MKS 270C User manual

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195-111664A-1/89
!IL • MKS
Instruction
Manual
I
Ii
I
TYPE 270C
HIGH ACCURACY
SIGNAL CONDITIONER
SIX SHATTUCK ROAD. ANDOVER, MASSACHUSETTS 01810 x508) 975-2350 TELEX 910-240-9005 FAX [5081975-0093

TABLI OF CONT' NTS
ct ion gal Descr otio
Section nsta at
Section Operation 3-
Section Theory of Operatio
Sectio Maintenance and Troubleshooting 5-1
Section Parts Lists
LIST OF F GORE.
Fig ire Interconnection Cables 2-i
Fig ont eI Control 3-3
Fig C Rear Panel Controls 3-3
Fi ire D270 Block Diagra 4.2
Fig ire Schematic, Mai Electronics, Sht. 6-1
Fig ire Schematic Mai Electronics, Sht 6-
Fig ire GSchematic, Digital DVM nterface 6-3

MODEL DIFFERENCES 270B TO 270C
The 270B has been replaced by the 270C. The only differences between the two units is found in the power
input components. The changes are:
1. The power line input connector, line voltage selector switch and fuse holder have been replaced by a
single Power Entry Module which is UL/CSA and VDE approved.
2. The power transformer has been redesigned to VDE specifications.
3. The power switch has been replaced by a VDE approved type.
These are the only changes between the two models. The 270C specifications are identical to those of
the 270B. Likewise all connector inputs/outputs are identical.
INSTALLATION AND OPERATING INSTRUCTIONS
FOR
200 SERIES TYPE 270 HIGH ACCURACY
SIGNAL CONDITIONER
Section 1 GENERAL DESCRIPTION
The 270 is a complete signal conditioner and 41/2 or 51/2 digit display, packaged in the standard MKS 1/2 rack
mount unit. This unit contains the power supplies, oscillator and heater supply necessary to interface with MKS
300 Series Pressure Sensors. A pressure units display switch provides for a front panel display in various pressure
units. With the 300 Series Pressure Sensor, this unit forms a high accuracy pressure measurement system.
The 270 can be combined with the 274 multiplex unit to provide pressure measurement for up to three 300 series
sensors. Also a complete pressure/flow system can be created by adding a series controller and valve. The con-
troller interfaces to the 270 through the REMOTE and ACCESSORY connectors.
Units that contain option boards will be shipped with additional manuals that reference those options.
1-1

SPECIFICATIONS
Input:
Ranges:
Output Voltage:
Display:
Displayed Pressure
Units:
BCD Outputs:
Package:
Weight:
Accepts signals from 398, 390, 370, 310, 315, and 317 pressure sensors, 25K to 1
mmHg.
X1, X0.1 and X0.01 of full range of the pressure sensor.
0to +10V into 10K ohms or greater, available at rear connectors.
41/2 Digit in 270-4 51/2 Digit in 270-5, 7 segment red LED's, 0.5 inches high.
in Hg, in H2O, mmHg, mbars, KPa, CmH2O and psi* (*psi not available on 270-5)
Pressure information is parallel line 3 state logic. Decimal points, channel ID and range
ID outputs are buffered TTL levels. All outputs can drive 1 TTL load.
1/2 rack size cabinet
8lbs.
Operating Temperature: 15 to 40°C
Operating Voltage:
Power Consumption:
Output Impedance:
Output Zero Drift:
Output Linearity:
Output Noise:
Display Accuracy:
Display Update Rate:
100-135/200-270VAC 50/60 Hz.
40 watts @ 115VAC (21 watts without heater)
Less than 1 ohm
Less than 0.05 my/°C
±(0.005% of RD +0.001% of FS)
Low frequency, less than 70 microvolts all ranges @ 0.4 Hz bandwidth
High frequency, 4 My peak-to-peak (1 KHz - 1 MHz)
41/2 Digit ±0.01% of RD ±1 Count
51/2 Digit ± 0.001% of RD ± 1 Count
2 samples/sec for both displays
Specifications subject to change without notice.
1-2

