Amptec Research 620A-4 Manual

MODEL 620A-4 OPERATION/
MAINTENANCE MANUAL
Phone [phone] // Fax [phone]
www.amptec.com // [email protected]
REV. H

620A4
620A-4
TABLE OF CONTENTS
SECTION A: RECEIVING AND INITIAL INSPECTION
A-1 Introduction to the AMPTEC 620A-4
A-2 Receiving, Unpacking, and Initial Inspection
A-3 AC Battery Charger- Power Requirements
A-4 Setup and Use
A-5 Serial Number, Revision and Safety Board Approvals
SECTION B: IGNITERTESTER/
FAILSAFE OHMMETER SPECIFICATIONS
Table B-2. Specifications
SECTION C: OPTIONAL ITEMS AND ACCESSORIES
C-1 Available Accessories andOptions
C-2 Test Lead and Probe Sets
SECTION D: OPERATION AND USE
D-1 General
D-2 Front Panel
D-3 Rear Panel
D-4 4-Wire ResistanceMeasurement
D-5 Connections
D-6 Failsafe Operation
D-7 Battery Monitoring Circuitry
Figure D-1. Error Sources in Resistance Measurements

SECTION E: THEORY OF OPERATION
E-1 General
E-2 Troubleshooting
E-2-1 Localizing the problem
E-2-2 Component Replacement
E-3 Circuit Description
E-4 Analog-to-Digital Converter
E-4-1 Reference Voltage
E-4-2 LED Display
E-5 Ohms-to-DC Converter
E-5-1 Constant Current Source
E-5-2 Constant Current Circuit Operation
E-6 Failsafe Design
E-7 Ultra-Safe Power Supply Scheme
E-8 Relay Board General Operation
SECTION F: ROUTINE MAINTENANCE
F-1 General
F-2 Required Test Equipment
F-3 Calibration Procedure
F-4 Battery Replacement Instructions

620A-4
620A-4
The AMPTEC 620A-4 represents the latest in
ultra-safe Igniter Tester measurements. The
620A-4 (serial numbers 620A4-800 and higher)
Igniter Testers all use the same main printed
circuit board (PCB) referred to as Revision D.
Please check the Appendix of this manual for
any addendums that may apply to any
differences between the newer (rev D.) version
620A-4s and older (rev C.) 620A-4 Igniter
Testers with serial numbers less than 620A4-800
(i.e. 620A4- 525).
620A 4
0A
0A
A-1. Introduction to the AMPTEC
The AMPTEC 620A and the 630 Series Igniter
Testers/Failsafe Ohmmeters have become the
standard in the Safety Igniter Circuit Test
industry, and are designed to provide extremely
safe and reliable resistance testing of electrically
explosive devices (EEDs) or “current sensitive”
blasting devices. Approvals from various Safety
Boards include, the U.S. Air Force ( 620A-4) for
use on Non-Nuclear munitions provided the
appropriate T.O. is available. In addition the US
NAVAL ORDNANCE CENTER has approved
the AMPTEC 630AN, 630BN and 640N Igniter
Testers and other versions are pending. Some of
the devices the 620A-4 Failsafe Ohmmeter may
be used on include: fuses, squibs, igniters,
warheads, explosive bolts, rocket motor squibs,
drone parachute squibs, automobile air-bag
initiators and many others.
The AMPTEC 620A-4 is a 4-wire Kelvin (i.e.
eliminates contact resistance errors) failsafe
digital ohmmeter which has been designed to
use safe levels of DC current for its DC
resistance measurement. Failsafe Output
Circuitry proprietary to AMPTEC RESEARCH
ensures that test current levels do not exceed the
specified "failsafe current" in a worst-case
component failure situation. The failsafe feature
is tested in every instrument before shipment.
Should the rechargeable batteries reach a low
charge level, a minus sign will appear on the
display. The 620A-4 has a battery monitoring
circuit that indicates when to charge the
batteries.
A-2. Receiving, Unpacking, and Initial
Inspection
Should the AMPTEC shipping box appear
damaged upon arrival, request that the carrier's
agent (i.e. UPS) be present when the unit is
unpacked. If the 620A-4 appears damaged, the
carrier's agent should authorize repairs before
the unit is returned to the factory. Even if the
instrument appears undamaged, it may have
suffered internal damage in transit that may not
be evident until the unit is operated or tested.
If the unit fails to operate or fails to meet the
performance specifications
of Section B, notify the
carrier's agent and the
nearest AMPTEC Sales
Office. Retain the shipping
carton for the carrier's
inspection.
DO NOT return equipment
to AMPTEC RESEARCH or any of its sales
offices without first obtaining an RMA number
(Return Material Authorization). We need to
know who to contact and how to contact (i.e.
phone number and FAX number) in order to
properly coordinate the return of the repaired
AMPTEC product. After getting an RMA #
from AMPTEC, as an added precaution against
loss, be sure to also affix an ID label (contact
info) on the lid of the 62 Igniter Tester with
your phone number, name, address and FAX
number, and email address. By calling
AMPTEC RESEARCH, prior to returning
the 62 A-4, we can often troubleshoot and
identify the problem (i.e. battery loose in the
battery holder), over the phone.
620A-4

