Guildline 6634A Resistance Standard User manual

OPERATORS MANUAL
FOR
MODEL 6634A
RESISTANCE STANDARD
NOTICE
The contents and information contained in this manual are proprietary to
Guildline Instruments Limited. They are to be used only as a guide to the
operation and maintenance of the equipment with which this manual was
issued and may not be duplicated or transmitted by any means, either in
whole or in part, without the written permission of Guildline Instruments
Limited.
OM6634A-B-02
4 January 2019

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TABLE OF CONTENTS
1. INTRODUCTION...................................................................................................1-1
1.1. SCOPE................................................................................................................................................... 1-1
1.2. GENERAL DESCRIPTION................................................................................................................ 1-1
1.2.1. Resistance Ranges...............................................................................................................................1-1
1.2.2. Standard Model Configurations..........................................................................................................1-1
1.3. FRONT PANEL INDICATORS AND CONTROLS......................................................................... 1-2
1.3.1. Power..................................................................................................................................................1-2
1.3.2. Heater..................................................................................................................................................1-2
1.3.3. Over Temperature...............................................................................................................................1-2
1.3.4. PRT.....................................................................................................................................................1-2
1.4. REAR PANEL CONNECTORS AND CONTROLS......................................................................... 1-3
1.4.1. Line Input Connector..........................................................................................................................1-3
1.4.2. Resistor Terminals..............................................................................................................................1-3
1.4.3. Guard Terminal...................................................................................................................................1-3
1.4.4. Chassis Ground Terminal....................................................................................................................1-4
2. SPECIFICATIONS................................................................................................2-1
2.1. GENERAL SPECIFICATIONS.......................................................................................................... 2-1
2.2. RESISTANCE SPECIFICATION ...................................................................................................... 2-2
3. OPERATING INSTRUCTIONS.............................................................................3-1
3.1. INSTALLATION.................................................................................................................................. 3-1
3.2. OPERATING NOTES.......................................................................................................................... 3-1
3.2.1. Connection to Standard Resistors .......................................................................................................3-1
3.2.2. Monitoring of the Internal Temperature..............................................................................................3-1
4. MAINTENANCE....................................................................................................4-1
4.1. PREVENTIVE MAINTENANCE....................................................................................................... 4-1
4.1.1. Cleaning..............................................................................................................................................4-1
4.1.2. Temperature........................................................................................................................................4-1
4.1.3. Resistor Stability.................................................................................................................................4-1
4.2. CERTIFICATION OF RESISTANCES............................................................................................. 4-1
4.3. CORRECTIVE MAINTENANCE...................................................................................................... 4-1
4.3.1. Preliminary Checks.............................................................................................................................4-1
4.3.2. Temperature Control...........................................................................................................................4-2
4.3.3. Over Temperature...............................................................................................................................4-4

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5. PRINCIPLES OF OPERATION ............................................................................5-1
5.1. INTRODUCTION ................................................................................................................................ 5-1
5.2. TEMPERATURE SENSITIVE OSCILLATOR................................................................................ 5-2
5.3. AMPLIFIER AND DETECTOR......................................................................................................... 5-2
5.4. HEATER DRIVER............................................................................................................................... 5-3
5.5. OVERTEMPERATURE CIRCUIT.................................................................................................... 5-3
5.6. POWER SUPPLY................................................................................................................................. 5-4

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TABLE OF FIGURES
FIGURE 1-1: FRONT VIEW....................................................................................................................................1-2
FIGURE 1-2: REAR VIEW ......................................................................................................................................1-3
FIGURE 5-1: HEATER CONTROL UNIT ..............................................................................................................5-1
FIGURE 5-2: TEMPERATURE SENSITIVE OSCILLATOR.................................................................................5-2
FIGURE 5-3: AMPLIFIER AND DETECTOR........................................................................................................5-2
FIGURE 5-4: HEATER DRIVER.............................................................................................................................5-3
FIGURE 5-5: POWER SUPPLY...............................................................................................................................5-4