Section 2 INSTALLATION AND OPERATION
INSTALLATION
THE 270 MUST BE MOUNTED IN SUCH A MANNER AS TO PROVIDE ADEQUATE AIR
CIRCULATION ABOUT THE UNIT!!
The 270 can be mounted in a panel cutout or a 19" rack when supplied with the RM-6 Rack Mount option.
FIGURE A shows the cables necessary to connect between the 270 and other instruments.
111
N..
270 200 CONTROLLER
() BCD
HEAD ACCESS.(.
REMOTE
()
CB-254-6
CB-270-4
CB-270-2-3
FIGURE A
INTERCONNECTION CABLES
2-1
()

INTERCONNECTION DATA
Pin No. Pin No.
1. DC Output Voltage 8. DC Output Return
2. N.C. 9. N.C.
3. Digital Common 10. N.C.
4. X1 Range ID 11. N.C.
5. X0.1 Range ID 12. N.C.
6. X0.01 Range ID 13. N.C.
7. N.C. 14. Chassis Ground
Pin No. Pin No.
1. N.C. 14. N.C.
2. 10K 15. N.C.
3. 1K 16. N.C.
4. 100 17. N.C.
5. 10 18. N.C.
6. 119. N.C.
7. N.C. 20. N.C.
8. CH.1 ID Input 21. Remote Read
9. CH.2 ID Input 22. Digital Ground
10. CH.3 ID Input 23. Digital Ground
11. Multiplex Select 24. N.C.
12. N.C. 25. Chassis Ground
13. N.C.
Pin No. Pin No.
1. DC Output Voltage 6. Chassis Ground
2. N.C. 7. N.C.
3. Select X0.1 Range 8. DC output Return
4. Select X0.01 Range 9. N.C.
5. Digital Common
J401
ACCESSORY CONNECTOR
J405
MULTIPLEX INTERFACE CONNECTOR
J402
REMOTE INTERFACE CONNECTOR
2-2

1M
1. 8000 18. +5V 025 mA 35. 40
2. 2000 19. HOLD 36. 10
3. 80000 20.. Good Data* 37. 4
4. 20000 21. Decimal +4P 38. 1
5. Data Ready 22. Decimal 4-3P 39. N.C.
6. Overrange 23. Decimal +2P 40. N.C.
7. Digital Ground 24. Decimal -4P 41. N.C.
8. 800 25. CH.3 ID 42. N.C.
9. 200 26. 4000 43. CH.1 ID
10. 80 27. 1000 44. Decimal + 1P
11. 20 28. 40000 45. Decimal OP
12. 829. 10000 46. Decimal -3P
13. 230. Polarity (+) 47. Decimal -2P
14. N.C. 31. 100000 48. Decimal - 1P
15. N.C. 32. Enable BCD 49. +5V Return
16. N.C. 33. 400 50. CH.2 ID
17. N.C. 34. 100
•
1. N.C.
2. N.C.
3. Digital Ground
4. Digital Ground
5. 10000
6. N.C.
7. Enable Digit 2
8. 1
9. N.C.
10. 100
11. 200
12. 40
13. 400
14. 20
15. 1000
16. 10
17. Enable Digit 1
*Used only by MKS.
270-4
BCD OUTPUT CONNECTOR
(41/2 DIGIT DPM)
18. +5V 025 mA
19. HOLD
20. Good Data*
21. Decimal +4P
22. Decimal +3P
23. Decimal +2P
24. Decimal -4P
25. CH.3 ID
26. Overrange
27. Polarity ( + )
28. Digital Ground
29. N.C.
30. Data Ready
31. Enable Digit 5
32. Enable Digit 4
33. 8000
34. Enable Remainder
270-5
BCD OUTPUT CONNECTOR
(51/2 DIGIT DPM)
35. 80
36. 2
37. 4000
38. 4
39. 2000
40. 8
41. Enable Digit 3
42. 800
43. CH.1 ID
44. Decimal +1P
45. Decimal OP
46. Decimal -3P
47. Decimal -2P
48. Decimal - 1P
49. +5 Volt Ground
50. CH.2 ID
*Used only by MKS.
XXXXXX When used with inputs: This indicates that the line must be pulled low.
XXXXXX When used with outputs: This indicates that the output is active low.
2-3