620A 4
620A-4
620A-4
620A-4
620A 4
620A-4
620A-4
A-3. AC/DC Battery Charger - Power
Requirements
The AMPTEC uses an internal
rechargeable heavy-duty nickel-cadmium
battery pack (4 D cells - 5.0 AHr).Replacement
batteries may be purchased, contact the sale
department at AMPTEC RESEARCH.
The battery charger is an external AC/DC
converter that plugs into a standard 115VAC
receptacle. The battery charger outputs 6.0
VDC @ 100 mA. The battery charger circuitry
is configured such that the 620A-4 main power
switch must also be in the “OFF” position.
The 620A-4 main power “On/Off” switch
design makes it impossible to be powered (in
operating - measurement mode) directly from
the AC/DC Battery Charger. When the
main power switch is in the “On” position the
The “OFF/ CHARGING” power switch position
is for use when the batteries need charging or
the 620A-4 is not in use. As mentioned earlier
the Battery Charger (115 VAC 60 Hz powered)
must be plugged into the 620A-4’s rear panel
charging jack to facilitate charging the batteries.
Although the batteries are fully charged prior to
shipment, it may be desirable to refresh the
charge for 24 hours before use. As a rule of
thumb, the 620A-4 requires twice as much time
to fully recharge as the amount of discharge
time. For example, if the instrument was used
continuously for 2 hours, the AC adapter must
be connected for 4 hours to fully restore the
charge.
620A-4
rear panel charging jack is electrically
disconnected from all 620A-4 circuitry.
A fully charged battery pack typically powers
the 620A-4 for approximately 8 hours before
requiring a recharge. AMPTEC installs a quality
set of 4 each Heavy Duty (5000 mAHr) Ni-Cad
batteries (option 620-BAT) set of 4 each. The
620A 4 will also operate on a 4000 mAHr D
cell Ni-Cads with a shortened operating time
between charges. Recharge time is typically
twice the “Power On” time. An “Overnight”
charge usually restores the 620A-4 to a “Fully
Charged” ready to use state.
If you turn on the 620A-4, and the display does
not come on, it may indicate the batteries need
charging.
The diagram on the right shows a rear panel
view of the 620A-4 Igniter Tester. To check the
fuse, remove the cap from the fuse holder. The
620A-4 uses a 2-ampere fast blow (3AG size).
Only replace with an identical rated fuse.
A spare or replacement battery charger (option
“620-DC”) or replacement set of 4 heavy duty
NICAD batteries (option "620-BAT") may be
purchased if needed, contact the sales
department at AMPTEC RESEARCH.
620A-4
620A-4
620A-4
620 Battery Charger
Option "620-DC"

