Tektronix DM 502 User manual

Tektronix, Inc.
P.O. Box 500
Beaverton, Oregon 97005
TEKTRONIX®
DM 502
DIGITAL
MULTIMETER
INSTRUCTION MANUAL
Serial Number
070 1726 00 First Printing Aug 1974

WARRANTY
All TEKTRONIX instruments are warranted against
defective materials and workmanship for one year.
Any questions with respect to the warranty should
be taken up with yourTEKTRONIX Field Engineer or
representative.
All requests for repairs and replacement parts
should be directed to the TEKTRONIX Field Office
or representative in your area. This will assure you
the fastest possible service. Please include the
instrument Type Number or Part Number and Serial
Number with all requests for parts or service.
Specifications and price change privileges reserved.
Copyright <© 1974 by Tektronix, Inc., Beaverton,
Oregon. Printed in the United States of America. All
rights reserved. Contents of this publication may not
be reproduced in any form without permission of
Tektronix, Inc.
U.S.A. and foreign TEKTRONIX products covered
by U.S. and foreign patents and/or patents pending.
TEKTRONIX is aregistered trademark of Tektronix,
Inc.

DM 502
SECTION 1
SECTION 2
SECTION 3
SECTION 4
SECTION 5
CHANGE INFORMATION
TABLE
OF CONTENTS
Page
OPERATING INSTRUCTIONS
Introduction 1-1
Instrument Description 1-1
Installation and Removal 1-1
Operating Considerations 1-2
Input Connections 1-2
Functiorfs Available at Rear Connector 1-2
Sine Wave Response 1-3
Temperature Measurements 1-3
Making Measurements 1-4
DC Voltage Measurements 1-4
DC Current Measurements 1-4
AC Voltage, Current and Resistance Measurements 1-5
Decibel Measurements 1-5
Temperature Measurements 1-5
Using aTransistor as aTemperature-Sensing Device 1-5
Accurate Current Source 1-6
Specifications 1-6
THEORY OF OPERATION
Introduction 2-1
Attenuator and Input Switching 2-1
AC Converter 2-1
DB Converter 2-1
Ohms Converter 2-2
Temperature Measurement Circuitry 2-2
Clock 2-3
Analog-To-Digital Converter 2-3
Display 2-5
Power Supplies 2-5
ELECTRICAL REPLACEABLE PARTS LIST
SERVICE INFORMATION
Internal Adjustment Procedure
Rear Interface Connector Assignments
Rear Interface Connector Wire Locations
Front Panel Controls and Connectors
Block Diagram
Parts Location Grid
Attenuators and Conversion Circuits Schematic
Integrator, A/D Converter and Display Schematic
Power Supply and Switch Logic Schematic
MECHANICAL REPLACEABLE PARTS LIST
Accessories
Repackaging
Exploded View
REV. A, OCT. 1974 i

DM 502
hi r-V/manomar'.sqhiH.com

Section 1—DM 502
OPERATING
INSTRUCTIONS
INTRODUCTION
Instrument Description
The DM 502 Digital Multimeter measures dc and ac
voltage and current. dBm, dBV, resistance and
temperature. The ac functions are average responding
and rms calibrated for sine waves. Asingle front panel
control selects all ranges. Afront-panel pushbutton
selects dB readout of ac functions in lieu of voltage or
current. Rear interface-connector input is also pushbut-
ton selected from the front panel. Values of dB are
obtained by adding the selected dB range value to the
display reading. Readout in dBm or dBV is chosen by an
internal jumper. The unit is shipped with the jumper in the
dBm position.
An internal jumper also permits selection of fet input (>
1000 MO) or 10 MO input impedance on the two lowest dc-
voltage ranges. The unit is shipped with the jumper in the
10 MO position. See Fig. 1-2.
Temperature measurements are made using a
Tektronix P6430 probe (Tektronix Part Number 01 0-6430-
00) or other suitable sensing devices. Option 2in-
struments delete the temperature measuring capability.
The readout is a3-1/2 digit display using seven-
segment LED's. The decimal point is automatically
positioned by the range-function switch. Polarity indica-
tion is automatic. Ausable 50% overrange exists in all
measurement modes except ac voltage and current, 1000
Vdc, and the 2Adc current range. In overrange, the
display blinks and the most significant digit is suppressed.
Upon exceeding the available 50% overrange, a1appears
in the most significant digit position. Display readings are
invalid beyond this point.
Interlaced bed output (1, 3, 2, 4) referenced to the low
input, is available at the rear interface connector if user
wired.
NOTE
The DM 502 is not directly compatible with the 153
Calculator Interface.
The maximum input voltage is 1kV, 350 Vat the rear
connectors. The front-panel HI and LO connectors may be
floated 1kV maximum above ground, the rear inputs 350 V.
Installation and Removal
The DM 502 is calibrated and ready for use when
received. It operates in any compartment of aTM 500
series power module. See the power module instruction
manual for line voltage requirements and power module
operation. Fig. 1-1 shows the DM 502 installation and
removal procedure.
Turn the Power Module off before inserting the DM
502. Otherwise, arcing may occur at the rear inter-
lace connectors, reducing their useful life and
damage may be done fo the plug-in circuitry.
Check that the DM 502 is fully inserted in the Power
Module. Pull the PWR switch on the Power Module. One or
more characters in the LED display should now be visible.
The Controls, Connectors and Adjustments foldout
page in Section 3gives acomplete description of the front
panel.
REV. A, OCT. 1974 1-1