REMOTE METER HOLD
This line is brought out for remote reading control of the meter. Left open, the meter will free run at a rate of 2.5
samples/second. Holding this line low prohibits the meter from reading. To command the meter to read, generate
apositive going pulse with a duration of 50 micro sec. minimum to 2 msec. maximum.
DATA READY
This line goes high when the data is valid and ready to read.
DECIMAL INFORMATION
This information is presented as the exponent of 10, example: The decimal point for a Full Scale reading of 1000.0
is +3P where the bar indicates a negative true. Likewise the decimal for .10000 is displayed as —1P.
ENABLE LINES
The BCD information is controlled by enable lines. The 41/2 digit dpm has six enable lines while the 51/2 digit dpm
has only one. When these lines are connected to ground all of the BCD information is present at the rear connector
and can be read when the DATA READY line goes high. When these lines are left open or connected to +5V then
all of the BCD outputs are open. This is done when several meters are connected in parallel to a common bus.
Each meter is strobed for a reading by grounding these control lines and then when the reading has been taken
the lines are forced high again and the outputs are open.
IMPORTANT!! CHANNEL ID AND DECIMAL POINT INFORMATION IS NOT CONTROLLED BY
THESE ENABLE LINES. THIS DATA WILL BE ACTIVE AT ALL TIMES.
OVERRANGE OUTPUT ON THE 51/2 DIGIT DPM
The panel meter used by MKS overranges at 130000 counts. This would normally cause it to flash on and off. Since
this would not allow the proper full scale reading in mbars, Kpa or cmH20 the flashing feature has been disabled.
The meter will continue to read correctly until it reaches approximately 138000 counts. When it reaches its max-
imum count it will simply stay locked at that reading. Because of this you must be careful of readings greater than
Full Scale on these ranges. Also the BCD OVERRANGE signal WILL BECOME ACTIVE AT 130000 counts.
2-4

Section 3 OPERATION
FRONT PANEL CONTROLS (See Figure B)
RANGE SELECT (A)
This switch controls the measurement mode of the instrument. The Remote, X1, X0.1 and X0.01 positions are active
measurement ranges while the System Check, Null and Full Scale are functional check positions.
NULL (B)
When the range switch is placed in the NULL position, the input to the 270 is placed at ground level. This control
is then adjusted to produce a zero reading at the DC output. (Signal Conditioner Null)
FULL SCALE (C)
When the Range Multiplier switch is placed in the Full Scale position, the input to the 270 is connected to an internal
calibration signal. This control is then adjusted to produce a Full Scale (+10V) at the DC output. (Signal Conditioner
Full Scale)
SENSOR ZERO-COARSE (D)
This control is used in conjunction with the Sensor Coarse Zero to provide an approximate zero setting.
SENSOR ZERO-FINE (E)
This control is used in conjunction with the 270 Coarse Zero to provide high resolution in the zero setting.
HEATER (F)
This lamp comes on when power is being applied to the heaters.
REG/OFF (G)
This switch controls power to the heaters. When placed in the REG. position, power is applied to the heaters in
the sensor to maintain a preset temperature. This minimizes zero and span drift due to ambient temperature varia-
tion. This switch is a locking type and the handle must be PULLED BACK to change positions.
RESPONSE (H)
This switch controls the electrical bandwidth of the instrument. The three positions are:
FAST165HZ1mSEC
NORMAL4Hz40 mSEC
SLOW 0 4Hz400 mSEC
ON/OFF (I)
This switch controls power to the 270.
SENSOR RANGE SELECTOR (J)
When switched to the range of the sensor in use, this switch combines the sensor range, display units and the
multiplier level of the 270 to properly position the decimal point.
3-1