Additional note - Main switch Power “O . n” When
the 620A-4 Igniter Tester is first turned on, the
unit briefly draws more internal power then cuts
back to less than 100 mA, after a few seconds.
The initial Power “On” battery drain is to heat up
the unit’s ovenized zener voltage reference. If the
“Low Battery” Indicator only comes on (i.e. 10 to
15 seconds) for a few seconds (i.e. when the
620A-4 is first turned on), then goes out, the
battery levels are starting to indicate the charge
level is starting to get lo w .
A-4. Setup and Use
Once the AMPTEC 620A-4 has had its batteries
charged for 12 to 24 hours it is ready for use. The
620A-4 only draws a little electrical power
(internal battery based) and generates virtually no
heat. Consequently, it may be used in any area
where the environment does not exceed the
specifications of Table B-2.
You may also have a test procedure or technical
order (T.O.) that has you inspect the calibration
sticker on the AMPTEC 620A-4 Igniter Tester to
determine that it is still valid, or within its
calibration interval. Avoid exposing the 620A-4 to
extreme temperatures, hot or cold which will affect
accuracy and shorten battery lifespan.
A-5 Serial Number, Revision and Safety Board
Approvals
The AMPTEC 620A-4 Igniter Tester represents
the latest in ultra-safe Igniter Tester resistance
measurements. The AMPTEC 620A-4 (serial
numbers 620A4-800 and higher) Igniter Testers
use the same main printed circuit board (PCB)
referred to as “Revision D”. This manual is
specific for the “Rev D” versions of the 620A-4.
The AMPTEC 620A-4 Igniter Tester is Mil Std
810 Method 511 Compliant for use in Explosive
Atmospheres Use to +50 degrees C.
This product is also
certified for Use in Explosive Fuel Air Mixtures per
Mil Standard 810 Method 511.

SECTION B - 620A-4 TESTER SPECIFICATIONS
20Ω/ 1 mΩ
200Ω/ 10 mΩ
2000Ω/ 100 mΩ
20 KΩ/ 1 Ω
5 mA / <8 mA
0.5 mA / 1.5 mA
50 µA / 150 µA
5 µA / 20 µA
Actual fail-safe currents vary with each instrument and may be ±20% from the typical value.
Table B-2. Specifications
Accuracy: (for 1 year @25°C ± 10°C)
20 ohm through 20Kohm ranges...............................±0.02% of reading ±0.02% of range
(i.e if you a using a 10.000 ohm Resistance Std. - the high limit for the 620A-4 is a ohm reading
and the low limit is ohm reading )
Temperature Range
Operating 0°C to 50°C
Storage -10°C to 70°C
Temperature Coefficient
20ohm through 20Kohm ranges ±0.002% per °C (from 0°C-15°C and 35°C-50°C)
Test Current Level
(Model 620A-4 version) 20ohm range is <5mA when operating normal, <8mA worst case when faulty
Instrument Display . . (20,000 count) 4½ digit Super Bright Light Emitting Diodes (LED)
Over-Range Indication .............................. ( select next higher range) 620A-4 Display flashes
Measurement Update Rate.........Approximately 300ms
Voltage Protection - Maximum Input........250VDC or ACpeak without damage
Open Circuit Current Source Compliance Voltage...........clamped at ~1.6 volts
Power.........(4 "D" 5.7AHr Heavy Duty) 1.2V rechargeable nickel-cadmium batteries
AC/DC Battery Charger (Option DC) provides 6VDC at 100mA nominal
Dimensions......................................................9.5" (24cm) W x 11" (27cm) D x 3"(8cm) H
Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 lbs net; 6.5 lbs shipping (not counting transit case and
accessories)
AMPTEC RESEARCH continually strives to make improvements with it's instrumentation and as such reserves the right for
general operation of its products and specifications to be subject to change on rare occasion.

