EAI 580/680 Scientific Computing System Maintenance Series: Digital Voltmeter — Chapters 4–5 (Maintenance and Calibration)
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CHAPTER 4
MAINTENANCE
4.1 GENERAL CONSIDERATIONS
The Model 26. 268 DVM is designed for long-term accuracy and trouble free operation. The
use of solid-state and conservatively rated components, in conjunction with high-quality etched-
circuit boards, ensures long-term reliability.
In addition to the material contained in this paragraph, schematic and wiring diagrams of the
DVM are provided in Appendix 2 (including component location diagrams) and a complete list
of replaceable parts, including descriptions and part numbers, is contained in Appendix 1.
(Information on ordering spare or replacement parts from EAI is found immediately after the
title page in the front of this handbook. )
It should be noted that these maintenance instructions and data cannot cover all possible troubles
and malfunctions; it is, therefore, essential that maintenance personnel become thoroughly fami-
liar with the DVM circuitry. General purpose maintenance data cannot be substituted for tech-
nical knowledge of the instrument.
NOTE
Whenever the 26.116-1 Summing Resistor Network,
the 26.242 Comparator and Diode Gate Unit, or the
12,937-1 Network of the DVM are serviced, the DVM
must be recalibrated to insure specified accuracy. If
the 6.736 Reference Amplifier or 43.141 Regulator of
the DVM power supply are serviced or replaced, the
bower supply output levels must be carefully and ac-
curately checked,
4.2 TROUBLE ANALYSIS
To assist in analyzing malfunctions, the DVM circuits should be considered as subdivided into
four groups: the input circuits (including both de amplifiers and associated circuits), the resis-
tor matrix and comparator circuit, the digitizing circuits (including the programmer, and the
four BCD counters),
and the reference and power circuits, All measurements of signal circuits
4-1
CHAPTER 4 MAINTENANCE
should be made with a high impedance device, such as an oscilloscope, to prevent circuit load-
ing and possible misleading results,
4.2.1 Test Connector Functions
The test connector (J3) provided at the rear of the DVM is a very useful troubleshooting aid,
Many signals within the DVM are terminated at this connector, permitting easy access with a
sharp test prod, If desired, a test cable may be constructed to mate with J3 and terminate the
signals at a remote chassis or panel equipped with tip jacks or their equivalent. The mating
connector for this purpose is a 25 pin male Cannon connector (EAI Part Number 542 098-0),
The following table (Table 4.1) lists the signals present at each pin of 53,
Table 4,1. Test Connector J3 Functions
:
Signal Description
Reference Amplifier Balance Output
-100 Volt Reference Level +. 005% with Respect to +10 Volt Reference
440 Volt Level 41%
No Connection
-20 Volt Level +1%
-110 Volt Level +. 5%
Conversion Complete Signal (Waveform e, Figure 3, 4)
Amplifier B Output
Amplifier A Output
| +2 Volt Level 415%
tL 45 Volt Level 45%
1 420 Volt Level +1%
[sf =. Volt Level 21%
= a revel 45%
, ul (Waveform a, Figure 3S. 4)
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CHAPTER 4 MAINTENANCE
4.2.2 Reference Circuit and Power Supplies
All power and reference supplies for the DVM are contained in the DVM chassis, The levels
and tolerances are listed in Paragraph 1.3 of Chapter 1, (See Figure 5,2, Chapter 5 for Con-
trol Location.) A rough check of the -100 volt reference supply can be made with a VTVM or
scope; however, for an accurate check a test circuit of at least +0.005% accuracy should be
used,
Check the reference amplifier balance as follows:
1. Allow 1/2 hour warm-up if the power supply has been off.
2. Connect a multimeter (Triplett, Model 630A, or equal) between pins 1
and 16 of J3, The meter should fluctuate about zero (10.05 volt) on the
3-volt range*.
3, If the meter does not read within these limits, adjust R9 on the 6,736
Card, (See Figure 5.2, Chapter 5.)
Relatively high levels of voltage may be generated
as the balance control is rotated. Use high voltage
ranges of meter first, reducing to the 3-volt range
as the voltage mull is approached.
