through the same
circuits over and over again) costing time and energy. The circuit needs to be
eliminated as a source of the problem. Only then can we go on to the next circuit
and keep reducing the circuits until we have eliminated all the working
components. The problem circuit is the only one left.
Another part involves the use of measurement tools. Use all available tools
including your own senses (sight, touch, hearing, smell) and the basic instruments
involved in the troubleshooting of medical instrumentation. When using these
instruments, use the simplest ones available that is capable of making the
necessary check. Do not fall into the trap of using the most sophisticated
instrument to do a simple bias voltage check. Always start simple and only go as
complex as you need to solve the problem. Remember ohms law, most of the
time you will be able to make the measurements with a simple VOM.
FRONT PANEL ANALYSIS
Front panel analysis is the first act to be performed in troubleshooting. It includes
talking to the staff members to determine what they think is wrong as well as
setting up and running the device to detect from its operation likely points of
trouble. In most instances, front panel analysis lets you set the initial limits to the
problem.
The following paragraphs illustrate the use of front panel analysis on three
different instrument systems. They are taken from actual technician experience
and are not hypothetical.
1.2.1 Example one: Floating trace on physiological monitor
The cardiac care nursing supervisor called for the technician at 0800 hours
because she could not get a stable trace or steady rate meter indication on the
bedside or central monitor. When the technician arrived at the ICU he asked her
what the problem was. She said, “The monitor is defective and doesn’t respond as
I would like.” The nurse was “too busy” to give further details.
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The technician went to the bedside monitor and observed the following facts:
1. The instrument had line voltage (pilot light was on).
2. The switches were in the right position (power, lead selector, gain and
polarity).
3. The trace was moving all over the scope face in an erratic manner but
QRS complex was present.
4. The rate meter readout was also erratic.
5. The patient was approximately 65 years old, appeared to be
diaphoretic and somewhat overweight.
From these five facts observed by the technician, the following conclusions were
Reached.
1. The pilot light was on therefore line voltage was present.
2. The instrument’s amplifier and power supply were probably
functioning properly because the:
a. Trace was present even though erratic;
b. QRS complex was present and amplified.
3. No fundamental operator error had been made because all switch
positions and cable connections were correct. This reduced the areas
about which initial limits could be placed. The next step in the front
panel analysis was soon taken.
The technician disconnected the patient leads from the patient and shorted them
together. A stable trace appeared, the rate meter went to zero and the low alarm
limit sounded. He then repeatedly depressed the one millivolt (cal) switch and a
square wave appeared on the stable trace and the rate meter rose and settled
between 60 and 80 beats per minute; the alarm stopped sounding.
From this brief exercise, the technician set his initial limits with absolute
confidence that he was right he could:
1. completely eliminate the monitor because, from the front panel
observation, he could tell it was working correctly.
2. Set his initial limits around those parts of the system from the cable
Connector to the patient.
The problem could still be in one of four possible areas. These would be:
1. Cable set, connector or lead defective.
2. Electrodes on the patient defective.
3. Electrodes placed on the fatty areas causing a constantly changing
electrode characteristic.
4. Insufficient electrode paste because a diaphoretic patient’s skin is a
poor conductor.
The technician used the half split rule which is explained later and isolated the
problem to diaphoretic patient skin conduction. He explained to the cardiac care
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supervisory nurse what he had done to find and correct the problem and has not
had a repeat call for this problem since.
1.2.2 Example two: Flame Photometer-erratic readout
The blood chemistry lab supervisor called the technician at 2300 hours stating that
she had a stat flame procedure and that the flame photometer was erratic. The
technician was on the scene within ten minutes and asked the lab supervisor
“what is the problem” She said that machine does not stabilize.
He asked her if she or one of her lab techs had set the aspiration rate.
She answered, “No, but the day shift had set it in the morning and had obtained
stable readings all day.”
The technician went to the flame, turned on the gas cylinder and observed the
following facts:
1. The gas, air and power pilot lights were on but the gas and air lights
Went out when the flame fired.