SECTION C OPTIONAL ITEMS AND
ACCESSORIES
C-1. Available Accessories and Options
Listed below are the options available for use with the
AMPTEC 620A-4 Series FailSafe Ohmmeters.
Option 620-DC: Battery Charger
Option "DC" is an AC/DC converter that converts
115VAC line voltage to 6VDC at 300mA. One charger
is provided as a standard accessory with every 620A-4.
A 220 VAC 50 Hz powered Battery Charger adapter is
also available.
Replacement Batteries (Option 620-BAT)
The rechargeable Nicad batteries installed in the 620A
should provide years of trouble-free operation.
Replacement, however, will eventually be necessary.
The 620A-4 uses four 1.2V cells (Heavy Duty)
installed in the unit’s battery box. The batteries are
held in place by reusable tie-wraps. When ordering
replacement batteries, please specify AMPTEC option
"620-BAT" (a new set of 4 each NiCAD D cells will
be supplied).
Option 100: Carrying Case
Option "100" is a hardshell (impact resistant plastic)
shock absorbing foam lined meter and accessory
carrying case for the 620A-4 Ohmmeter with extra
room for test leads, battery charger, Kelvin Clip Test
Leads, Connector Adapters, single pointed probes and
operator manual etc.
Test Leads and Probes - All AMPTEC Igniter Tester
Leads and Probe sets are a minimum 48" length, (call the
Sales Department for any custom requirement).
Option “620RACK”: Rack Mount Adapter
Option "620RACK" includes an adapter tray that
allows any single AMPTEC 620 series tester to
be flush panel mount installed in a standard 19"
equipment rack.
C-2. Test Lead Sets and Probes
Option “290" Alligator Clip Lead Set
Option "290" is the recommended general
purpose Alligator Clip test lead set (red and
black) for models not having a 2.0 Ohm range
(due to a small resistance “offset” at the tips -
OK for 20 Ohm and higher range units ) . Option
“290" is supplied as a standard item with all
620A-4 (USAF version) ohmmeters as part of
the accessory package. Option “290" is a 48"
long cable set with dual banana plugs at one end.
The other end is terminated with an alligator clip
(red and black). Some “Squib Resistance” test
panel jacks have a plastic exterior with a
conductive socket center. These alligator clips
will measure (2 wire method) properly with
either upper or lower jaw connected to the
conductive socket center of plastic test panel
jacks. Kelvin Clips see Option “300” (using the
4-wire method) may have difficulty only if both
jaws (both upper and lower) do not make
electrical contact with the conductive “resistance
under test” center.
See next page for Kelvin Clips and other
accessories.

Option 300: 4-Wire Gold Plated Kelvin Lead Set
Option "300" is a general-purpose Gold-Plated
Kelvin four wire Leads for all AMPTEC 620 series
Testers. Kelvin clips provide a 4 wire Kelvin Gold
plated low thermal EMF connection (minimizes the
thermocouple effect) with most connections. The
Option “300” is the recommended test lead set for
any AMPTEC 620 Series Igniter Tester for
calibration or precision measurements (i.e
AMPTEC 620ES and 620RK). The 4 wire Kelvin
connection is important when measuring less than
1.0 ohm, and also automatically eliminates test
lead length offset and test lead contact resistance
errors. Option “300” is a shielded 48" lead set
terminated in ½" opening gold-plated Kelvin clips.
The option "300" can clip easily to wires, pins, and
medium size (up to ½" diameter conductors). The
dual banana plug ends connect directly to the 620
tester's front panel input terminals. (see Option
"320" for replacement Kelvin clip ends only)
Option 305: Banana-to-Banana Cable
Option "305" is a 48" shielded cable terminated in
dual banana plugs at both ends.
Option 320: Kelvin Clips
Option "320" are the gold-plated kelvin clips used
on the Option "300" cable set. These clips may be
used when making custom cables or when repairing
Option "300".
Option 360: Heavy-Duty Clips
Option "360" are Gold-plated jumper cable (large
jaws) type clips used on the Option "350" cable.
They are still Kelvin Clips because the insulated
upper and lower jaw (jumper cable appearance)
integrity is maintained. These may be used when
making custom heavy-duty cables that open to 1
1/2 " wide (i.e bolt heads).
Option 401: Handheld Single Probe Leads
Option "401" is a gold plated 620 series compatible
handheld probe (one black probe & one red probe) lead
set terminated in single points. The OP401 handheld
probes allow for easy access to connector socket wiring
(i.e. drone parachute squib sockets), recessed surfaces,
and parts (i.e. flares) that alligator clip can't reach.
Option 430 4000-ITS Mating Connector
AMPTEC offers an adapter connector that provides
equivalent circular socket connections (J9 Connector)
found on SIMPSON 4000 ITStm, and Alinco 101 series
Circuit Igniter Testers. In some applications (i.e. AIM-7
Missile harnesses) regular alligator clip leads cannot
properly connect to (screw on ring collar type
connectors) the small recessed electrical sockets inside a
connector harness that normally mates with the J9 style
connector. This item is included in the AMPTEC 620A-
4 NSN 6625-01-460-1499NM package. The “OP430” has
the AMPTEC 620A-4 Ohmmeter act as a 2-wire
ohmmeter from the OP430 connector out to the resistance
under test.
Option "247" - Isolated Continuous Operating Power -
Allows the AMPTEC 620A-4 series tester safely operate
continuously 24 hours a day 7 days a week. It includes an
isolated AC/DC wall adapter. This option uses a 3.0 KV
AC and DC isolated circuitry (i.e. medical grade DC to
DC convertor) to safely supply the 5.0 VDC needed to
operate the unit’s main measurement electronics
normally supply by batteries. This 3 KV medical grade
electrical isolation is the same safety isolation that cardiac
monitors use. They are AC line powered (i.e. 120 VAC)
while electrodes are connected to a human patients' chest.
The unit’s Safety Board Approved failsafe ohmmeter current
source is still running independent of the meter’s 3.0
KV DC supply isolation circuitry. UL 61010 AC Line Isolation
Test Report confirms Isolation to 3 KV AC input protection.
Option “500” Optically Isolated Analog Output
This rear panel mounted isolated analog DC Voltage
output signal is directly proportional to the 620 tester's
ohm display measurement. (i.e. 1.2345 VDC for 12.345
ohms). It is used to track 620 tester measurements for
datalogging purposes with an external device (i.e chart
recorder, system DMM with GPIB). Isolation protection
is rated @2000 volts peak to ensure any instrument
connected to "Option 500" has virtually no impact on
the 620 series tester's safety.