DISPLAY UNITS SELECTOR (K)
This switch selects the type of units used to display pressure on the front panel display.
X10-3 LAMP (L)
Since it is not possible to display the decimal point to the left of the MSD, a Full Scale reading of +.10000 is displayed
as +100.00 and the X10-3 lamp is illuminated.
X10-6 LAMP (M)
Similar function as the X10-3 lamp. A Full Scale reading of +.10000 X10-3 would be displayed as +100.00 and the
X10-6 lamp is illuminated.
DIGITAL READOUT (N)
Displays the pressure signals in the pressure units selected by the display unit selector.
REAR PANEL CONTROLS (See Figure C.)
REMOTE (A)
Provides connection to range selection circuits when used with a controller or other external device to range change.
The three ranges (1, 0.1, 0.01) along with a 0.10VDC output on each range is available through this connector.
ACCESSORY (B)
Interfaces with accessory units. Provides 0-10VDC output and range ID.
INPUT (C)
Sensor cable or other sensor conditioning cable is inserted in this connector to receive excitation, heater power, etc.
POWER ENTRY MODULE (D)
This module contains: 1. RFI power line filter.
2. IEC power line connector.
3. Line voltage selector.
4. Power line fuses (2).
The fuses and line voltage selector drum are accessed by opening the cover with a small screwdriver.
The module is matched to the line voltage by removing the selector drum, turning it to the proper position, and
re-inserting it. The value of the selected voltage will be visible through the window in the cover when it is closed.
The fuses are contained in fuse carriers. These carriers are removed by inserting a small screwdriver blade behind
the cover tabs and pulling them straight out.
The fuse values are marked on the rear panel. They are:
100/120 Vac = 0.6 Amperes Slow Blow Type
220/240 Vac = 0.3 Amperes Slow Blow Type
BCD CONNECTOR (E)
This connector provides BCD and decimal point outputs. See Section 2 for the pin assignments.
MULTIPLEX CONNECTOR (F)
This connector provides access to the logic circuits of the meter. When the SENSOR RANGE switch is in the MPX
position, the multiplex unit controls the placement of the sensor range and channel ID through this connector.

FIGURE B
FRONT PANEL CONTROLS
0
lc*
a
•
REMOTE
ZERO
CD
OUT OF
RANGE
ENABLE 0 D
ON
OFF
PWR
PRESSURE
n1-1 1-1 1-1
LL L1 1_1 LI 1_1
REG
OFF
HEATER
FAST.R-1 STD
SLOW
RESPONSE
O
010.1 xla TYPE 270
sacova CONDITIONER
11( 100
-1
OAPS
mmHg Rolm
PHI°, 1 / APB
a"S°
SENSOR RANGE DISPLAY UNITS
REW)TE XI
MOW / xl
\1 / /
- X 01
O
NULL ES
RANGE SELECT
0O
FIGURE C
REAR PANEL CONTROLS
LSE: 11011 613/S0
(SO VA MAX.)
NAM: 100/100 • OA ASS/SSOV
210/240 • 0.3 ASS/100Y
3-3
O