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LIST OF TABLES
TABLE 2-1: GENERAL SPECIFICATIONS...........................................................................................................2-1
TABLE 2-2: RESISTANCE SPECIFICATION........................................................................................................2-2
TABLE 4-1: CONTROL CIRCUIT BOARD TESTING..........................................................................................4-2
TABLE 4-2: TRANSISTOR VOLTAGES................................................................................................................4-4

Section 1
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1. INTRODUCTION
1.1. SCOPE
This manual contains technical specifications, a detailed description, maintenance information,
and diagrams for the Guildline Instruments model 6634A Resistance Standard.
1.2. GENERAL DESCRIPTION
The model 6634A Resistance Standard is a rack mountable or bench top instrument which
provides a set of precision resistance standards enclosed in a multi-walled constant temperature
chamber. The internal temperature is maintained by a solid-state control system at 30 C 0.01
C. The design assures minimum thermal gradients within the inner chamber containing the
standard resistors. The resistors are immune to normal shock and vibration in commercial carrier
shipment.
To protect the standard resistors from overheating due to a malfunction of the temperature
control, there is an overriding over-temperature control that will disable the heating system if the
temperature exceeds 34 °C 3 C.
The model 6634A is intended to be used as a Working Standard Resistance Reference for an
automated resistance measurement system.
1.2.1. Resistance Ranges
The ten resistor elements within the model 6634A-10 cover the range of 0.001 Ωto 100
MΩ in decade steps. Each resistor element is isolated and has a 4-terminal connection at
the back panel. The normal power to be applied to each resistor element is 10 mW.
Other models are as described in section 1.2.2.
1.2.2. Standard Model Configurations
The following standard models are available with the resistance element ranges specified.
Other variations can be supplied as a requested special build with any values of resistance
from 0.001 Ωto 100 MΩ.
Model 6634A-10 comprises a full complement of 10 elements from 0.1 Ωto 100 MΩ
ohms in decade values.
Model 6634A-9 comprises 9 elements from 0.1 Ωto 10 MΩin decade values.
Model 6634A-8 comprises 8 elements from 1 Ωto 10 MΩin decade values.
Model 6634A-5 comprises 5 elements from 1 Ωto 10 kΩ ohms in decade values.

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Note that if 0.001 and/or 0.01 values are ordered they physically take more space so are
delivered with fewer elements.
Figure 1-1: Front View
1.3. FRONT PANEL INDICATORS AND CONTROLS
The front panel indicators and controls are illustrated in Figure 1-1. They are provided to
indicate power status, heater operation and temperature status.
1.3.1. Power
The POWER indicator is lit when the AC power is applied to the unit and the power
ON/OFF switch on the rear panel is “ON”.
1.3.2. Heater
The HEATER indicator is lit when power is being applied to the heater.
1.3.3. Over Temperature
The OVER TEMP indicator is lit when the internal temperature has exceeded 34 °C 3
C. This may occur when the unit is operated in an ambient temperature in excess of the
operating temperature, or in the event of control failure.
1.3.4. PRT
The platinum resistance thermometer, imbedded within the temperature controlled
chamber, is configured as a four terminal resistor. The four terminals terminate in four
low thermal binding posts labelled “PRT”. The set of terminals consists of two black
binding posts for the 'P1' and ‘C1’ terminals, and two red binding posts for the ‘P2’ and 'C2'
terminals of the PRT. The ‘C’ terminals are to be connected to the current source and the
‘P’ terminals are to be connected to the potential measuring circuit of the resistance
measuring device. The temperature sensitivity of the PRT is 0.392 /C. The resistance
at 30 C is provided in the Report of Calibration. For accurate temperature measurement,
the resistance must be measured in four wire mode with an excitation current of 1 mA or
less.