SECTION D - OPERATION AND USE
D-1. General Operation
This section contains operating instructions for the
AMPTEC 620A-4 Explosive Safety Igniter Tester. A
description of the front panel controls, connection
instructions and theory behind 4-wire Kelvin
resistance measurement is discussed in this section.
D-2. Front Panel Features and Operation
When the front panel power switch is in the “ OFF ”
or charging position, all power is removed from the
(output terminals) ohmmeter measurement circuitry
and the unit's internal battery pack is connected to
the rear panel charging jack (see diagram on next
page).
When the AMPTEC 620A-4 main power switch
(single throw double pole) is placed in the " ON " or
up position, the battery pack is disconnected from the
rear panel charging jack. The possibility of a voltage
between the device under test and a AC/DC Voltage
from the rear panel AC/DC Charging jack is
therefore eliminated. The operator need not be
concerned if the battery charger is plugged in while
making resistance measurements.
The AMPTEC 620A-4 ohms ranges are
manually selected by pressing the desired range
switch on the front panel. The range select
pushbutton for the
lowest resistance range 20 Ohms (left-most
resistance range button) indicated above the
range select buttons. When a given range is
selected (pushed in) the other range switches
"pop-out" and inform the user. It is obvious to
the 620A-4 user which resistance range has
been selected, as it is pushed in. Also note that
a resistance range should be selected after
powering up the 620A-4 to place it in an
operational mode. After turn "ON", a range
should be selected. If the resistance being
measured (including "Open Circuit/
Disconnected states") is a higher value than the
selected range, the instrument's display will
flash (blink), which indicates "overrange". If
the front terminals are disconnected, the
display may even wander around; this is normal
for the disconnected 4 Wire Kelvin "Open
Circuit" mode.
D-3 Rear Panel
Range Switch
Option "620-DC"
Fuse Holder
AMPTEC 620A-4 Range Switches shown
The fuse holder is mounted on the rear panel
of the unit. The fuse for the AMPTEC 620A-4
is a 2-ampere fast blow 3 AG type (1 ¼"
long).
This fuse is designed to protect the battery pack
from excessive currents. On some AMPTEC
620A-4 versions a fuse holder is mounted on