Operating Instructions—DM 502
Fig. 1-1. DM 502 Installation and removal.
OPERATING
CONSIDERATIONS
Input Connections
Three binding posts provide measurement connec-
tions. The HI and LO posts are normally used for all
measurements except temperature. Normal measurement
conditions will be with the LO terminal ungrounded. A
connection between the LO and GND posts may be made
to reference the input against DM 502 chassis ground. Use
caution, as the LO terminal is then connected to earth
ground through the three-wire power cord. False readings
may be obtained due to ground loops.
If the INPUT button is pushed in, signal inputs are made
via the rear interface connector; if the button is out, signal
inputs are made via the front panel.
Functions Available at Rear Connector
HI, LO, GROUND, temperature output at 10 mV per
degree and bed connections are available at the rear
interface connector. The Rear Interface Connector
Assignments page in the Service Section at the rear of this
manual gives data on the various connections. Only the
Temp Out. Temp Gnd and HI and LO inputs are factory-
wired. Jumper holes are provided for making the desired
bed output connections. See the wire locations illustration
in the foldout pages for the correct circuit board locations.
Simply insert and solder an insulated wire strap in the
proper locations. Buffers are required before the data
outputs are used to drive externai loads. See Fig. 1-3 fora
typical buffer circuit.
The DM 502 has aslot between pins 17and 18. Abarrier
in the corresponding position of the power module jack
allows only compatible plug-ins to be used in that
compartment. This protects the plug-in, should specializ-
ed connections be made to that compartment. Consult the
Building ASystem section of the power mddule manual
for further information.
1-2

Operating Instructions—DM 502
FET Input
jumper position
10 MO Input
jumper
position
Connect jumper on
reverse side of board
as shown to hard wire In
°F range.
dBV jumper
position
Fig. 1-2. Locations of dB reference jumper, input impedance jumper, and hard wired °C or ‘*F component or wire changes.
Sine-wave Response
The DM 502 responds to the average value of a
sinusoidal current or voltage. The readout indicates the
equivalent rms value. The effective or rms value of asine
wave is 0.707 times the peak voltage or current. The
average value is 0.636 of the peak value. The scale factor of
the DM 502 is 0.636/0.707 or 0.9. Multiply the DM 502
readout by 0.9 to obtain the average value of asinusoidal
input voltage or current.
Temperature Measurements
Connect the P6430 (or other sensing device) to the
connector marked TEMP PROBE. Use care to align the
connector pins properly. Select calibration in “F or “C
with the range-function switch. For best accuracy,
calibrate the individual instrument to the probe used. See
the Internal Adjustment Procedure in the service section
of this manual for the proper procedure.
1-3

Operating Instructions—DM 502
^5 V
The Temp Out terminals at the rear interface connector
are always connected to the output of the temperature
circuitry. This feature provides continuous temperature
monitoring capabilities in °C even while using other
functions of the DM 502. To avoid amomentary scale
change when arecorder or other device is connected to
these terminals avoid switching through the °F position.
The DM 502 may be hard wired in either the or °F
range independent of the range-function switch setting.
To hard wire in the °Frange, connect ajumper as shown in
Fig. 1-2. To hard wire in °C lift either end of R285 as shown
in Fig. 1-2.
Fig. 1-3. Typical buffer circuit for rear connector data outputs. Up
to 20 mA may be used from the DM S02 +5 V supply.
MAKING MEASUREMENTS
With the DM 502 properly installed in the power
module, allow twenty minutes warmup time for operation
to specified accuracy. When the value of the quantity
being measured is unknown, select the highest range first.
Decrease the range setting until the display blinks in-
dicating over-range. Then increase the range switch to the
next higher position. This method obtains maximum
resolution. Resolution of the DM 502 is 0.05% of full scale
setting, except temperature, which is 0.1°. Do not exceed
the maximum voltage ratings. With the range-function
switch in the 1KDC VOLTS or 500 AC VOLTS position,
internal damage may result before overrange is indicated.
DC Voltage Measurements
Select an appropriate full range dc voltage position
with the range-function switch. Apply the voltage to be
measured to the INPUT binding posts. Observe the
maximum voltage ratings as indicated on the front panel.
The readout displays a+if the HI input is positive with
respect to the LO input. A—is displayed if the LO input is
more positive. With the input shorted, the display reads
zero. ±one count.
To prevent possible damage to switch contacts by
high currents, do not switch into, out of, or through
the current measurement modes from the other
measurement modes with ahigh current source, or
large inductive or capacitive load, connected to the
DM 502 input. This caution does not apply to
switching between individual current ranges up to
the maximum stated input current, as the switch
contacts are adequately protected for this operation.
DC Current Measurements
Select an appropriate full scale DC mA position on the
range-function switch. Apply the dc current to be
measured to the INPUT binding posts. Acurrent (electron
flow) into the LO connector and out of the HI connector
indicates +on display. Foroppositecurrentflow, a—will
be displayed.
1-4 REV. A, OCT. 1974

Operating Instructions—DM 502
AC Voltage, Current and Resistance Measurements
Select an appropriate full scale AC VOLTS, AC mA or
OHMS position on the range-function switch. Connect the
unknown voltage, current or resistance to the INPUT
binding posts.
Decibel Measurements
To use this feature, switch to an ac voltage or current
range and push the button marked dB on the front panel.
Connect the voltage or current source to the DM 502. To
arrive at the correct dB reading, algebraically add the
number on the outside scale to the display reading. V\/hen
the dB switch is depressed, the decimal point is fixed and
the maximum display reading is 19.99 on all scales.
For example, if the range-function switch isbn the2 AC
VOLTS position and the display reading is +13.98, then
the correct dB reading is:
+13.98
-20.00
-6.02 dB
If the range-function switch is on the 200 AC mA
position and the display reads —4.04, then:
-4.04
4-20.00
-15.96 dB
Selection of dB readout in dBV or dBm is accomplished
by ajumper shown in Fig. 1-2. The reference for dBV when
measuring ac voltage is one volt and when measuring
current one milliampere. The reference for dBm in the
voltage range is one milliwatt dissipated in 6000 or 0.7746
volts: in the current measuring range the reference is
0.7746 milliamperes. When dBm or dBV is applied to
current measurements, they have no significance other
than to the reference current as stated above.
Useful dB formulas are:
El IP2
dB =20 log 10-^, 20 log 10 -p and 10 log 10
li ri
For calculations substitute the reference voltage or
current listed as the denominator.
Temperature Measurements
With the P6430 probe connected to the front panel
connector labeled TEMP PROBE, set the range-function
switch to the ®F or °C in the TEMP position. Apply the
probe sensor tip to the device being measured. For
optimum temperature transfer, coat the surface of the
device being measured with silicone grease and apply the
probe tip squarely to the surface. Allow sufficient time for
the probe tip to stabilize before taking areading. The time
required depends upon several factors. Generally, when
the tip is first applied to the device under test, the readings
change rapidly. As the probe tip temperature approaches
the temperature of the device under test, the readings
change less rapidly, and finally stabilize. The readings are
in °C or °F, depending on the position of the range-
function switch. See the P6430 manual for more informa-
tion on temperature measurements and probe use.
Using aTransistor as aTemperature-Sensing
Device
Certain npn transistors such as a2N2484 can be used
as separate sensors in place of the P6430 probe with little
or no selection of the transistor. Connect the temperature-
sensing transistor to the DM 502 through the TEMP
PROBE connector as shown in Fig. 1-4. Accuracy without
recalibration of the DM 502 is within ±8®C to 150°C. If the
Connector shell
Connector
TEKTRONIX Part Number
131-1653-00
Fig. 1-4. Schematic diagram of temperature-sensing transistor
connected to probe connector.
REV. A. OCT. 1974 1-5