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Figure 1-2: Rear View
1.4. REAR PANEL CONNECTORS AND CONTROLS
The rear panel connectors and controls are illustrated in Figure 1-2. They include power entry,
resistor terminals and guard/ground terminals. The terminals are not installed for resistance
element values that are not provided in specific models.
1.4.1. Line Input Connector
The Line input connector is a combination standard male 3-prong AC connector, power
ON/OFF switch and fused line input voltage selector. The line input voltage selector
consists of a four-position selector and a fuse holder. The selector allows switching for
100 VAC, 120 VAC, 220 VAC and 240 VAC. A ½ amp time delay fuse is provided for
100/120 VAC operation and a ¼ amp time delay fuse is provided for 220/240 VAC
operation.
1.4.2. Resistor Terminals
There are a set of four terminal input/outputs for each installed resistance element. Each set
consists of four low thermal binding posts labelled with the nominal value of each of the
standard resistors. Each set of terminals consists of two black binding posts for the 'P1' and
‘C1’ terminals, and two red binding posts for the ‘P2’ and 'C2' terminals of the specific
standard resistor. The ‘C’ terminals are to be connected to the current source and the ‘P’
terminals are to be connected to the potential measuring circuit of the resistance measuring
device.
1.4.3. Guard Terminal
The GUARD terminal consists of a four-way white binding post. The GUARD terminal is
connected to the isothermal chamber which contains the standard resistors. This chamber is
isolated from chassis ground and may be guarded through the guard terminal separately
from the power ground.

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1.4.4. Chassis Ground Terminal
The CHASSIS GND terminal consists of a four-way green binding post. The CHASSIS
GND terminal is bonded to the chassis of the model 6634A and to the power ground of the
line input connector.

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2. SPECIFICATIONS
2.1. GENERAL SPECIFICATIONS
MODEL 6634A RESISTANCE STANDARD
General Specifications
Operating Temperature
+23 5
°C
+73 9
°F
Storage Temperature
-20 to +60
°C
-4 to +140
°F
Operating Humidity (non-condensing)
< 70
% RH
Storage Humidity (non-condensing)
< 90
% RH
Temperature Stability (24 hours)
(exclusive of the self heating effects
of the resistors)
30 0.003
°C
86 0.005
°F
Temperature Stability (1 year)
(exclusive of the self heating effects
of the resistors)
30 0.01
°C
86 0.02
°F
Cold Power on Stabilization Time to
Specified Accuracy
36
hours
Power Requirements
15
VA
Voltage Requirements
100, 120, 220 or 240 10%
VAC
Line Frequency
50 or 60 5%
Hz
Dimensions (Nominal) (1)
D 503, W 440, H 132
mm
D 19.8, W 17.4, H 5.2
in
Weight Bench Top
11
kg
24
lb
Rack Mount
9.4
kg
20.7
lb
Table 2-1: General Specifications
Notes:
1. The width including rack-mount ears is 480 mm (18.9 in). The height including bench
top feet is 142 mm (5.6 in).

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2.2. RESISTANCE SPECIFICATION
Nominal
Resistance
(Ω)
Nominal
Initial
Tolerance
(ppm)
(Note 1)
Stability
over any 24
hour period
(ppm)
Stability
12 months
(ppm)
Temperature
Coefficient
(ppm/°C)
Maximum
Voltage
(volts)
0.001
20
0.4
15
0.04
0.01
0.01
10
0.2
10
0.02
0.03
0.1
10
0.1
4
0.01
0.1
1
10
0.01
2.5
0.005
0.32
10
10
0.01
2.5
0.005
1.0
100
10
0.01
2.5
0.005
3.2
1k
10
0.01
2.5
0.005
10
10k
10
0.01
2
0.005
32
100k
15
0.02
5
0.01
100
1M
25
0.04
5
0.02
320
10M
35
0.2
6
0.2
1000
100M
50
0.5
15
(Note 5)
0.2
1000
Table 2-2: Resistance Specification
Notes:
1. Nominal initial tolerance is defined as the maximum variation of resistance mean values
as initially adjusted at the point of sale.
2. Calibrated in ambient conditions of 23 °C, referred to the unit of resistance as maintained
by the National Research Council of Canada or the National Institute of Standards and
Technology, and expressed as a total uncertainty with a coverage factor of k = 2.
3. Stability is exclusive of the effects of applying power above 20 mW, but not exceeding
the maximum voltage, in terms of hysteresis and short term temperature stabilization.
4. See section 1.2.2 for specific resistance values installed in various model configurations.
5. For all ohmic values, except 100 MΩ, 2nd year and subsequent years 12 month stability
remains the same. For the 100 MΩ 2nd year and subsequent year 12 month stability is
±10ppm.