4,2.3 Input Circuit
The general operating condition of the two unloading amplifiers can be checked by performing
the followin g balance procedure: (Inability to balance usually indicates an amplifier circuit
malfunction. )
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1. Allow 1/2 hour warm-up time if unit has been off.
a= Connect a multimeter (Triplett, Model 630A, or equal) between J3-17 and
and J3-16, (Check amplifier A.)
it
stabilizer output at this point is pulsating de.
t |
1
MAINTENANCE
CHAPTER 4
0,05 volt, adjust R10 on the 12. 937-1
(See Figure 5.2, Chap-
3. If the meter does not read between +
Network Card until the needle vibrates about zero.
ter 5.)
Relatively high levels of voltage may be gener ated
as the balance control is rotated. Use high voltage
ranges of meter first, reducing to the 3-volt range
as the voltage null is approached,
4. Repeat this procedure for amplifier B; connect the meter between J3-18
and J3-16. Adjust R11 on the 12,937-1 if necessary.
If the DVM functions normally with negative inputs but malfunctions with positive inputs, amp- ii
lifier B is probably at fault (the polarity relay, K1, could also cause this effect). Witha
known input, the output of amplifier A can be checked at J3-9, The amplifier A gain should
be 1,25; the output of amplifier B (equal but opposite to amplifier A) can be checked at J3-8.
4.2.4 Resistor Matrix and Comparator Circuit
The resistor matrix is best checked by performing the accuracy check outlined in Paragraph
4.3 of this chapter.
Malfunction of the comparator is indicated (but not confirmed) if the DVM, with no input con-
nected, displays a maximum readout of +1.1999. Also if the DVM registers all zeros with a
signal input, the comparator could be at fault. A suspected comparator malfunction can be
verified by monitoring pin 47 of connector A8 with a scope; compare to waveform (i) of Fig-
ure 3.4,
4.2.5 Digitizing Circuit
al readout indicator serves as one of the best trouble indicators for checks of the
;eircuitry, Accordingly, the troubleshooting procedures of this sub-paragraph are
with respect to possible indicator cienleye, If the DVM is removed from the compu-
should be used to reconnect J1 and J2 to the computer mating rece as
rn display (assum-
88 possible trouble sources) usu-
CHAPTER 4 MAINTENANCE
ally indicates that the digitizing cycle has stopped. The loss of the master clock pulses (12-
18 ke clock) will stop the digitizing cycle. Check for the clock pulses at pin F of connector A5.
If one or more of the lower order display readouts are at zero, the counting cycle probably has
stopped in the lowest order decade that is displaying a digit higher than zero. The suspected
decade counter can be checked by interchanging it with the next lower order counter. If the
count again stops at the suspected card, the fault is isolated to that card. Ali of the counters are
interchangeable including the Model 38,032-1 1000's counter. However, as previously noted,
if the 1000's counter is repositioned during trouble analysis it should be returned to the 1000's
position, The DVM will function with this card in any position, but, since it normally drives
two lamps, it should be returned to the 1000's position to insure longer lamp life.
4.2.5.2 Indicator Displaying Continually-Changing Random Digits. This display
indicates the digitizing circuits are functioning but trouble may exist in the input circuits.
4.2.5.3 Two Digits Displayed Simultaneously on a Single Indicator. This display
usually indicates a faulty diode in the BCD to decimal conversion matrix, The faulty diode can
be isolated by noting the particular combination of digits.
NOTE
Noise on the input signal may cause one or
more of the lowest order displays to flicker
between digits.