2. Spark was present because he could hear it ignite the gas/air mixture.
3. The flame looked normal.
From this evidence the technician can assume that the:
1. Gas and air supplies are working and there are no blockages.
2. Combustion is normal.
3. Electrical supply is normal.
This reduces the areas of concern to the atomizer, electronics or standardization
chemicals around which he placed his initial limits.
The technician next tried to aspirate the lithium/water solution into the flame and
noticed that the aspirated fluid had large drops and further noticed that the cup
bottom was gently pushing up on the aspiration tube pushing it out of position.
Resetting the aspiration rate and sample stage elevator brought the instrument into
a stable condition.
It should be noted that the electronics in both flame photometers and blood gas
analyzer instruments are so much more reliable than the mechanical elements
(aspirators and electrodes) that nearly 90% of the problems encountered are with
the mechanical elements
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1.2.3 Example three: Blood gas analyzer; PO² meter pegging at the top of the
scale.
The blood chemistry lab supervisor called the technician at 1300 hours stating that
PO² meter began to have an unstable readout then went to the top of its scale.
She further stated, “We have changed the electrode membrane but this has had no
Effect.”
The technician went to the instrument (no operators were in the lab) and set it up
For a normal run. Turning the blood gas analyzer on (separate meter PO² and
PCO², electrode bank, and ph electrode bank) he noticed that:
1. ph readout was stable and within normal range.
2. PCO² readout was stable and within normal range.
3. when the po² readout was switched in, the meter began a steady, fast
climb up to the top of the scale.
This is a direct indication that either the meter input is shorted or the electrode
chamber is defective. The internal limits were set between the meter readout
amplifier input to the PO²electrode and PO²electrode mounting hole on the
electrode bank.
The troubleshooting problem was solved using half split analysis. The problem
was a high resistance in the order of 109 ohms in the electrodes connector.
1.2.4 Example four: Trace on bedside monitor but no trace on the central station
monitor.
The night supervisor of the cardiac care unit called the technician in at 2200 hours
stating that she could not obtain a trace on the central station from the patient in
bed four.
The technician arrived in the CCU at 2300 hours, approached the night supervisor
and said, “If I can solve the problem in five minutes, will you buy me a cup of
coffee?” She said, “sure, I just put this monitor into the system and could get a
trace on the patient but the trace doesn’t show on the central monitor.”
The technician looked at the central monitor and observed that the other seven
beds had good traces. He looked at the monitor for bed four and observed a good
he trace, He immediately set his initial limits between the central station junction
box to the back panel connector on the bedside monitor. A quick check of the
monitor’s cabling found that the plug was inserted in the wrong receptacle.
Elapsed troubleshooting time --- approximately two minutes; reward: one cup of
coffee.
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The preceding examples may seem trivial to the experienced troubleshooter, but
to the individual with less experience, they are fundamental to the understanding
of front panel analysis.
1.2 Half split analysis
The use of half split analysis is fundamental to logical troubleshooting. It is
relatively simple and always applicable when troubleshooting an instrument or
system which consists of a series of functions such as the one shown in figure
one. The block diagram shown represents an ECG amplifier.
Patient
Leads
→
Switching
Input
→ Amplifier
1
Signal
→ Amplifier
↑
2
Power
→ Amplifier
→
Recorder
1'
Figure one: Applications of half split analysis
From the front panel analysis, the initial parameters are around the whole instrument; at
point one to point three. We now desire to reduce those limits as rapidly as possible. To
do this, we will apply half split analysis which is based on the rule:
When troubleshooting a functional series circuit, make your first check at the
point where the circuit divides (split the circuit in half). If the desired response is present,
the problem is located between the checkpoint and the output and the limits can be moved
to encompass this area. If the desired response is not present, the problem is probably
located between the input and the checkpoint and, again, the limits can be moved to
encompass this area.
In the case of the circuit in figure one; our first check should be at point 2. The question
is what type of check should we make and what should we see?
In this case, an oscilloscope measurement of the output of the input amplifier should
probably be made and if you used either an ECG simulator or a square wave (one mill
volt test button); you should expect to see an amplified output (20 to 40X) at this point.