Section 3
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3. OPERATING INSTRUCTIONS
3.1. INSTALLATION
The model 6634A has been shipped with the line input voltage set to 240 volts with the fuse
removed unless specifically requested to be set to another voltage level. The line input selectors
must be set to the correct line voltage before power is applied to the instrument. Remove the
warning label positioned across the power cord only after checking the setting of the operating
voltage. The line input voltage selection must be set correctly. The settings available are 100 V,
120 V, 220 V, and 240 V. Figure 3-1 details the line input voltage selector setting.
The instrument is supplied with a North American style line cord, unless otherwise specified at
time of order. Ensure that the line cord is plugged into a wall socket or extension cord that has a
protective or safety ground. Where 3-contact power supply outputs are not available, a suitable
protective ground connection must be made before switching the instrument power on. Any
interruption of the protective ground may possibly render the instrument unsafe.
To set the correct line input voltage pry open the power receptacle on the rear panel as shown in
Figure 3-1.
Place the instrument on a vibration-free bench or install it in an equipment rack as required.
Connect the line cord to the appropriate line power outlet and switch the instrument on using the
power switch at the back of the instrument at the line input connect.
The red POWER lamp should light, together with the green HEATER lamp.
When the inner chamber containing the standard resistors reaches its operating temperature, the
HEATER lamp will indicate periodic cycling. Due to the thermal lagging, it will take several
hours to raise the inner chamber and the standard resistors to the operating temperature. The
standard resistors should be allowed to stabilize at the operating temperature for at least 36 hours
before measurement or use.
3.2. OPERATING NOTES
3.2.1. Connection to Standard Resistors
Connection to the resistor terminals should be made with solid copper wire having low
thermal EMF characteristics. The wires to each C-P pair of standard resistor terminals
should be twisted and preferably shielded to avoid spurious pick-up. Use Guildline Type
SCW Wire or equivalent. The certified resistance of each resistance element installed
and the date of calibration are indicated in the Report of Calibration.
3.2.2. Monitoring of the Internal Temperature
The PRT which comes to the front panel is used to measure the internal temperature.
Connect an ohmmeter capable of four wire resistance measurement with low excitation
current. Connect the current leads to C1 and C2, and the respective potential leads to P1

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3-2
and P2. Set the ohmmeter to the best range for measuring 112 with an excitation
current of 1 mA or less. Measure the resistance of the PRT (Rt). The change in internal
chamber temperature from the previously measured resistance value:
( )
C
RR
tot 







−
= 392.0
Where Ro is the previously measured resistance value.
Figure 3-1: Opening the Power Receptacle
Check to see that the fuses inserted in the receptacle correspond to the correct type specified in
Section 1.4.1 Line Input Connector
100V
120V
220V
240V
100V
120V
220V
240V
EXPOSE LINE SELECTOR SLIDER
REMOVE SELECTOR (120V SHOWN)
PRY HERE TO REMOVE COVER
ROTATE SELECTOR AND REPOSITION
SLIDER TO DESIRED LINE INPUT
(240V SHOWN) AND REINSERT
REPLACE COVER
DISCONNECT POWER