OPERATION
1. Connect the 300 Series Pressure Sensor to the 270 with the appropriate sensor cable. Should there be
any more accessories they are wired in as outlined in Figure A.
2. Plug the power cable from the 270 into a 115 VAC-60Hz power source. For 230-50Hz sources, the power
entry module, Figure C-item D, must be set to the proper voltage.
3. Apply power by turning on the Power switch. (Figure B-item I)
4. Place the Sensor Range switch, Figure B-item J, to the full scale value of the pressure sensor in use.
5. Place the Display Units switch, Figure B-item K, to the mmHg position.
6. Place the Heater switch, Figure B-item G, to the REG position. This applies power to the proportional heater
circuit in the sensor. There will be a small delay between the closing of this switch and the lighting of the
HEATER lamp, Figure B-item F. The lamp should come on brightly, go out and then come on and remain
on at a lower intensity. Should the system contain a 274 multiplex unit, the heater supply is included in that
unit and the REG switch on the 270 must be placed in the OFF position. Allow a minimum of 4 HOURS
to stabilize a "cold" sensor. For sensors that have just been received, it is recommended that they be left
on heat overnight. To achieve the best possible accuracy the heater should always be on.
7. Place the Response switch, Figure B-item H, to the STD position. This provides the best all around response
vs. noise for pressure measurement. When using a series 200 controller, use the FAST position. The SLOW
position is used when low noise on the DC output is more important than fast response.
8. Place the Range SELECT switch, Figure B-item A, to the NULL position. If necessary, adjust the Null Con-
trol, Figure B-item, to produce a zero reading on the Digital Display. (Figure B-item N)
9. Place the Range SELECT to the F.S. position. If necessary, adjust the F.S. control, Figure B-item C, to pro-
duce a full scale reading of +10000* on the panel meter. *(100000 on a 270B-5)
10. Place the Range SELECT to the X0.01 position. Pump the pressure sensor below its resolution (or cross-
port differential units) and check the zero. Correct, if necessary, with the Coarse and Fine zero controls,
Figure B-item D&E. Should these controls lack the range for this adjustment, refer to the Zero and Span
adjustment section in the MKS Instruction Manual for 170 Series Sensors.
NOTE: SHOULD A MULTIPLEX UNIT BE IN USE, THE ABOVE CONTROLS ARE BYPASSED AND
THE ZERO CORRECTION IS DONE AT THE MULTIPLEX UNIT.
11. Place the Range SELECT in the System Check position and compare this reading with the system check
number on the sensor calibration sheet. If the reading is incorrect, refer to the maintenance section.
12. Place the Range SELECT in the X1 position. The system is now ready to display the pressure range of
the sensor. For 1/10 of that range, use the X0.1 position, for 1/100 use the X0.01 position.

•
RANGE TO BE SELECTED REMOTE CONNECTOR
J402-3 J402-4
1HI HI
0.1 LO HI
0.01 HI LO
REMOTE RANGE SELECTION
114
NC•
Iq
ray
10
it
le •
.,
The X1, X0.1 & X0.01 ranges can be remotely selected by placing the Range Select in the Remote position and
applying the proper logic levels to the Remote interface connector.
3-5

Section 4 THEORY OF OPERATION
The first portion of this section will deal with the signal flow through the instrument while later portions will cover
the operation of specific circuits in more detail.
Referring to Figure D, it is seen that the 270 interfaces with the pressure sensor via the sensor connector. This
connector also provides the preamp voltage, heater voltage and oscillator voltage to run the pressure sensor.
The pressure signal is applied to the zero circuit when it comes directly from the pressure sensor. Should it
come from the multiplex unit, the zero circuit is bypassed so that the zero controls in the multiplex unit override
the zero controls on the 270.
The output from the zero circuit is applied to the Input Selector switch. This switch connects the input of the
AC amplifiers to the pressure signal or to the internal cal signals. Another pole of this switch selects the gain
of the AC amplifiers.
The Input Selector switch applies the signal to the 1st AC amplifier where it is amplified and passed on to the
divider circuit and 2nd AC amplifier. The divider circuit controls the amplitude of the AC output signal by com-
mand of the range select circuit which in turn is controlled by the Input Selector switch.
The output of the 2nd AC amplifier is applied to the demodulator transformer which interfaces it to the demodulator
circuit. The demodulator fullwave rectifies the AC output and applies it to the DC amplifier. This demodulator
is synchronized to the oscillator to block quadrature signals.
The DC amplifier acts as an integrator and filters the input signal to produce a smooth DC output signal. This
circuit also has a Response Speed Selector switch which selects one of three possible bandwidths for this amplifier.
The signal from the DC amplifier goes to the Remote and Accessory connectors and is applied to the panel
meter via the Display Units Divider circuit. The panel meter is interfaced with a Digital Interface circuit which
converts the setting of the Input Selector switch, Display Units switch and the Sensor Range Selector switch
into the proper decimal point selection. The decimal point information plus the BCD output from the panel meter
is then applied to the BCD output connector.
Power for the heater and voltage for the electronics is supplied by the main power supply circuits. These sup-
plies also provide power for the oscillator which produces a highly regulated 10KHz sinewave output. This voltage
is used for excitation of the Pressure Sensor and for internal calibration signals as well as synchronization of
the demodulator. A more detailed circuit description follows.
—13 VOLT SUPPLY: See figure E - Section A
Both power supplies are identical floating 13 volt power supplies. The negative terminal of one is connected
to the positive terminal of the other and grounded. This produces the required +13v and —13V at the other
remaining terminals.
The regulations are the three terminal adjustable type. An adjustment range of approximately 1V is obtained
from potentionmeters R19, R22. These regulators provide current limit and thermal overload protection. The
rectifiers CR4, CR5 protect against voltage reversal.
4-1