We make the test and there is an output of approximately what we think should be there.
The next step is to forget the input circuits and concentrate on the output side’s circuit
and repeat the half split analysis.
SIGNAL
AMPLIFIER
→ POWER
AMPLIFIER
→ RECORDER
↑
2
3
1'
Figure two: Taking the next step with half split analysis
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In this case the limits have been reduced to those circuits between two and three. There
are three functional blocks; therefore they can not be split evenly. So we arbitrarily select
a point between two and three to make our next check. Again, we make a scope check to
see if we have an amplified signal or a square wave. If the desired signal is not present,
the problem is between two and three and the limits are reduced again. If the desired
signal is present at point three the problem is in the recorder. After we reduce the limits
of the problem to the smallest circuit area, we look at that section of the circuit schematic
and make individual resistance, voltage, and current checks to determine the particular
faulty components.
Signal
amplifier
Power
amplifier
Recorder
↑
2
4
3
Figure three: Localizing the problem with half split analysis
1'
The following paragraphs illustrate the use of the half split rule on three different
instrument systems. They are taken from actual technician experience and are not
hypothetical.
1.3.1 Example five: Electrocautery tip not heating
The electrocuatery was delivered to the technician’s shop with the note that its tip did not
heat up. The technician set up the device for operation, performed front panel analysis
and by all indications (power, light, and switch feel) the unit should have worked. He set
his initial brackets from the line voltage controller, variable transformer to the cautery
blade as shown in figure four. The initial brackets set at 1 indicate that the first check
should be at point 2, the junction of the heating element and the step-down transformer.
The check chosen was a VOM measurement of the AC voltage controller. The check was
positive with a variable voltage obtained at point 2. This isolated the problem to the
heating element which was plugged into a pistol grip handle with a series switch.
Line voltage →
Voltage
controller
→ Step-down
transformer
-------------------→
↑
1
2
Heating
element
1'
Figure four: Half split analysis of the electrocuaterizer
The next check was again a VOM measurement of the AC voltage at the junction of the
pistol grip and the heating element. With the switch depressed, the voltage was present.
The problem was reduced to the heating element which had no visible indication of a
fault. A resistance check of the heating element was negative with an open indicated. The
technician crimped the junctions of the heating element with his long nose pliers and the
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resistance dropped. He plugged the heating element back into the handle, tested the unit
and returned it to the outpatient department.
1.3.2 Example six: Manual/ automatic hypo hyperthermia overheats
This particular instrument system is difficult to work with because the large quantity of
water used presents a large thermal load to the heating/cooling system. If circulation of
this water is blocked, the tendency is for the water temperature to rise to a point just
above ambient.
In this case, the intensive care nurse called the technician after the water temperature in
the blanket and reservoir exceed 105°F and the overheat alarm sounded.
The Technician, with the help of the nurse, removed the hypo hyperthermia unit and
replaced it with another unit.
Taking the defective unit out of the ICU area, the technician set it up with a new blanket
and performed his front panel analysis. The following indications were observed:
1. Water was circulating.
2. Cooling cycle worked in both manual and automatic modes.
3. Temperature indication (circulating water and control) were in agreement and
correct.
4. Heat cycle was out of control in both manual and automatic modes.
5. All the pilot lights worked in correct sequence.
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6. Overcool and overheat alarms worked appropriately. (40°F and 105°F,
respective
Reservoir
Cooler
Cool
Thermostat
PATIENT
BLANKET
Circulating
pump
Heat
Thermostat
Heater
1
2
1'
Figure five: Simplified functional block diagram of the hypo hyperthermia unit
From this, the technician determined that the control circuits were functional because the
cooling cycle and pilot lights were responding to control variation. He also thought
temperature measurement elements were okay because the controller indications were the
same as the circulating water temperature. Finally, he eliminated the possibility of
circulation problems because the pump was working. With these facts in mind, he set his
initial limits from the circulating pump’s water input (1) to the water heater’s water input
(1'). This is the area between 1 and 1' in Figure five. The first check selected was an AC
heater voltage test (115 volts) made with a VOM. It was found that the 115 volts applied
to the heating elements did not drop to zero volts when the controller circuits were set at
a temperature below the circulating water temperature when the heat mode was selected.