Section 4
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4. MAINTENANCE
4.1. PREVENTIVE MAINTENANCE
4.1.1. Cleaning
The instrument requires very little maintenance other than periodic dusting of the outside
of the enclosure. Use alcohol to clean around the standard resistor terminals to avoid
formation of leakage paths.
4.1.2. Temperature
Check the internal temperature with the imbedded PRT before using the resistors as
references and also periodically.
Compare the calculated temperature with the value given in the Report of Calibration.
4.1.3. Resistor Stability
Resistor stability is determined by comparison against known reference resistors.
4.2. CERTIFICATION OF RESISTANCES
If the resistance of the standard resistors is to be determined by direct comparison with a
standards laboratory, the comparison should be made with the resistors inside the enclosure at the
operating temperature. After transportation, or if the resistors’ temperature stabilized enclosure
has been allowed to cool, the resistors should be allowed to stabilize at the operating temperature
for at least 36 hours before measurement or before use.
4.3. CORRECTIVE MAINTENANCE
Symptoms appear at the beginning of each of the following paragraphs as a guide to detailed
fault location. Refer to Section 5 of this manual for the PRINCIPLES OF OPERATION.
4.3.1. Preliminary Checks
Check POWER indicator - This should be lit when the instrument is connected to the AC
line and the power ON/OFF switch in set to “ON”. If not, check that both of the other two
HEATER and OVERTEMP indicators are also not lit. If they are both not lit, then a
general loss of power is indicated. If either indicator is lit, then it indicates that only the
power indicator circuit is defective. In addition, check the power outlet. The fuse in the
Power Entry module may be blown. It may have been a defective fuse.
Check the HEATER indicator - It should indicate the cycling on and off of the heater.
Check the OVERTEMP indicator - If it is lit, then the temperature inside the enclosure
has exceeded 34 °C 3 C, either from a high ambient temperature or failure of the

Section 4
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4-2
control circuit. Proceed to paragraph 4.3.3. Check the internal temperature with the
imbedded PRT to compare the calculated temperature with the value given in the Report
of Calibration.
4.3.2. Temperature Control
If the temperature is low and the HEATER indicator is lit, check the continuity and
resistance of the heater circuit. See drawing M6634A-01-22 for details.
If the temperature is low and the HEATER indicator is lit, check the operation of the
control circuit as follows:
1) Locate Rl connected from pin C to pin D of P2.
2) Short out Rl and the HEATER indicator should light. If not, then check for a fault in
the control circuit. If the indicator lights, then check the resistance of Rl and the
thermistor pair RT1/RT2. Rl should be 3k ohm to 5k ohm. RT1/RT2 is equal to Rl
at 30°C, and is larger than R1 if the temperature is less than 30°C. It has a
temperature coefficient of approximately -4%/C°. If necessary, replace Rl and
RT1/RT2 as a matched pair.
Control circuit board testing - Measure the voltage at the pins of connector P1 as
indicated in Table 4-1.
Between PIN
And PIN
Should Read
Components To Check
20
16
14.0 V 60 Hz
T1
21
16
14.0 V 60 Hz
T1
7
16
-12 V D.C
CR106, CR107
9
7
-3.2 V
see NOTE 1
10
7
-3.2 V
see NOTE 1
12
7
-3.2 V
see NOTE 1
14
7
-0.3 V
when heater off
-0.7 V
when heater on: see NOTE 2
Table 4-1: Control Circuit Board Testing
NOTE 1: If this reading is incorrect, then check the voltage across C101, 470 F capacitor. It
should read 8.2V 5%. If not, then check R107, CR101, CR102 and C101. If
correct, then check R102, R105, C103 and Q101.
NOTE 2: If this reading is not obtained, check Q1.