Operating Instructions—DM 502
measurement to be made requires greater accuracy,
recalibrate the temperature circuitry or determine a
correction factor. Place the sensing device in an environ-
ment having aknown ambient temperature. Use any
difference between the known temperature and the DM
502 readout as acorrection factor throughout the entire
measurement range.
Accurate Current Source
The OHMS positions on the range-function switch
provide accurate (typically 0.2%) constant currents at the
INPUT terminals. SeetheOhmmeterspecificationsforthe
value of the currents and maximum voltages across the
INPUT terminals for full scale display readings. The
maximum voltage available across the INPUT terminals in
the OHMS position is about 10 volts.
ELECTRICAL
SPECIFICATIONS
Performance Conditions
The electrical characteristics are valid only if the DM
502 has been calibrated at an ambient temperature
between f20° Cand I30° Cand is operating at an ambient
temperature between l15°Cand <-40°C. Warm uptimeto
specified accuracy is twenty minutes.
Display
The measurement rate is 3.33 per second. Resolution
on all ranges is 0.05% of full scale setting; except for
temperature, which is 0.1°.
DC VOLTMETER
RANGES;
200 mV, 2V, 20 V, 200 V, 1000 V.
ACCURACY:
±0.1% of reading, 11 count.
COMMON MODE REJECTION:
^100 dB at dc, 80 dB at 50 or 60 Hz with a1kQ
unbalance.
NORMAL MODE REJECTION:
^80 dB at 50 or 60 Hz.
STEP RESPONSE TIME:
^1/2 s.
INPUT RESISTANCE:
10 MQ ±0.25% (jumper selectable for >1000 MO on 0.2
Vand 2Vranges).
MAXIMUM INPUT VOLTAGE ON ANY RANGE:
Front Panel: 1kV.
Rear Connector: 350 V.
AC VOLTMETER
RANGES:
Voltage: 200 mV, 2V, 20 V, 200 V, 500 V.
dB: -40, -20, 0, 20, 40. Reference is dBV (1 V) or
dBm (1 mW dissipated in 600 Q, 0.7746 V).
ACCURACY:
Voltage: ±0.5% of reading, ±1 count, for a40 Hz to 10
kHz sine wave.
±1 .0% of reading, ±1 count, for a20 Hz to 20 kHz sine
wave.
Usable to 100 kHz. Typically <10% down between 40
mV and 500 Vat 100 kHz.
1-6 REV. A, OCT. 1974

Operating Instructions—DM 502
DB:
Display Frequency Maximum
Reading Range
0to -20 20 Hz to 20 kHz 0.5 dB
0to -10 20 Hz to 2kHz 0.5 dB
2kHz to 20 kHz 1.0 dB
-10 to -20 20 Hz to 2kHz 0.5 dB
2kHz to 7.5 kHz 1.0 dB
7.5 kHz to 20 kHz 2.0 dB
COMMON MODE REJECTION;
60 dB at 50 or 60 Hz with a1kO unbalance.
RESPONSE TIME:
^5s.
IMPUT IMPEDANCE:
10 Mfi <1% paralleled by ^60 pF.
MAXIMUM INPUT VOLTAGE ON ANY RANGE:
Front Panel: 500 Vac rms or l56 dBm or 600 Vdc, not
to exceed 1kV dc plus peak ac.
Rear Connector: 350 Vdc plus peak ac or +50 dBm.
OHMMETER
RANGES;
200 O, 2kn, 20 kQ, 200 kO, 2MO, 20 MO.
ACCURACY:
200 Orange, 0.5% ±1 count, •0.1 O.
2kO through 2MO ranges, ±0.5%, 11 count.
20 MO range, 1.0%, J1count.
MEASUREMENT CURRENT AND VOLTAGE:
Range Current Full Scale
Voltage
200 O1mA 0.2 V
2kQ 1mA 2V
20 kO 100M 2V
200 kQ :0fjA 2V
2MO 1/jA 2V
20 MO 0.1 fjA 2V
RESPONSE TIME:
200 Othrough 2MO ranges, ^1 /2 s. 20 MO range, <5 s.
MAXIMUM INPUT VOLTAGE ON ANY RANGE:
130 Vdc and ac rms indefinitely.
250 Vdc and ac rms for one-half hour.
DC AMMETER
RANGES:
200 fjA, 2mA, 20 fnA, 200 mA, 2A.
ACCURACY:
±0.2% of reading, ±1 count.
RESPONSE TIME:
<1/2 s.
INPUT IMPEDANCE:
Range Setting
except <2.0 kO on the 200 /jA range.
MAXIMUM INPUT CURRENT ON ANY RANGE:
Fused at 2.5 A.
AC AMMETER
RANGES:
Current: 200 pA, 2mA, 20 mA, 200 mA, 2A.
dB: —40, —20, 0, 20, 40. Reference is 1mA (dbV) or
0.7746 mA (dBm).
REV. A, OCT. 1974 1-7