This indicated that the heating element was working but the thermostat was not. The
second check was the removal of the thermostat element and a resistance check across its
contacts, it was found to be welded shut (short circuited) regardless of the temperature
that it was subjected to. Replacement of the thermostat restored the unit to normal
operation.
1.3.3 Example seven: Spectrophotometer – unstable output, constant ±.2 absorption unit
fluctuation on meter.
The chemistry lab supervisor called for assistance at 1300 hours complaining that she
was being driven to distraction by a constant “flutter” of the output of her
spectrophotometer.
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The technician responded quickly and from a conversation with the lab supervisor, found
that this fluctuation had been present for a considerable length of time (one week) but
staff thought that the problem was caused by the power line. This was in spite of the fact
that they had not experienced similar fluctuations on the other systems.
The technician went to the machine and set it up for a test run. He observed the following
facts:
1. The lamp was on at the spectra and was correct with a yellowish green output
(sample cell holder) at 575nm.
2. The unit responded to adjustment of its linearity
Source
power
supply
Source
Sample
cell
Detector
Amplifier
Readout
1'
1
3
Amplifier
Power
supply
2
Figure six: Troubleshooting the noisy spectrophotometer
Initially, the technician set his limits from the detector through the readout, point 1 in
figure six. The first check was made at point 2 and consisted of a DC voltmeter reading.
The indication was a DC voltage the level of which was proportional to the energy level
striking the detector. There was, however, slight fluctuations of the DC test meter. A
scope check was made at point 2 and a high value of AC ripple was found to be riding on
the normal DC output. The technician removed the amplifier load from the power supply
and the ripple was eliminated. Reloading the supply with the amplifier caused the ripple
to reappear.
The technician then made a substitution check of the power supply output filter capacitor.
This greatly reduced the ripple from the loaded supply and the spectrophotometer’s
output was stable again.
NOTE: Substitution check means that another capacitor was substituted for the original
capacitor to see if the problem was resolved.
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1.4 Converging branch analysis
The converging branch analysis techniques can greatly simplify troubleshooting
instruments or systems where two or more separate function branches are brought
together, converging into a single branch. In the hospital, the most obvious example of a
converging branch system is found in intensive and coronary car monitoring. In this case,
the bedside monitors from the branches which converge into the central station displays.
AC source
Signal processor
Branch one
Branch three
Amplifier
Amplifier
Branch two
Signal processor
Test point
Figure seven: Converging branch analysis
The analysis of converging branches is made at the point of convergence, shown as the
test point in figure seven.
If a check at this point shows the output of branch one but not the output of branch two,
the trouble is in branch two.
If a check at this point shows the output of branch two but not of branch one, the trouble
is in branch one.
If a check at this point shows the output of both branch one and branch two the problem
is in branch three.
In any case, the one check should eliminate two of the branches and allow you to limit
your troubleshooting to the defective branch.
The following examples illustrate the use of converging branch analysis. They are taken
from actual technician experience and are not hypothetical.
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1.4.1 Example eight: No ECG traces at the central station monitor, all patients straight
line.
The intensive care night nursing supervisor called the technician at 2300 hours stating
that the central station scope was not displaying any of the patients’ ECG traces. There
were four straight lines across the display. The nursing supervisor further stated that the
bedside monitors all had good displays, the technician went to the ICU as rapidly as
possible and found an entirely new shift of nurses who could not help because the didn’t
know what the problem was.
The technician began troubleshooting by first looking at each bedside monitor and
finding an adequate response. The system, shown in figure eight, was a hybrid
installation with new transistorized bedside monitors driving a vacuum tube central
station scope. The technician next visually checked the central station and found that
there were four traces but they were absolutely straight lines. From his observances, the
technician could say:

The central station’s power supplies and horizontal circuits are working.

The bedside monitors are functioning.