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Check the operation of the temperature sensitive oscillator. If the resistance of RT1/RT2
is greater than the resistance of Rl, then the oscillator should be running. Check with an
oscilloscope between TP1 (collector of Q102) and Pl-7. The waveform should be as
shown. If not, then check the following components:
Q101, Q102, C104, T101, C102, R103, R125, R104, R110
TP1
If oscillation is observed, then check between TP2 (collector Q103) and Pl-7. The
waveform should be as shown. If not, then check the following components:
Q103, C106, R117, R112, R106
TP2
If this waveform is observed, then measure the D.C. voltage between TP3 (CR104 anode)
and Pl-7. It should read -2.5 volts. If not, then check C105, C107, CR103 and CR104.
If it is correct, then check Q104, Q105, R121, R122 and R114.
* Indicated waveform typical, if RT1 is at ambient temperature of about 25 °C.
As a further guide to trouble shooting, typical D.C. voltage at the transistor terminals are
given in Table 4-2.

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DEVICE
VOLTAGE READINGS (Pl-7 Common)
HEATER ON
HEATER OFF
E
B
C
E
B
C
Q101
-3.05
-3.2
-5.2
-3.05
-3.2
-5.2
Q102
-5.4
-5.2
-3.05
-5.4
-5.2
-3.05
Q103
-2.95
-3.05
-5.3
-2.95
-3.05
-5.3
Q104
-1.4
-2.0
-1.45
-----------Variable--------
Q105
-0.75
-1.42
-1.45
-----------Variable--------
Q1
0
-0.75
-0.25
0
-0.25
-12.8
Table 4-2: Transistor Voltages
4.3.3. Over Temperature
If the temperature within the constant temperature chamber exceeds 34 °C 3 C, then
the OVER TEMP indicator will be lit. Allow the enclosure to cool to less than 30 °C.
Check the resistance of R2 (2.2 ). Check between P2-F and P2-H to ensure that the
temperature sensitive switch S2 is open. If these conditions are satisfied, then replace
fuse F1 and check that the HEATER indicator is lit. Monitor the increase in temperature
with the PRT. If the lamp does not indicate heater cycling when the temperature
approaches normal, then check the operation of the control circuit as follows:
- Locate pins A and C of P2.
- Short pin A to pin C. The HEATER indicator should go out. If it does, then
check the resistances of RT1 and R1 as detailed in section 4.3.2. Replace them
as a matched pair. If not, then check at TP1 and TP2 with an oscilloscope
(common to P1-7). With P2-A and P2-C shorted, there should be no
oscillation. Check the voltage at TP3. It should be zero. Refer to Table 4-2
for voltage measurements to aid in locating the fault in the heater driver.

Section 5
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5. PRINCIPLES OF OPERATION
5.1. INTRODUCTION
The heater control unit in the model 6634A enclosure consists of the following functional blocks.
a) Temperature Control - consisting of a sensor, oscillator, amplifier, detector and
heater.
b) Over temperature Circuit
c) Power Supply
Figure 5-1: HEATER CONTROL UNIT
OSCILLATOR
AMPLIFIER/
DETECTOR
HEATER
DRIVER
POWER
SUPPLY
AC IN
HTR
CONTROL
O/T
SWITCH

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5-2
5.2. TEMPERATURE SENSITIVE OSCILLATOR
Figure 5-2: TEMPERATURE SENSITIVE OSCILLATOR
Temperature changes are sensed by the thermistor RT1/RT2 connected with Rl across the 1:2
ratio autotransformer T101. As the temperature decreases, the thermistor resistance increases
and the voltage divider output to the base of Q101 increases. As soon as this voltage exceeds
half the transformer output, the circuit gain becomes greater than one and oscillation occurs.
5.3. AMPLIFIER AND DETECTOR
Figure 5-3: AMPLIFIER AND DETECTOR
The output of the oscillator is amplified by Q103 and rectified by a voltage doubler, consisting of
C105, CR103, CR104 and C107. The resistor R115 gives current limiting protection for the next
stage.
T101
Q101
P1-12
P1-9
P1-10
C104
R110
R1
RT1/RT2
t
P2-C
P2-D
P2-A
Q102