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