,1-:15 iriii 1111-2-11-11- )11--a
••
Pressure
Signal
(Sensor)
Pressure
Signal
(mux)
Oscil-
lator
Sensor
Connector
Zero
Circuit .•••••••110.1
Input
Selector
Switch
System
Check
Pre Amp
Filter
1st
AC amp
Divider
Circuit
Heater
Controls
+13v
—13v +13
Ana Gnd —13
+5v Gnd
Digital Gnd
+40v
—40v
CAC and
Zero
XFMR
Range
Selection
Circuit
2nd
AC amp
Demodu-
lator
10 KHz
Oscil-
lator
Multiplex
Connector
Digital
Interface
Circuits
Display
Units Switch
and Divider
Circuit
Panel
Meter
DC
amp
Response
Time
Circuit
BCD
Accessory
Connector
Sensor
Range
Switch
BCD
Connector
FIGURE D
270 BLOCK DIAGRAM

At the output of each supply there is a jumper bus. This bus doubles as a test point and a means of isolating the
power supply when troubleshooting the circuit.
CAUTION: To avoid damage to internal circuits, never operate the instrument with one
power supply disconnected. Both supplies must be disconnected at the same
time.
+5VOLT SUPPLY: See Figure E - Section A
The logic supply consists of an adjustable three terminal regulator similar to the above supplies. It also provides
internal current limit and other protection circuits. The voltage level is adjusted by R16 and CR3 provides reverse
voltage protection.
HEATER POWER SUPPLY: See Figure E - Section A
The heater supply consists of bridge rectifier DB1 and the RC filter R12 & C7. The voltage at no load is approx.
110 volts which will be approx. 85V under full heater load. (approx. 12 Watts) The heater supply is totally above
ground so some care must be taken not to ground the heater return. Power is supplied through VR1 and the heater
lamp. This parallel combination provides a visual heater indication when used with the proportional heaters in the
sensor.
10KHz OSCILLATOR: See Figure F - Section A
Oscillator is a Wein Bridge type which has a buffered output and amplitude stabilization to insure constant output
voltage under all load conditions. The oscillator is formed by amplifier U12 and the bridge network C45, C47 and
R90, R93. The frequency is factory adjusted to 10 KHz by adjustment of R91. The output of U12 passes through
the buffer amplifier formed by 010-013. This output is fed back to the oscillator input by R94. Diodes CR16-17 act
to shut down the buffer amplifier in the event of a short circuit at the output. Amplitude stabilization is accomplished
by rectifying the output and comparing the DC level against the temperature regulated precision reference VR2.
This reference voltage is applied to the positive input of the integrating amplifier U14. The negative input of this
amplifier is connected to precision half wave rectifier U13. The integrator compares the difference between the
reference voltage and the rectified output and produces a control signal for Q14. Should the rectified output exceed
the reference voltage, 014 is biased OFF and the gain of the oscillator amplifier is descreased. When the rectified
output falls below the reference voltage, 014 is biased ON and the gain of the oscillator amplifier is increased.
This technique of buffering and amplitude stabilization produces a high purity sine wave capable of driving loads
on the order of 100 ohms or less with almost no change in the output amplitude.
AC AMPLIFIERS: See Figure F - Section B
The input signal is applied to the input of the first stage AC amplifier, U9. This amplifier multiplies the signal by
again of 3 and drives the range transformer, T2. Depending on the range selected, the signal is either passed
through directly (X0.01 range), or divided by 10 for the X0.1 range, or divided by 100 for the X1 range. Selection
of the proper taps is done by turning ON or OFF FET switches 03-A5. The signal passes from the transformer
to the high impedance input of the 2nd AC amplifier, U10. U10 multiplies the signal by a gain of 31 and couples
it through C26-C27 to the primary of T3.
AC TO DC CONVERTER: See Figure F - Section C
T3 along with the switching FET's Q8 and 09 form the synchronous detector. The FET's are driven 180 deg. out
of phase by a 25V P-P switching squarewave which is synchronized perfectly to the 10KHz oscillator. The FET's
are turned ON and OFF, first grounding one end of the secondary of T3 and then the other, this causes the center
of the transformer to produce a full wave rectified signal. By switching in this manner, all 60Hz and all 90 deg.
quadrature signals are blocked from the output.
4-3