From this, he set his initial limits from the central station crt’s vertical deflection plates to
the outputs of the bedside monitors. He chose to make a scope check at the output of the
preamplifier, point 2 used to drive the central station and found no signal. He reduced his
limits to the output of the preamplifier. He then took a scope check at cannel one’s input
of the preamplifier, point 3 and found the signal. The problem was located in the
preamplifier. The preamplifier was opened and the problem located using the half split
analysis method of troubleshooting. The problem was an open transistor amplifier which
was replaced.
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Transistorized
bedside monitor
#1
3
Transistorized
bedside monitor
#2
Preamplifier
Four channel
Vacuum
Tube
Central
station
Transistorized
bedside monitor
#3
2
Transistorized
bedside monitor
#4
1
1'
Figure eight: Hybrid ICU monitor with no ECG trace
1.4.2 Example nine: SGOT channel readout unstable on three channel autoanalyser.
The chemistry la supervisor caught the technician in the hall outside the chemistry lab
and said that the SGOT readout on the auto analyzer had been unstable the day before.
She further stated that she had replaced the tubing in the SGOT chain and checked the
colorimeter lamp but the unstable readout was still there.
The technician looked at the entire auto analyzer and found that all of the tubing was
clear, not yellowed by reagent, and appeared to be correct. The proportioning pump was
operating correctly as was the sampler. Each colorimeter appeared to be on and the
SGOT output was not stable.
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1'
Printout
Bun, SGOT, glucose readout
2
Colorimeter
Bun
Colorimeter
SGOT
Colorimeter
Glucose
1
Incubator
Incubator
Incubator
Dialyzer
Dialyzer
Dialyzer
Proportioning pump
Figure nine: Three channel autoanalyser, SGOT readout erratic
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The technician set his initial limits from the input of the colorimeters to the output of the
digital printout unit. (Point 1 to 1' in figure nine) He decided to make his first check at the
input of the BUN, SGOT, Glucose readout unit (point 2). He disconnected the BUN
colorimeter and substituted it into the SGOT line. To do this, he had to switch filters and
the sample flow cell. The SGOT readout remained unstable. The technician then put the
colorimeter which was originally in the SGOT line into the BUN line, placing the BUN
filters and the flow cell into the colorimeter. The BUN readout was stable.
From this, the technician determined that the problem had to be in the SGOT line and not
in the BUN or Glucose line. Furthermore, the trouble could either be in the readout
circuits which serve only the SGOT line (dedicated) or somewhere back in the SGOT
line. As the SGOT portions of the autoanalyser looked all right and the lab supervisor
(who was highly skilled and extremely competent) had just changed all the tubing, the
technician reduced his limits around the SGOT, readout circuits.
He opened the readout cabinet and observed that each channel had a separate but
identical amplifier board. His next check, made in the readout, was a substitution of the
BUN board for the SGOT board. The SGOT output, though not correct stabilized. His
conclusion was that the problem was on the SGOT board. Note: the technician has not
used his test equipment to this point in the analysis.
The technician pulled the SGOT board for testing and replaced the BUN board in its
correct slot. Using half split analysis; the technician reduced the problem to a partially
defective transistor. The technician replaced the transistor and this resolved the problem.
1.5
Diverging branch analysis
Diverging branch analysis techniques greatly simplify troubleshooting instruments or
systems where two or more separate functional branches are brought out, diverged, from
a single branch. In the hospital, there are two principal examples. One, the power
distribution that makes an ever expanding tree with branches brought out from its feeder
trunk, and two, some central stations which have scope display, delay recorder and
electrocardiograph brought out from a junction box or patient selector.
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Scope
Display
Signal
Time
Delay
Amplifier
Strip Chart
Recorder
Figure ten: Diverging branch analysis
The analysis of diverging branches can be performed at the junction where the signals
diverge or at each of the divergent branches. The rule is that if a check at the divergent
junction shows the desired signal present, the problem must be in one of the divergent
branches, furthermore, if a check of one or more of the branches indicates that the desired
signal is present, the problem lies in that branch or branches without the desired signal
present.
If however, a check at the junction does not show the desired signal or if none of the
braches have the desired signal, the problem will lie in the system, prior to the point of
divergence.