Operating Instructions—DM 502
ACCURACY:
Current: ±0.6% of reading, ±1 count for a40 Hz to 10
kHz sine wave. Usable to 100 kHz.
DB:
Display Frequency Maximum
Reading Range
0to ±20 20 Hz to 20 kHz 0.5 dB
0to -10 20 Hz to 2kHz 0.5 dB
2kHz to 20 kHz 1.0 dB
-10 to -20 20 Hz to 2kHz 0.5 dB
2kHz to 7.5 kHz 1.0 dB
7.5 kHz to 20 kHz 2.0 dB
RESPONSE TIME:
$5 s.
INPUT IMPEDANCE:
Range Setting
except <2.0 KQ on the 200 ^/A range.
MAXIMUM INPUT CURRENT ON ANY RANGE:
Fused at 2.5 A.
THERMOMETER
RANGES:
Centrigrade: —55° to +150°.
Fahrenheit: —67° to +200°.
ACCURACY:
With any P6430 probe. ±8'’C or ±14.4°F.
With instrument calibrated to probe, ±2“C or ±3.6° F.
MAXIMUM INPUT VOLTAGE:
Up to 1kV may be safely applied to the front panel
INPUT terminals (350 Vto the rear input connectors) when
the range-function switch is in the TEMP positions.
ENVIRONMENTAL SPECIFICATIONS
TEMPERATURE:
Operating: +15°C to +40°C.
Non-operating: —40°C to +75°C.
POWER CONSUMPTION:
6.5 watts.
1-8 REV. A, OCT. 1974

Section 2—DM 502
THEORY
OF OPERATION
THEORY OF OPERATION
Introduction
The DM 502 is basically an analog-to-digital converter
with the logic, display devices, and power supplies
necessary to display, in digital form, the value of adc
voltage. The analog-to-digital converter is an integrating
amplifier. Current is measured by passing the unknown
current through aknown resistance and measuring the
resultant voltage drop, using afour terminal configura-
tion. Resistance is measured by passing aconstant
current through an unknown resistance and measuring
the voltage drop across the unknown resistance.
Temperature is measured by obtaining adc voltage
proportional to the temperature and measuring that
voltage. Ac voltages and currents are rectified and the
resulting dc is measured, then read out in appropriate ac
units. The dB circuitry converts the output of the ac
rectifier to alogarithmic output that is applied to the
analog-to-digital converter. Refer to the Block Diagram in
the foldout pages for an overall view of the DM 502
operation.
Use the following circuit description with the
schematics to gain an understanding of the operation of
the DM 502.
Attenuator and Input Switching
The HI and LO binding posts are connected across
R147, R146 and R145 serving as avoltage divider. The
various capacitors switched across these resistors are the
compensating capacitors used in the ac modes of opera-
tion. C100 is switched in series with the divider when the
unit is operating in the ac voltage modes.
R105, R106, R107, R110 and R142 serve as current
shunts in the current modes. Afour terminal technique is
used. The current passes through the shunts from the HI
terminal to the LO terminal. The voltage is applied to the
analog-to-digital converter through S110, 17, 16,9, 15and
18 to ground. FIDO, in series with shunts R105, R106, R107
and R1 10 along with CR1 lOand CR1 12, protect the shunts
from over current. The diodes conduct, opening the fuse
before the voltage across the higher value shunts damage
them. Lamp RT100 protects shunt R142, in the lowest
current range, and the Ohms Converter.
Ac Converter
In the ac measurement modes, the ac signal is applied
to fet 0165. CR165 and CR166 protect 0165 from over
voltage. 0170, connected from the source to the drain of
0165, provides bootstrapping.
The output of 0165 is fed to the negative input of U195,
an operational amplifier. Asmall amount of negative dc
feedback is provided by R192, R190andthefiltering action
of C190. The gain of U195 is extremely high until CR187
and CR195 conduct. This high gain allows rectification of
extremely small signals. More negative dc feedback is
provided through the action of CR187 and CR195. The dc
feedback stabilizes the operational amplifier gain and
compensates for amplifier offset voltages or capacitor
leakage.
The positive half cycle is applied through CR195 to the
filter network. The dc voltage from the filter output is
switched to the analog-to-digital converter input in the ac
measurement mode, and the dB converter in thedB mode.
The gain of U195 is set by R180 and R181 to produce adc
output equivalent to the rms value of asine-wave input.
DB Converter
This circuitry converts the linear dc current output from
the ac converter to alogarithmic output voltage. A
collector current change with respect to abase to emitter
voltage change in abipolar transistor is logarithmic.
VR310 serves as the voltage reference diode at 6.2 V.
This voltage is applied to the negative input of operational
amplifier U210 through R203 and R205. R205 sets the
current through Q120A. Should the current through
Q120A increase, the voltage at pin 2of U210 goes
negative. This causes the output of U210 to go positive
reducing the current through the transistor to the value set
by R205. Q120A and Q120B are housed in the same case
so that their base to emitter voltages remain constant with
temperature (for equal collector currents).
REV. A, OCT. 1974 2-1