4.3 ACCURACY CHECK
Due to the high accuracy of the DVM, a complete calibration procedure is necessarily an exact-
ing process. It is the purpose of this paragraph to provide an accuracy check to determine
which, if any, of the calibration adjustments are necessary to bring the DVM within the re-
quired specification. It is suggested that all results be double checked before attempting the
indicated calibration adjustments,
Allow a tn 2 oe warm-up Period of all bp oabizment to assure DVM circuit and test equipment
4-5
CHAPTER 4 MAINTENANCE
Table 4,2. Accuracy Check Inputs, First Test
Summing Resistor
Input Level DVM Readout Gates Checked
-9.999 VDC -0,9997 to -1.0001 8 and 1
-7.717 VDC -0,.7775 to -0..7779 4, 2 and 1
-6. 666 VDC -0, 6664 to -0. 6668 4 and 2
-5.555 VDC -0. 5553 to -0, 5557 4 and 1
-3.333 VDC -0. 3331 to -0, 3335 2 and 1
If certain combinations are in error, one particular summing resistor gate adjustment may
seem to be indicated as faulty, However, this should not be considered as conclusive since
two gates may be in error and their errors may cancel when used in combination with each
other. If erroneous readouts are obtained, apply the inputs listed in Table 4.3 and check for
the indicated readout,
Table 4.3. Accuracy Check Inputs, Second Test
Summing Resistor
Input Level* DVM Readout Gates Checked
-8.003 VDC -0, 8001 to -0. 8005 8000
-4,008 VDC -0, 4001 to -0. 4005 4000
-2,003 VDC -0, 2001 to -0, 2005 2000
-1.003 VDC -0, 1001 to -0. 1005 1000
-0,803 VDC -0, 0801 to -0, 0805 800
If all the gates are in error, with the 8000's gate error largest and the error progressively de-
creasing (the 800's gate error the smallest), the full scale setting may need adjustment, or, the
-100 volt reference may be out of tolerance. Check these adjustments as outlined in Chapter 5
(it is best to check reference first since adjustment of the full scale control interacts with the
are in error adjust these gates as outlined in Chapter 5,
iillivolt digit) should be 3 millivolts + the % eryoy
7” of the de source;
2, and 1 and the 100's 8 gates conduct. ee ae
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CHAPTER 4
NOTE
After making any adjustment to the DVM repeat
the check-out procedure starting with Table 4. 2.
If both checks are within specifications proceed as follows:
1. Apply an input of -10. 000 volts and note the readout (should be between -0, 9998
and -1,0002).
2. Apply an input of +10.000 volts. The readout should be between +0, 9998 and
+1.0002 units plus any error between the two voltage sources (if separate positive
and negative supplies are used) in addition to the basic full scale error of the power
sources. (If the positive readout is in error the de gain control may require adjust-
ment. )
If all of the voltages are read out within the tolerances specified, the DVM is with-
in the required accuracy specifications and no further check is required nor is cali-
bration needed.
4,4 DISPLAY INDICATORS
Only two troubles can occur directly involving the display unit. The numerals and symbols are
displayed by a multiple lens rear projection arrangement. The light source for each numeral
or symbol is an individual incandescent lamp. Since the lamps are operated at less than rated
voltage, filament life is exceptionally long under normal conditions.
4.4.1 Display Unit Checks
The display unit is best checked by applying a voltage to the DVM through a potentiometer, and
increasing the voltage slowly while observing the display. If any digit fails to light, either the
lamp is faulty or the associated lamp driver transistor is faulty. The easiest way to check is
to substitute a lamp bulb known to be good in the suspected position. If the trouble remains,
the associated lamp driver transistor should be checked.
4.4.1.1 Lamp Replacement. To replace an indicator lamp, proceed as follows:
1. Remove the left-hand side panel of the computer so that the rear of the
display unit is exposed.
MAINTENANCE
4-7
—
CHAPTER 4 MAINTENANCE
2. Apply an input to the DVM which causes an adjacent decade indica-
tor to display the same numeral as that which has failed.
3. Observe the rear of the display unit, and note the position of the
illuminated lamp on the adjacent indicator. Remove the bulb in the
corresponding position of the faulty decade indicator. Visual obser-
vation or an ohmmeter continuity check of the suspected bulb may
be used to determine whether bulb replacement is required or if the
driver transistor should be suspected.
4,4,1.2 Lamp Adjustment. Since a multiple lens optical system is used to focus
a symbol on the front screen of the display unit, proper positioning of the bulb filament is im-
portant to assure clarity of the associated symbol. If a digit or symbol appears blurred or fuz-
zy (and the DVM appears to be operating correctly) , proceed as follows:
1. Apply an input to the DVM which causes the blurred symbol to be
illuminated.
2. Remove the illuminated lamp in the position with the blurred sym-
bol, rotate the bulb 180° (the bulbs have a bayonet base), and re-
place it. If this procedure fails to correct the trouble, install a
new bulb.