3
AMPTEC
MADE IN USA
RESEARCH www.amptec.com
CAGE CODE 1CRL2
PHONE [phone]
1
4 5
OP247 Adapter
FUSE - 2 ampere
3AG FAST BLO
RS232C
Interface
revision 2.0
VHI VLO IHI ILO
AMPTEC 620A-4 REAR PANEL configured with Option "232", "247"
2 (not part of NSN 6625-01-460-1499NM package)
6
620A-4 Rear Panel with options RS232C I/O
The AMPTEC 620A-4 rear panel (shown above) may
contain many optional jacks, terminals, labels, and
stickers. Only the option "247" Continuous Isolated
Operating Power DC Adapter (item #1) plugs in the rear
panel jack for isolation and to continuously operate the
unit.
Item 2 is the fuse holder - replace with a 2 ampere 3 AG
type fast blow fuse (rarely needs replacing).
RS232C Serial Interface - Item # 3 is the safety isolated
RS232C serial I/O (9 pin D type Sub-min connector).
This RS232C Interface provides the meter’s resistance
measurements to a PLC fitted with a serial port. The
RS232C protocol settings should be 9600 Baud, 8
Bits, No parity, 1 Stop Bit, 9 pin D Sub-min
connection.
RS232C Command Set (Option 232)
(NOTE: All front panel range buttons must be de-
selected (press in halfway) with all range pushbuttons
out to disable with local lockout and enable RS232C
control. Note commands are case sensitive.
C Continuous Read Mode - RS232C I/O outputs a
data string every A to D conversion cycle, approx. 2.5
times per second.
S Single Read Mode - RS232C I/O outputs a
single data string upon reception of a “R” command.
R Read - Commands RS232C I/O to output a
single data string (1 resistance reading).
r0 De-Selects all Ranges
r1 Selects the 20 Ohm Range
r2 Selects the 200 Ohm Range
r3 Selects the 2K Ohm Range
r4 Selects the 20K Ohm Range
V Version commands board to output the firmware
version string.
Data Format - The RS232C I/O outputs a data
string with the following format:
1.2345E+3 The measurement is always in Ohms
(where E+3 = 10+3scientific notation style) . The
Exponent is defined below. 1.2345E+3 = 1.2345
KOhms (where E+3=10+3). 1.3700E+1 = 13.700
Ohms (where E+1=10+1)
Range Exponent
20.0 Ohm E+1
200.0 Ohm E+2
2.0 K Ohm E+3
20 K Ohm E+4
An overrange condition is indicated by
9.9999Enn..Where nn is the selected resistance
range exponent. A Range Error is indicated by
x.xxxxERR.
Item # 4 is the unit’s serial number sticker.
Item # 5 is the gold plated 4 terminal rear terminal
strip (they are wired in parallel with the front termi-
nals). If a “2 wire ohms” connection is made then
the V high and I high terminals should be shorted
together, and the V low and I low terminals should
be shorted. The AMPTEC 620A-4 Voltage High,
Voltage low, Current high and Current low wires
are permanently connected to the gold plated rear
terminal strip ( seel labeled gold plated terminal
strip on rear panel).
Item #6 Calibration Sticker - If the calibration
due date has expired (1 year) AMPTEC or a Cal.
Lab can contact to re-certify the AMPTEC 620A-4
Explosive Safety Ohmmeter/Igniter Tester. contact
AMPTEC customer service 001-512-858- 4045.
AMPTEC 620A4
Serial # 620A4-1278
DATE
CA TED
DATE DUE
LIBRA
23JUNE2004 23JUNE2005
CERTIFIED BY
DWH
MODEL NUMBER
620A4
SERIAL NUMBER
620A4-1278