fA
•
111
The rectified signal then passes through to the input of the DC integrator U11 which filters and amplifies the waveform
to produce a noise free DC output signal. The signal is adjusted to the proper full scale of 10 volts with the coarse
span adjust R70 and is filtered once more by L1 and C37 and then applied to the proper output connectors.
The demodulator drive provides a fast switching waveform that is synchronized with the oscillator to drive the switch-
ing FET's in the demodulator (synchronous detector), and is done by applying the oscillator input voltage to the
comparator U15. When the oscillator crosses through zero the comparator switches, one output goes high while
the other goes low. These outputs are applied to the high speed "flip flop" that is formed by 015-018. The demodulator
drive circuit increases the switching speed and produces a waveform that switches between ± 13 volts. This waveform
and its complement are connected to the switching FET's through C28, C29.
OVERRANGE DETECTOR: See Figure E - Section B
When the output from the AC to DC converter exceed ± 11 volts, the amplifiers are entering their saturation region
and may become non linear. This comparator will go to a high state whenever the input exceeds ± 11 volts and
will turn on transistor 01 which blanks the display on the panel meter.
RANGE SELECTION CIRCUITS: See Figure F - Section D
The range selection circuit provides a means to select the proper sensitivity range either by the manual range
switch or by the external control connector and to provide decimal point infomration to the panel meter. The switch-
ing is based on the principle that the X1 range is selected by the absence of a command to the X0.1 or X0.01 ranges.
The command to select the X0.1 or X0.01 range by the range switch is done by applying a ground closure via S1
while the REMOTE position is left in the hi state to "LOCKOUT" any commands from the REMOTE connector.
Nand gate U7 produces the range commands which are applied to U8. U8 interfaces the digital logic levels to the
higher levels required to drive the switching FET's. U5 selects the proper decimal command to be sent to the AC-
CESSORY connector.
In the REMOTE position the "LOCKOUT" is defeated and commands from the REMOTE connector can be entered
through U5. The outputs of U5 are passed through the "LOCKOUT" gates (U6) to the inputs of U7. At this point
the range selection is identical to the process described above.
DIGITAL INTERFACE CIRCUITS: See Figure G - Section A
The placement of the decimal point is determined by the position of the DISPLAY UNITS selector, RANGE SELEC-
TOR and the SENSOR RANGE selector. This is done by creating a BCD number for each switch position and then
adding these numbers together. These numbers are indicated on the schematic as logic code numbers and are
processed in the following manner:
1. The DIPSLAY UNITS selector generates a logic code which is applied to the A inputs of U1.
2. The RANGE ID's generate a logic code which is applied to the B inputs of U1.
3. The sum of the A&B inputs is applied to the B inputs of U2.
FOR EXAMPLE: The TORR setting of Si generates a code of 2L and X1 gain position generates a code of 2L.
The sum, 4L, is applied to the B inputs of U2.
4. The SENSOR RANGE selector, or the multiplex unit when in use, is connected to a digital interface circuit which
generates a BCD number for each position of the switch.
FOR EXAMPLE: The 1000 range generates a logic code of 4L which applied to the A inputs of U2,
5. The sum of U2, 8L, is then applied to the decoder, U3. This causes the 8 output to go high and activate the
1000.0 decimal point.
4-4