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CHAPTER 5
CALIBRATION
5.1 INTRODUCTION
This chapter contains instructions for a complete calibration of the DVM. It should be noted
that calibration of an instrument with the accuracy of the DVM is an exacting procedure re-
quiring precision equipment and extremely stable voltage sources not normally available in
field installations (to obtain +0. 01% of full scale +1 digit accuracy). Unless such equipment
is available, EAI recommends that the DVM be returned to the factory if re-calibration becomes
necessary.
Prior to attempting the calibration check, the accuracy check outlined in Chapter 4 should be
performed, The check performs a twofold function; it ascertains that calibration is necessary
and it localizes the exact calibration adjustment required. This last feature of the check per-
mits calibration of certain circuits without having to perform the entire calibration procedure.
5.2 REQUIRED EQUIPMENT
The equipment listed in Table 5.1 is the type recommended to calibrate the DVM to 0, 01% of
full scale +1 digit.
5.3 CALIBRATION PROCEDURES
Allow a one-hour warm up period for the DVM
andall test equipment to assure circuit stabili-
zation before proceeding. Ascertainthatthe +10
and -10 volt computer reference supplies are
carefully balanced before attempting any adjust-
ments.
5.3.1 Reference Level Adjustment
1. Balance the reference ampli-
fier as outlined in Chapter 4,
Set up the test circuit illu-
strated in Figure 5,1, (The
setting of the voltage divider
is 10 to 1.)
10 10:1 DIVIDER
(JULIE VDR 106
l OR EQUAL)
—& J2-3-)
G |
+
— (l!0 VOLT
i REFERENCE
Figure 5.1. Reference Level Adjustment
Test Setup
CHAPTER 5
CALIBRATION
Table 5.1. Recommended Test Equipment
Component Recommended Type Used For
Precision Voltage Divider Julie VDR 106 (Or Equal) 1. Reference Adjustment
(6-Place) 2. DC Gain Adjustment
3, Summing Network
Trimming Adjustment
Galvanometer (Capable Hewlett-Packard, Model 1. Reference Adjustment
of Detecting 1/10 Micro- 425A (Or Equal) 2. DC Gain Adjustment
volt or Better 3, Summing Network
Trimming Adjustment
Secondary Standard Volt Epley MIN II Calibrated Reference Adjustment
Cell (Calibrated at Against a Primary
0. 002%) Standard (Or Equal)
Two 30,000 Ohm, Ten- Helipot, Model 30, 000-Al Summing Network Trimming
Turn Potentiometers (Or Equal) Adjustment
Stable 10 Volt DC Source See Procedure in test for 1. DC Gain Adjustment
calibrating a stable 10 2. Summing Network
volt source to 0.005%. Trimming Adjustment
NOTE
Tt is assumed that all other power supply
levels have been checked and meet the
specifications listed in Chapter 1.
3, Adjust R29 and R30 on the 6. 736 Unit (Figure 5. 2) until a null is obtained on
the galvanometer. (This procedure sets the reference level at -100 vde
0.005% with respect to the +10. 000 volt reference.)
5.3.2 Test Reference Calibration (-10 VDC)
provides a -10 vde +0. 005% test reference source which may be used
ence for the full scale adjustment, summing resistor trim pot adjust-
adjustment. (This source may also be used for de gain adjust-
ur 3.3. (The setting of the voltage
CALIBRATION
CHAPTER 5
FULL SCALE
ADJ (R8)
LEVEL
AMPLIFIER A
waaT) BALANCE
(R10)
+20 VOLT
LEVEL ADJ
aD DC GAIN ADJ
(R5)
—20 VOLT UNLOADING ADJ
LEVEL ADs (R4)
(R11)
+40 VOLT
LEVEL ADJ AMPLIFIER B
(R23) N AT
$ R
(a) Controls Accessible from Top
/=H0V ADJ (R7) — 8000 TRIMMING
nee ADJUSTMENT
(R9p)
4000 TRIMMING
ADJUSTMENT
(Rip)
2000 TRIMMING
ADJUSTMENT
(RIi3p)
1000 TRIMMING
ADJUSTMENT
(RI5p)
800 TRIMMING
- ADJUSTMENT
{RIOp)
REFERENCE
_ BALANCE —
ADJ (R9)
Neel ed
(6) Controls Accessible from Bottom
Figure 5.2. Calibration Controls Location
CHAPTER 5 CALIBRATION
© a ~10 VOLT
STABLE ¥ TEST REFERENCE
DC
+
source 3 STANDARD
jie CELL JULIE
2
= ro VOR 106
All (OR EQUAL)
-
Figure 5.3. -10 Volt Test Reference Calibration Setup
Adjust the stable de source output for a null on the galvanometer. This sets
the test reference level at -10 vdc +0. 005%,
5.3.3 Full Scale Adjustment
The full scale adjustment can appear to correct summing resistor gate errors; likewise, ad-
justment of the summing resistor gates may appear to correct a misadjustment of full scale.