Theory of Operation—DM 502
The rectified and filtered dc current from the ac
converter is applied to the plus input of operational
amplifier U220, through R212. R212 causes areference
current shift in the dBm mode and is shorted out in the dBV
mode. As pin 3of U220 goes positive, the output goes
positive. The base of Q120B goes positive and the
collector negative providing negative feedback. The
collector current of Q120A always equals the current
through R212 at OdB. The necessary voltage at the output
of U220 is logarithmic due to the logarithmic
characteristics of Q120B in the feedback loop. As the
temperature increases the gain of Q120B decreases. This
is compensated for by RT220 decreasing in value with
increasing temperature.
Ohms Converter
U135 is an operational amplifier. The current setting
resistances are connected from the output to the plus
input terminal of U135. The unknown resistances are
connected from the plus terminal to floating ground. See
Fig. 2-1. Negative feedback occurs from the output of
U1 35 to the negative input terminal. R130 is the calibration
adjustment. R1 35 and R1 36 are switched in and out of the
negative feedback loop to provide bias current compensa-
tion.
The value of the current setting resistance is always
equal to one half of the full scale measurement value,
except on the 200 Orange, as shown on the range-
function switch. The 200 Oscale uses 1kO and the gain of
the analog-to-digital converter is increased by afactor of
ten. R130 adjusts the gain of the amplifier to produce one
volt across the current setting resistance (0.1 Vin the 200
Qrange). As long as the unknown resistance is within the
measurement range, the circuitry adjusts the current so
that the drop across the current setting resistance is one
volt. This current flows through the unknown resistance,
varying the voltage at the plus terminal of U135. This
voltage is then fed to the analog-to-digital converter input.
Temperature Measurement Circuitry
This circuitry operates on the principle that achange in
base to emitter voltage of abipolar transistor is propor-
tional to the temperature of the junction as the collector
current is changed between two levels.
Asquare wave (about 2kHz) is generated by Q230 and
Q235 operating as an astable multivibrator. The square
wave is applied to the gates of Q255 and Q260.
The collector to base voltage of the temperature
sensing transistor in the probe is held constant. When fet
Q255 is off, the current to the probe through R255 and
R257 is approximately one-tenth the on current.
Fig. 2-1. Simplified diagram of Ohms Converter.
2-2 REV. A, OCT. 1974

Theory of Operation—DM 502
When Q230 is off and Q235 is on, Q255 is off and the
voltage at pin 2of U250A goes negative. The output of
U250A goes positive, reducing the temperature-variable
base-emitter current of the sensing transistor until pin 2of
U250A is at ground. Apositive-going square wave results
at the output of U250A, with an amplitude proportional to
the temperature of the base-emitter junction. Q260 is on
and Q245 is off, due to the action of Q230 and Q235. Adc
voltage, whose amplitude is proportional to temperature,
occurs across C263. The opposite occurs when Q230 and
Q235 change state, with the exception that Q245 turns on,
shorting the negative voltage to ground; and Q260 is off,
preventing the discharge of C263. The dc voltage across
C263 is applied to operational amplifier U250B. R280
adjusts the gain. Offset voltage in U250B is compensated
for by R270. R268 and R285, through S1 10, 44, change
offset and increase gain for °F readout vs °C. The output
of U250B at 10 mV/° is applied to the analog-to-digital
converter input and the rear interface connectors.
Analog-To-Digital Converter
U290 comprises the analog-to-digital converter and
U335 contains the necessary control logic. U290 contains
the integrating amplifier, comparator, auto-zero amplifier
and input buffer amplifier. See Fig. 2-2. C300 is the
integrating capacitor. The clock frequency is divided by
the time base counter into groups of 6144 pulses.
Measurement takes place for 4096 of these pulses and
automatic zeroing takes place for 2048 pulses. The auto-
zeroing feature provides current to compensate for offset
currents in the buffer amplifier.
Basic sensitivity of the analog-to-digital converter is 2V
for afull scale display reading. When S1 10, 36 is closed,
the sensitivity becomes 0.2 Vfull scale.
Clock
U325 is amonolithic timing circuit used to generate the
clock frequency for the DM 502. The frequency is
20.48 kHz. C330, the timing capacitor, charges through
R328, R329 and R330, and discharges through R330. R328
sets the frequency so that it is amultiple of the power line
frequencies. This insures minimization of line frequency
interference in the analog-to-digital converter.
During auto zero, the input of the buffer amplifier is
grounded. The buffer offset current also offsets the output
of the integrator. The offset at the integrator output passes
through alow-pass filter comprised of C300, R300 and
R302 to the plus input of the auto-zero amplifier. The
output of the unity gain auto-zero amplifier compensates
for this offset current at the summing node (pin 9) at the
integrator input. The reference current through R304 and
R305 from the reference current source, is applied for four
clock cycles and disconnected for the next four cycles by
the U/D control logic. The reference current is connected
to R304 when the up-down logic is low. Equilibrium for the
"a
Fig. 2-2. Analog to digital converter and control logic block diagram. Reprinted by permission of Slliconix Inc.
REV. A, OCT. 1974 2-3

Theory of Operation—DM 502
entire integrating and auto-zero system is obtained when
the sum of the average currents at the integrator summing
junction equals zero. C316 charges to the equilibrium
voltage and maintains this voltage at the auto-zero
amplifier input during the measurement interval when the
switch from the integrator output is open. See Fig. 2-3 for
functional timing during the auto-zero interval. Notice that
the fifty percent duty cycle of the up-down counter is
overridden at the start of the auto-zero interval. This
override period permits the output of the integrator to
come to V„r, and C316 to assume this voltage.
Upon completion of the 2048 clock pulse auto-zero
interval, the measurement interval commences. The
measure-zero logic switches the analog input voltage to
the buffer input and disconnects the integrator output
from the auto-zero amplifier input. The additional current
resulting at the summing junction of the integrator causes
the integrator output voltage to move away from the
equilibrium voltage obtained during auto-zero interval
and maintained during the measurement interval by C316.
The comparator senses and transmits this deviation to the
control logic. The control logic changes the duty cycle of
the reference voltage in an attempt to re-establish
equilibrium at the integrator summing junction. The up-
down logic is up (integrator output voltage is also up) for
one clock cycle and down for seven cycles when the
comparator output was high during the preceding set of
eight clock cycles. This is shown as duty cycle Ain Fig. 2-
4. When the comparator output is low in clock cycle seven,
the up-down logic is up for seven cycles and down for one
cycle during the following eight clock cycles. This is
shown as duty cycle B. Fig. 2-4 shows the result of these
actions on the integrator output.
Acounter, synchronous with the up-down logic, in-
crements by each clock pulse when the up-down logic is
up and decrements by each clock pulse when the up-down
logic is down. The net count increases by six counts for
each Bduty cycle and decreases by six counts for each A
duty cycle, to amaximum count of about 3100. This
counting procedure is reversed for negative input
voltages.
Polarity of the input voltage is determined by the state
of the up-down logic when the bed counter state is zero.
This information is stored in aflip-flop and loaded into the
static latch once each measure-zero cycle.
The bed counter accumulates anumber of counts
proportional to the input voltage during the measure
interval while the control logic works to maintain
equilibrium. It can be seen that equilibrium is achieved in
steps and usually aresidual voltage remains at the end of
the measurement cycle. This residual voltage is compen-
sated for by ashort override interval at the beginning of the
auto-zero period. The counter continues until the in-
tegrator output equals the auto-zero equilibrium voltage
and the up-down logic is down. The bed counter is now
put on hold and its contents loaded into the latches. The
counter is then cleared.
CVCIE finnmmnnjuiMnjuinjinnmuinMjmnj^^
mz
OVERfttOf
u/o
“I
J
J
COMPARATOA
Fig. 2-3. Auto-zero timing intervals. Reprinted by permission of Siliconix Inc.