Section 5 MAINTENANCE AND TROUBLESHOOTING
GENERAL
Should any difficulties be encountered in the use of your instrument, it is recommended that you contact any author-
ized MKS Sales Office or Home Office for repair instructions.
IF IT IS NECESSARY TO RETURN THE INSTRUMENT TO MKS FOR REPAIR, PLEASE
CONTACT MKS' CUSTOMER SERVICE REPRESENTATIVE AT THE NEAREST MKS SERVICE
CENTER FOR AN ERA NO. (EQUIPMENT RETURN AUTHORIZATION NUMBER) TO EXPEDITE
HANDLING AND ASSURE PROPER SERVICING OF YOUR INSTRUMENT. SEE LIST OF
SERVICE CENTERS INSIDE BACK COVER.
TROUBLESHOOTING
Should difficulties be encountered in the use of any 270 system, information in this section should help to isolate
the source of the problem. Whenever similar equipment is available the method of substitution is recommended.
If the pressure reading is overranged >11V, or otherwise suspect, make the checks listed below:
Check to be sure that all vacuum fittings are tight and that the sensor is properly pumped down. Low range absolute
sensors (100mmHg and below) will be overranged at atmospheric pressure.
Connect another meter, analog or digital, to the output to verify the proper operation of the panel meter in the 270.
Place the RANGE SELECTOR in the SYSTEM CHECK position, DISPLAY UNITS selector in the mmHg position,
and perform the following checks:
Does the system check read
properly? See cal sheet for
system check value.
NO
11P
Problem may be in sensor
electronics, cable or 270.
Do test A. Does it pass?
NO
Problem may be cable or
270. Do test B.
Does it pass?
NO
*
Problem is in the 270.
Place RANGE SELECTOR
in Full Scale. Does it work
properly?
NO
The problem is internal to
the 270.
Problem is in the
YES sensor mechanics
YES -10
YES
Problem is in the
sensor mechanics
olProblem is in the
sensor cable
Problem is in the input
YES - circuits of the 270
5-1

tw
111
N
M
M
SENSOR CABLE
PIN NO. VOLTAGE FUNCTION
IOR (13)* +12 to +13 SENSOR ELECTRONICS
SUPPLY VOLTAGE
D OR (14)* +12 to +13 SYSTEM CHECK COMMAND
J OR (15)* 6V RMS EXCITATION VOLTAGE
TEST A
LOCALIZING MALFUNCTION TO CABLE OR ELECTRONICS UNIT
Equipment required: VOM
1. Disconnect the sensor from the sensor cable.
2. Place the RANGE SELECTOR in the system check position.
3. Place the REG switch in the OFF position.
4. Use the VOM to check the voltages at the sensor end of the cable.
Reference the VOM to pin H (2)*
SENSOR CABLE VOLTAGE CHECKS
*Some sensor cables use number while other use letters.
5. Connect two resistors to the sensor cable as shown. This should produce a negative Full Scale ± 10% with
the RANGE SELECTOR in the System Check Position.
PIN J (15)
PIN A (7)
PIN B (6)
PIN H (2)
5-2
3K
1K