The following paragraphs illustrate the use of divergent branch analysis. They are taken
from actual technician experience and are not hypothetical.
1.5.1 Example ten: The mysterious change in outlet polarity in the ICU
The technician was performing his quarterly inspection in an older intensive care unit and
started to perform the leakage test on the first instrument. Plugging his line polarity tester
into the wall outlet brought a quick reaction. The instrument indicated that the power
distribution polarity was reversed. He immediately opened the outlet and found that the
wiring color code was correct. He began testing all of the other outlets in the area and
found that their polarities were also reversed.
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From this observation, the technician knew that:

The receptacles were wired correctly.

All of the receptacles were indicating reversed polarity.

The polarity of these receptacles was correct when the last pm was performed.

The problem had to be in the main feeder lines coming from the power distribution to
the ICU.
The technician called the electrician and together they removed the cover from the power
distribution box. The technician performed his second check measuring the potential to
ground of each hot (black) and neutral (white) wire at its termination in the junction box.
All of the black leads were at zero volts, and all of the white leads were at 115 volts. The
technician and the electrician then tested the outlets in the adjacent area and found that
the hallway receptacles had reversed polarity and that two adjacent rooms had correct
polarity. These rooms had been recently remodeled (work finished four weeks prior to the
incident) which involved rewiring. Opening one of the receptacles in this area, the
technician found that the color code was reversed (white to hot and black to neutral) yet
the polarity showed correct. The panel feeding the remodeled area was opened and the
voltages tested, it was found that the whites were hot and the black wires were at neutral
polarity. Both the feed branch to the remodeled area and the feed branch to the ICU area
came from the same central feed box in an electrical closet one floor below the area in
question. The technician stated that the problem had to e in this distribution box or one of
the distribution braches feeding it.
The technician and electrician opened the central feed box in the electrical closet and
found that the electrical contractor had “switched” the hot and neutral mains because in
corrected a wiring mistake easily. The fuses and the circuit breakers didn’t blow because
the hot and neutral leads were being fed from a power transformer.
The remodeled area’s distribution was removed from the ICU’S distribution and
reconnected, with correct polarity, to the mains so that correct polarity was again present
in the ICU areas. Finally, the incorrectly wired outlets in the remodeled area were
rewired. Luckily, no patient or staff incidents occurred during the time that the lines were
reversed. Both the engineering officer who performed the acceptance inspection and the
contractor were at fault.
1.5.2. Example eleven: ICU central station not recording traces
The ICU in question was an older unit which used tape deck ECG storage. It had a long
history of frequent malfunctions and had been placed under tight engineering control. At
1100 hours one morning, the technician received a call from the unit’s supervisor
complaining that the central station was not working correctly. The technician went
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immediately to the ICU and asked the nurse what the problem was. The nurse said that
the central station recorder was not printing out traces for beds three and four but beds
one and two were working correctly.
The technician went to the central station and found that he had all four traces on the
scope display. From this, he assumed that all four bedside monitors were working and
were delivering their signals to the central station. The technician then keyed the recorder
(delay mode) using the patient selector. He obtained traces for channels one and two but
none on channels three and four. He then switched the recorder mode from delay to direct
printing and obtained traces on channels one, two, three, and four. From this, he
determined that the recorder and the patient selector were functional and that the problem
had to be located in the tape delay assembly.
Central
Station
scope
Initial limits
Bedside
monitors
Patient
selector
Time delay
tape
Recorder
Figure eleven: No tracings on channels three and four on central station recorder
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Input
amp
Record
head
Replay
head
amp
output
2
Figure twelve: Block diagram of the defective tape delay
The technician set his initial limits around the tape delay, Opening this unit, he made his
first check by substituting tape loops. This did not correct the problem. He also observed
the capstan and motor and they were operating correctly. The technician used the half
split rule and checked the alignment of the record, replay heads (point 2, figure twelve0
and found them t be slightly misaligned. Realigning the heads returned the system to
normal.