Theory of Operation—DM 502
Fig. 2-4. Measurement interval timing. Reprinted by permission of Siliconix, Inc.
Display
The bed data from the data buffers is applied to U340, a
bed to seven segment deeoder. All segments of digits one,
two and three are eonneeted in parallel to U340. The high
outputs from the digit buffers are inverted by U345A, B, C
and D, whieh drive Q350 for digit one, Q355 for digit two,
Q360 for digit three and Q365 for the most signifieant digit.
When the eolleetors of these transistors are high, the
particular digit is displayed. This is accomplished in aone,
three, two, four sequence. The common terminal for the
polarity signs is also activated by Q365 at the most
significant digit time. S1 10, 37, 38 and S200 disconnect the
polarity signs when not appropriate for the measurement
mode in use. S200 is on the dB switch. When pin 5of U335
is high the positive sign is lighted. When pin 5of U335 is
low, the negative sign is illuminated by the action of Q375
and U345F.
The decimal positions are changed by contacts in the
range-function and dB switches through Q370. As pin 16
of U335 is the bed one line to the seven segment decoder,
when the most significant digit is present, pin 16 is high.
This is converted to alow and connected to the cathodes
of the ones segments in the most significant digit module.
The contents of the static latches are multiplexed to the
data output buffers. The data output format is an interlac-
ed scan of digits one, three, two and four. Four is the most
significant digit. With acount of 2000 or greater in the
counter (100% of full scale) the 3-1/2 digit display blinks
off during the zero cycle.
Power Supplies
The entire DM 502 circuitry, with the exception of the
temperature circuitry can be elevated up to IkV above
chassis ground. Isolation is accomplished with
transformer T410. 50 Vac from the power module (two 25
Vac windings in series) is applied to the transformer
primary. Ac for the (5 Vand ±12 Vsupplies is bridge
rectified. U420, for the ±12 Vsupplies, is adual tracking
voltage regulator. R418 is acurrent shunt for the •12 V
supply and R427 is the same for the —12 Vsupply. R422
sets the levels. U420 is internally protected against short
circuit damage. Power for the display modules is not
electronically smoothed, regulated or filtered.
Q440 serves as the series pass transistor for the )5V
supply. The output voltage is set through the divider
consisting of R452 and R453 connected to the base of
Q450. Should the -^5 Voutput drop, Q450 increases
conduction. This causes Q440 to also increase conduc-
tion restoring the output voltage to the preset level.
The temperature measuring circuitry obtains ±15 V
from ±33.5 Vthrough zener diodes VR400 and VR405.
@2-5

Section 3—dm 502
REPLACEABLE
ELECTRICAL PARTS
PARTS ORDERING INFORMATION
Replacement parts are available from or through your local Tektronix, Inc. Field Office
or representative.
Changes to Tektronix instruments are sometimes made to accommodate Improved
components as they become available, and to give you the benefit of the latest circuit
improvements developed in our engineering department. It is therefore important, when
ordering parts, to include the following information in your order: Part number, instrument
type or number, serial number, and modification number if applicable.
If apart you have ordered has been replaced with anew or improved part, your local
Tektronix, Inc. Field Office or representative will contact you concerning any change in part
number.
Change information, if any, is located at the rear of this manual.
SPECIAL NOTES AND SYMBOLS
XOOO Part first added at this serial number
OOX Part removed after this serial number
ITEM NAME
In the Parts List, an Item Name is separated from the description by acolon (:).
Because of space limitations, an Item Name may sometimes appear as incomplete. For
further Item Name identification, the U.S. Federal Cataloging Handbook H6-1 can be
utilized where possible.
ABBREVIATIONS
ACTR ACTUATOR PLSTC PLASTIC
ASSY ASSEMBLY QTZ QUARTZ
CAP CAPACITOR RECP RECEPTACLE
CER CERAMIC RES RESISTOR
CKT CIRCUIT RF RADIO FREQUENCY
COMP COMPO*!ITION SEL SELECTED
CONN CONNECTOR SEMICOND SEMICONDUCTOR
ELCTLT ELECTROLYTIC SENS SENSITIVE
ELEC ELECTRICAL VAR VARIABLE
INCAND INCANDESCENT WW WIREWOUND
LED LIGHT EMITTING DIODE XFMR TRANSFORMER
NONWIR NON WIREWOUND XTAL CRYSTAL
3-1