An indication of full scale misadjustment, therefore, should be verified by the accuracy check
procedure outlined in Chapter 4, Paragraph 4.3. Also for this reason, the initial control
position, as indicated in the following procedure, should be marked. Thus the full scale ad-
justment can be returned to its original position if the final check of this procedure indicates
that full scale adjustment was not the cause of the error.
1,
4,
NOTE
Before attempting to adjust the full scale
adjustment check the -100 volt reference
level as outlined in Paragraph 5.3.1 above.
Check the amplifier balance as outlined in Chapter 4,
Set up the test circuit shown in Figure 5.4, (The -10 voit test reference ig
obtained from the computer or as described in Paragraph 5.3.2 abov e.)
Zero the DVM indicators as outlined in Chapter 2, Paragraph 2.1.1
Set the voltage divider for 9, 998 volts; adjust the unloading potentiometers
for a galvanometer null, Note and carefully mark the present
Position of the
full scale control (Figure 5.2, R8 on the 12, 937-5 Network, )
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CHAPTER 5
—l0V COMPUTER
CALIBRATION
<2
i
OR
TEST REFERENCE
VOLTAGE
DIVIDER
JULIE VOR 106
(OR EQUAL)
30K 30K
TO COMPUTER
Lom
Table 5.2.
vy
(OR DVM) GROUND
TO DVM INPUT
vy
Figure 5.4. Test Setup for Full Scale and Summing Resistor Trim Pot Adjustment
5. Adjust the full scale control for a DVM readout of -0. 9998.
6. Once the full scale adjustment has been completed check that the DVM makes
a smooth counting transition through each of the trim gates as indicated in
Table 5.2. DVM Counting Transition Check
Apply Input For DVM Increase Input For Further Increase Input For
Readout Of Readout Of Readout Of
-. 7998 -. 7999 -. 8000
-. 3998 -. 3999 -. 4000
-. 1998 -. 1999 -. 2000
-. 0998 -. 0999 -. 1000
-. 0798 -. 0799 -. 0800
-. 0398 -, 0399 -. 0400
ee dat
If the unit does not make a smooth counting transition, (for 6xample if the DVM switches from
-. 7998 unit to -. 8000 unit thus skipping -.7999) it indicates a summing resistor trim pot is
improperly set. Recheck all of the gates as outlined in the accuracy check in Chapter 4. If it
is found that all of the gates are out, reset the full scale control to the recorded initial position
and repeat the accuracy check to isolate the faulty summing resistor trimming adjustment(s).
CHAPTER 5
is
2.
2.
_ Vary input to ascertain that the unit counts Smoothly from -
Apply an input of -4, 0015 volts to the DVM. (Check
for a null condition. )
|
|
alll aT PP
CALIBRATION
5.3.4 Summing Resistor Gate Trim Pot Adjustment
Check the DVM zero adjustment as outlined in Chapter 4, Use the test setup of Figure 5.4
for the following adjustments,
is in error it is only necessary to adjust that particular gate. (Calibration of the -100 volt
reference should be checked before adjusting any gates.) The location of the individual trim
pots is shown in Figure 5, 2,
If the accuracy check in Chapter 4 indicates a specific gate
5.3.4.1 8000 Trim Pot Adjustment
Set up the Figure 5. 4 test circuit, check the zero adjustment.