1.6
Switching analysis
When a technician encounters switching in troubleshooting, the problem always becomes
confusing. Switching always implies that several signals are brought into a branch
individually and in some sequential manner. To analyze a switching circuit, we make a
check at the wiper’s output as in figure thirteen. If the correct signal is present for each
switch position, the problem has to lie in the wiper’s circuit. If however, there is one
signal missing, the problem has to lie in that position’s driving circuit. Finally, if there is
no signal present on the wiper and a second check on the position contactors shows
signals present, the switch is defective.
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Signal
#1
Wiper
Output
Signal
#2
Signal
#3
1
Figure Thirteen: Switching analysis
Switching analysis is especially helpful when diagnosing an ECG apparatus. In this case,
the switch (usually dual and complex) is driven by the five leads from the patient. There
are, however, twelve positions on the switch. Each lead is used in more than one position.
The following example illustrates how a technician isolates an instrument problem from a
lead problem using switching analysis.
Example twelve: switching analysis used to isolate lead problems from instrument
problems.
The technician was called to the heart station because the ECG machine would not record
for lead II. The technician connected the instrument to his five-lead cardiac simulator.
His first check was to attempt to record a trace for each limb position and one chest
position. He obtained a trace for leads I, AVR, AVL, and chest. He could not get a trace
on II, III, and AVF. This was at firs puzzling until he realized that the one common to
positions II, III, and AVF was the left leg. This lead was broken inside the cable. Table
one shows the ECG lead connections which are used and table two is a truth table which
can be used to analyze a patient cable’s leads.
Einthoven’s triangle with lead connections is shown in figure fourteen A.
Figure fourteen B shows the connections as listed in table one.
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+
Lead 1
Right arm
-
+
-
Left arm
-
Cardiac
vector
Lead III
Lead II
+
+
Left leg
Figure fourteen A: The Einthoven Triangle
Table one: cable test sequence
Lead
I
II
III
AVR
AVL
AVF
C
connections
RA to LA
RA to LL
LA to LL
RA to LA, LL
LA to RA, LL
LL to RA, LA
C to RA, LA, LL
Table two: cable truth table
Broken lead
RA
LA
LL
C
RL
Tracing on
III, AVL, AVF, C
II, AVR, AVF, C
I, AVR, AVL, C
I, II, III, AVR, AVL, AVF
NOISE, unstable tracings
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No tracing
I, II, AVR
I, III, AVL
II, III, AVF
C
I, II, III, AVR, AVL, AVF
1.7
Feedback analysis
Systems which utilize feedback control purposes have always presented a problem to the
troubleshooter. The principle reason for this difficulty is that the technician (or engineer)
troubleshooting the feedback network invariably attempts to figure out, on the spot, how
the feedback network functions. In most instances, this “circuit analysis” is wasted time
and effort. It is not important to troubleshooting to know the function of the feedback
network to troubleshoot it. One must only remember that the feedback network
controls the function of the circuit. Therefore, a feedback controlled system (as shown
in figure fifteen) can be troubleshot by the following technique.
Feedback
input
output
System
Figure fifteen: Feedback analysis
When analyzing a feedback controlled system or network, break (disconnect) the
feedback loop. If the output of the system or network does not change, the problem is in
the feedback loop. If the output changes radically, the problem is in the system or the
circuit.
The first check of the feedback controlled system is restricted to its output and consists of
measuring it as you disconnect the feedback loop. This measurement might be a
voltmeter, oscilloscope or, as in the case of a servo system, motion check. Exact outputs
are not looked for, but rather, large changes.
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Always remember that feedback is used to control. When you remove effective
control, you have rapid and large change. When you remove ineffective control, you
have slow and minute change.
1.8
Conclusions
The expert troubleshooter utilizes all of his senses to isolate and eliminate a problem as
rapidly as possible. He will use all techniques available to him, including:

Front panel analysis

Half split analysis

Converging branch analysis


Diverging branch analysis
Switching analysis

Feedback analysis
To set initial limits within which his problem is located and then relentlessly reduce those
limits until he has reduced the problem to a specific defect.
With practice of the techniques detailed here, you can improve your effectiveness and,
therefore, your value within the hospital or organization where you work.
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