Electrical Part* List—DM 502
CROSS INDEX MFR. CODE NUMBER TO MANUFACTURER
MFR.CODE MANUFACTURER ADDRESS CITY,STATE,ZIP
00853 SANGAMO ELECTRIC CO., S. CAROLINA DIV. P. 0. BOX 128 PICKENS, SC 29671
01121 ALLEN-BRADLEY CO. 1201 2ND ST. SOUTH MILWAUKEE, NI 53204
01295 TEXAS INSTRUMENTS, INC.,
SEMICONDUCTOR GROUP P. 0. BOX 5012 DALLAS, TX 75222
03508 GENERAL ELECTRIC CO,, SEMI-CONDUCTOR
PRODUCTS DEPT. ELECTRONICS PARK SYRACUSE, NY 13201
03888 KDI PYROFILM CORP. 60 S. JEFFERSON RD. WHIPPANY, NJ 07981
04713 MOTOROLA, INC., SEMICONDUCTOR
PROTUCTS DIV. 5005 E. MCDOWELL RD. PHOENIX, AZ 85036
07263 FAIRCHILD SEMICONDUCTOR, ADIV. OF
FAIRCHILD CAMERA AND INSTRUMENT CORP. 464 ELLIS ST. MOUNTAIN VIEW, CA 94042
07910 TELEDYNE SQ1ICONDUCTOR 12515 CHADRON AVE. HAWTHORNE, CA 90250
14752 ELECTRO CUBE INC. 1710 S. DEL MAR AVE. SAN GABRIEL, CA 91776
15281
15454
GENERAL INSTRUMENT CORP., DELETED,
BUSINESS DISCONTINUED
RODAN INDUSTRIES, INC. 2905 BLUE STAR ST. ANAHEIM, CA 92806
15818 TELEDYNE SEMICONDUCTOR 1300 TERRA BELLA AVE. MOUNTAIN VIEW, CA 94040
17856 SILICONIX, INC. 2201 LAURELWOOD RD. SANTA CLARA, CA 95054
18324 SIGNETICS CORP. 811 E. ARQUES SUNNYVALE, CA 94086
19396 ILLINOIS TOOL WORKS, INC., PAKTRON DIV. 1321 LESLIE AVE. ALEXANDRIA, VA 22301
27014 NATIONAL SEMICONDUCTOR CORP. 2900 SAN YSIDRO NAY SANTA CLARA, CA 95051
28480 HEWLETT-PACKARD CO., CORPORATE HQ. 1501 PAGE MILL RD. PALO ALTO, CA 94304
34553 AMPEREX ELECTRONIC CORP., COMPONENT DIV. 35 HOFFMAN AVE. HAPPAUGE, NY 11787
49956 RAYTHEON CO. 141 SPRING ST. LEXINGTON, MA 02173
50522 MONSANTO CO., ELECTRONIC SPECIAL
PRODUCTS 10131 BUBB RD. CUPERTINO, CA 95014
56289 SPRAGUE ELECTRIC CO. 'NORTH ADAMS, MA 01247
58474 SUPERIOR ELECTRIC CO., THE 383 MIDDLE ST. BRISTOL, CT 06010
71400 BUSSMAN MFG., DIVISION OF MCGRAW-
EDISON CO. 2536 W. UNIVERSITY ST. ST. LOUIS, MO 63107
71590 CENTRALAB ELECTRONICS, DIV. OP
GLOBE-UNION, INC. 5757 N. GREEN BAY AVE. MILWAUKEE, WI 53201
71744 CHICAGO MINIATURE LAMP WORKS 4433 RAVENSWOOD AVE. CHICAGO, IL 60640
72982 ERIE TECHNOLOGICAL PRODUCTS, INC. 644 W. 12TH ST. ERIE, PA 16512
73138 BECKMAN INSTRUMENTS, INC., HELIPOT DIV. 2500 HARBOR BlVD. FULLERTON, CA 92634
75042 TRW ELECTRONIC COMPONENTS, IRC FIXED
RESISTORS, PHILADELPHIA DIVISION 401 N. BROAD ST. PHILADELPHIA, PA 19108
80009 TEKTRONIX, INC. P. 0. BOX 500 BEAVERTON, OR 97077
80031 ELECTRA-MIDLAND CORP., MEPCO DIV.,
ANORTH AMERICAN PHILLIPS CO. 22 COLUMBIA RD. MORRISTOWN, NJ 07960
80294 BOURNS, INC., INSTRUMENT DIV. 6135 MAGNOLIA AVE. RIVERSIDE, CA 92506
81483 INTERNATIONAL RECTIFIER CORP. 9220 SUNSET BlVD. LOS ANGELES, CA 90069
86684 RCA CORP., ELECTRONIC COMPONENTS 415 S. 5TH ST. HARRISON, NJ 07029
90201 MALLORY CAPACITOR CO., DIV. OF
P. R. MALLORY CO., INC. 3029 E. WASHINGTON ST. INDIANAPOLIS, IN 46206
91637 DALE ELECTRONICS, INC. P. 0. BOX 609 COLUMBUS, NB 68601
B JAN. 1975