Apply an input of -8.0015 volts to the DVM. (Check galvan-
ometer for a null condition. )
Adjust R9p on the 26.116-1 Summing Resistor Network (NW2)
until the readout flickers between -0, 8001 and -0. 8002*,
Re-check zero; if zero requires resetting repeat Steps 1 through 3,
- 7998
to -.8000. If not repeat Steps 1 through 4,
If this is the last trim pot adjustment to be made,
repeat accuracy
check of Chapter 4,
5.3.4.2 4000 Trim Pot Adjustment
1, Set up the Figure 5, 4 test circuit; check the zero adjustment,
galvanometer
in, : Resistor Network (NW2)
- 4001 and -0, 400g,
CHAPTER 5 CALIBRATION
5, Vary input to ascertain that the unit counts smoothly from -, 3998
to .4000. If not repeat Steps 1 through 4.
6. If this is the last trim pot adjustment to be made, repeat the
accuracy check of Chapter 4,
5.3.4.3 2000 Trim Pot Adjustment
Set up the Figure 5.4 test circuit; check the zero adjustment.
2. Apply an input of -2.0015 volts to the DVM. (Check galvanometer
for a null condition. )
3. Adjust R13p on the 26. 116-1 Summing Resistor Network (NW2)
until the readout flickers between -0. 2001 and -0. 2002*.
4. Re-check zero; if zero requires resetting repeat Steps 1 through 3.
5. Vary input to ascertain that the unit counts smoothly from -. 1998
to -. 2000. If not repeat Steps 1 through 4.
cd dd dd Ld ddd
6. If this is the last trim pot adjustment to be made, repeat the
accuracy check of Chapter 4.
5.3.4.4 1000 Trim Pot Adjustment
1, Set up the Figure 5.4 test circuit; check the zero adjustment.
2. Apply an input of -1,0015 volts to the DVM. (Check galvanometer
for a null condition. )
3, Adjust R15p on the 26, 116-1 Summing Resistor Network (NW2)
until the readout flickers between -0. 1001 and -0.1002*,
4. Re-check zero; if zero requires resetting repeat Steps 1 through 3.
Vary input to ascertain that the unit counts smoothly from -, 0998
CHAPTER 5 : CALIBRATION
6. If this is the last trim pot adjustment to be made, repeat the
accuracy check of Chapter 4.
5.3.4.5 800 Trim Pot Adjustment
1. Set up the Figure 5, 4 test circuit; check the zero adjustment.
2. Apply an input of -0. 8015 volts to the DVM. (Check galvanometer
for a null condition. )
3. Adjust R10p on the 26. 116-1 Summing Resistor Network (NW2)
until the readout flickers between -0. 0801 and -0. 0802*.
4. Re-check zero; if zero required resetting repeat Steps 1 through 3.
5. Vary input to ascertain that the unit counts smoothly from -. 0798
to -.0800. If not repeat Steps 1 through 4.
6. Repeat the accuracy check of Chapter 4.
5.3.5 DC Gain Adjustment (Figure 5. 2)
Prior to making this adjustment ascertain that the computer reference supplies are balanced.
1. Apply the -10 volt computer reference to the DVM input and note the read-
out display.
2. Apply the +10 volt computer reference to the DVM input. If the readout is
not the same as Step 1, adjust R5 on the 12. 937-5 Network (NW3).
3, Repeat Steps 1 and 2 if adjustment is necessary,
5.3.6 Amplifier Unloading Adjustment (Figure 5. 2)
1. Set up the test circuit shown in Figure 5.5,
2. Place the switch in position A. The DVM should read -1,0000 unit
*Approach all settings from the high side.
. 2-8
2 8 8.
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NN Ne lM lM poll oll pe
CHAPTER 5 CALIBRATION
3. Place the switch in position B, The DVM should read > -0,9980. (Indi-
cating a current flow of < 0.02 microamperes. )
4, If readout is less than -0. 9980 volts, adjust R4 on the 12, 937-1 Network
(NW8) for a readout greater than or equal to -0.9980 unit. This provides
the DVM with an input impedance > 500 megohms at full scale.
B
mee ae —> TO DVM INPUT
—10V COMPUTER 4 A
REFERENCE
Figure 5.5. DVM Unloading Adjustment Test Setup