Electrical Parts List—DM 502
Ckt No.
Tektronix
Port No. Serial/Model No.
Eff Dscont Nome &Description
Mfr
Code Mfr Part Number
A1 670-3495-00 BOlOlOO B019999 CKT BOARD ASSY: MAIN 80009 670-3495-00
A1 670-3495-01 B020000 CKT BOARD ASSYtMAIN 80009 670-3495-01
A1 670-3694-00 BOlOlOO B019999 CKT BOARD ASSYtMAIN OPTION 280009 670-3694-00
A1 670-3694-01 B020000 CKT BOARD ASSYtMAIN OPTION 280009 670-3694-01
A2 670-3509-00 CKT BOARD ASSY tDISPLAY 80009 670-3509-00
ClOO 285-1106-00 CAP. ,FXD,PLSTCt0.22UF,20%,600V 14752 230B1F223
Clio 283-0279-00 XB020000 CAP.,FXD,CER DItO.OOlUF, 20%, 3000V 72982 878Y5S102M
C120 290-0536-00 CAP. ,FXD,ELCTLTtl0UF,20%,25V 90201 TDC106M025NLF
C130 283-0003-00 CAP.,FXD,CER DItO.OlUF, +80-20%, 150V 72982 855-547E103Z
C142 283-0626-00 CAP. ,FXD, MICA Dtl800PF,5%,500V 00853 D195E182J0
C146 283-0626-00 CAP. ,FXD,MICA Dtl800PF,5%,500V 00853 D195E182J0
C150 281-0182-00 CAP .,VAR,PLSTC tl.8-10PF,300V 34553 2222-809-05002
C151 281-0515-00 CAP.,FXD,CER DIt27PF,+/-1.35PF,500V 72982 302-OOOCOG0270J
C152 281-0564-00 CAP.,FXD,CER DIt24PF,5%,500V 72982 301-0OOCOG024OJ
C154 281-0579-00 CAP.,FXD,CER DIt21PF,5%,500V 72982 301-050COG0210J
C1S5 281-0184-00 CAP. ,VAR,PLSTCt2-18PF,500VDC 34553 2222-809-05003
C157 285-0596-00 CAP. ,FXD,PLSTCt0.01UF,l%,100V 56289 490P10311
C158 285-0719-00 CAP. ,FXD,PLSTCt0.015UF,5%,100V 56289 410P113
C168 283-0003-00 CAP.,FXD,CER DItO.OlUF, +80-20%, 150V 72982 855-547E103Z
C170 283-0003-00 CAP.,FXD,CER DItO.OlUF, +80-20%, 150V 72982 855-547E103Z
C175 290-0535-00 CAP .,FXD ,ELCTLT t33UF ,20% ,lOV 56289 196D336X0010KA1
C190 290-0527-00 CAP .,FXD ,ELCTLT t15UF ,20% ,20V 90201 TDC156M020NLF
C192 281-0661-00 CAP.,FXD,CER DIt0.8PF,+/-0.1PF,500V 72982 3O1-OO0C0GO808B
C195 290-0723-00 CAP. ,FXD,ELCTLT:150UF, 20%, 6VDC,DIPPED TANT 56289 196D157X0006PE3
Cl97 290-0525-00 BOlOlOO B019999 CAP. ,FXD,ELCTLTt4.7UF,20%,50V 56289 196D475X0050KA1
C197 290-0536-00 B020000 CAP. ,FXD,ELCTLTtl0UF,20%,25V 90201 TDC106M025NLF
C198 290-0536-00 BOlOlOO B019999 CAP ., FXD ,ELCTLT tlOUF ,20% ,25V 90201 TDC106M025NLF
C198 290-0527-00 B020000 CAP ., FXD .ELCTLT :15UF ,20% ,20V 90201 TDC156M020NLF
C199 290-0512-00 BOlOlOO B019999 CAP. ,FXD,ELCTLTt22UF,20%,15V 56289 196D226X0015KA1
C199 290-0535-00 B020000 CAP .,FXD ,ELCTLT t33UF ,20% ,lOV 56289 196D336X0010KA1
C208 281-0513-00 CAP.,FXD,CER DIt27PF,+/-5.4PF,500V 72982 301-000P2G0270M
C220 283-0065-00 CAP.,FXD,CER DItO.OOlUF, 5%, lOOV 72982 805-505B102J
C230^ 283-0065-00 CAP.,FXD,CER DItO.OOlUF, 5%, lOOV 72982 805-505B102J
C235^ 283-0065-00 CAP.,FXD,CER DItO.OOlUF, 5%, lOOV 72982 805-505B102J
C251^ 281-0546-00 CAP.,FXD,CER DIt330PF,10%,500V 72982 301-0O0X5PO331K
C253 283-0114-00 CAP.,FXD,CER DIt0.0015UF,5%,200V 72982 805-509B152J
C254 283-0114-00 CAP.,FXD,CER DIt0.0015UF,5%,200V 72982 805-509B152J
C257^ 283-0065-00 CAP.,FXD,CER DItO.OOlUF, 5%, lOOV 72982 805-505B102J
C260^ 285-0808-00 CAP. ,FXD,PLSTC tO.lUF, 10%, 50V 56289 LP66A1A104K004
C263 290-0536-00 CAP .,FXD ,ELCTLT tlOUF ,20% ,25V 90201 TOC106M025NLF
C265^ 283-0065-00 CAP.,FXD,CER DItO.OOlUF, 5%, lOOV 72982 805-505B102J
C270^ 290-0525-00 CAP .,FXD ,ELCTLT 14.7UF ,20% ,50V 56289 196D475X0050KA1
C285^ 283-0001-00 CAP.,FXD,CER DItO.OOSUF, +100-0%, SOOV 72982 831-559E502P
C287^ 283-0065-00 CAP.,FXD,CER DItO.OOlUF, 5%, lOOV 72982 805-505B102J
C291 285-1101-00 CAP., FXD,PLSTC tO. 022 UF, 10%, 200V 19396 PT485-022-200-10
C300 285-1101-00 CAP. ,FXD,PLSTCt0.022 UF, 10%, 200V 19396 PT485-022-200-10
C310 290-0536-00 CAP. ,FXD,ELCTLT tlOUF, 20%, 25V 90201 TDC106M025NLF
C312 290-0525-00 CAP. ,FXD, ELCTLT t4. 7UF, 20%, 50V 56289 196D475X0050KA1
C315 281-0205-00 CAP .,VAR ,PLSTC t5.5-6 5PF ,lOOV 80031 C010GA/60E
C316 285-1102-00 CAP .,FXD ,PLSTC 10.lUF ,20% ,lOOVDC ,REEL PKG 19396 PT720B104M
C320 290-0536-00 CAP .,FXD ,ELCTLT tlOUF ,20% ,25V 90201 TDC106M025NLF
C325 290-0512-00 CAP .,FXD .ELCTLT 122UF ,20% ,15V 56289 196D226X0015KA1
C330 283-0626-00 CAP. ,FXD,MICA Dtl800PF,5%,500V 00853 D195E182J0
^Remove for Option 2.
REV. CFEB. 1975 3-3