Charging Jack
The battery charging jack is a barrel type and is
located on the 620A-4 rear panel. The center
pin of the connector is positive. The charging
requirements of the internal battery pack are
6VDC @300mA. The correct charging voltage
is supplied by the adapter included with the
instrument. Additional AC/DC Battery
Chargers are available as Option "620-DC".
with the AMPTEC 620A-5, the
charging jack is mounted on the unit’s front
panel.
D-4. 4-Wire Resistance Measurement
The four-terminal configuration of the 620A-
4 eliminates errors normally caused by in
series test lead resistance and contact
resistances. In many applications the contact
resistance can exceed the value of the load by
several orders of magnitude.
The 620A-4 avoids this potential error source
by providing two terminals of constant current
and an additional two terminals for high
impedance voltage measurement. The constant
current can be thought of as a current loop that
overcomes lead length resistance and contact
resistance encountered (all series resistance)
along its loop path. The result is a fast, accurate
resistance measurement of the load,
independent of the resistance of the current
carrying leads.
Figure D-1 (above) illustrates the 4-wire
principle and how it is used to eliminate lead,
wire, and contact resistances as potential error
sources. The internal current source inherently
overcomes all series resistance (within
compliance voltage limits) and delivers a
precise constant current.
The internal high-impedance DVM senses the
voltage drop across the load. There is
negligible contact and lead resistance error
created by the voltage measurement because the
high input impedance of the DVM limits
current flow in the voltage leads.
LEAD RESISTANCE
*
IGNITER
*
VOLTAGE
LEAD RESISTANCE
*contact resistance
Figure D-1. Error Sources in Resistance
Measurements eliminated with Four Wire
Kelvin Connections
D-5. Connections
Connections are made to the front panel
terminals using a 4-wire configuration as
described in section 4-4. When using AMPTEC
test leads, the tabbed or negative (-) side of the
banana jack is plugged into the current
terminals of the 620A-4. This ensures that the
current is carried in the largest conductor and
that the voltage input is shielded. The 620A-4
features five-way gold plated input jacks. The
gold plating on the input jacks helps minimize
thermal EMFs. Wire connections often generate
a DC offset voltage with the contact between
two dissimilar metals (also called the thermo-
couple effect).
In addition, the 620A-4 five-way input jacks
allow for clean, simple connections to bare
wires, cables terminated with spade lugs, and
banana jacks.
V high I high
V low I low
620A-4 CLOSEUP - INPUT JACKS (4 wire connection)
LIMITING
RESISTOR
LIMITED
CONSTANT
CURRENT
SOURCE
FAIL-SAFE
OHMMETER
DIGITALVOLTMETER
4.8VDC
V
Cu
l
r
o
re
w
ntLoop
I low
I hi

All AMPTEC ohmmeters use a high
impedance voltmeter as part of the
resistance measurement process. This
voltmeter is a highly accurate and stable 4½
digit analog-to-digital converter (A to D).
Unless it is receiving a definite input signal,
the output reading of this A to D is
ambiguous. The display may indicate a
randomly wandering number, or it may
indicate an overrange condition. This
unpredictable display may make it seem to
appear that the instrument is experiencing
some sort of malfunction. It is, in fact, just a
characteristic of the voltmeter circuit and
should not be mistaken for a fault in the
instrument.
The display indications should be ignored
unless there is a definite measurement being
taken. If this wandering display is not
acceptable, the ohmmeter can be made to
indicate an overrange condition whenever
the terminals are open by using a 4-wire
Kelvin type lead set or by shorting the V
HI
and I
HI
terminals together.
The display should indicate a stable reading
when the test leads are securely attached to
the device under test. If the display appears
to be erroneous when connected to a load,
recheck the test leads for integrity and
cleanliness. If all external items appear to
be functioning properly, the problem may be
the ohmmeter. In this case, please contact
your local AMPTEC RESEARCH Sales
Office.
D-6. Failsafe Operation
The AMPTEC 620A-4 Series Igniter
Tester/Ohmmeter incorporate a proprietary
current source design that renders them
incapable of delivering excessive voltage or
current to the device under test. The typical
fail-safe current for each range is indicated
under the corresponding range switch on the
620A-4 front panel. Please refer to section 5-
6 for a technical description of the failsafe
circuitry.
Every 620A-4 Series Failsafe Ohmmeter is
thoroughly tested before it leaves the
factory. Every resistance range is tested and
calibrated (a U.S. N.I.S.T. Certificate of
Calibration accompanies every 620A-4).
As a further precaution the 620A-4 is
isolated from the AC line whenever the
POWER switch is in the ON position.
The 620A-4 receives its power from an
internal rechargeable battery pack. The
620A-4 must be in the OFF/CHARGING
position to charge the batteries.
D-7. Battery Monitoring Circuitry
If the low battery indicator LED is
continuously illuminated, readings should
not be trusted. An overnight recharge
should be performed before using the
620A-4 for critical testing.
It is possible for the user to receive a low
battery indication on a single range only
(particularly the 20 ohm range), while the
620A-4 remains well within operating
limits
on other ranges. Unless the user observes a
continuous low battery indication during
measurement, readings are still valid.
Notice for Cal Lab: The variable
potentiometer - trimpot RV3 is factory
adjusted to have the low battery indicator
come on at 4.50 VDC. To make this
adjustment, refer to Section F Routine
Maintenance for further details.

Replacing the AMPTEC 620A-4 Handles
The AMPTEC 620A-4 handle access screws are
located behind the black circular end caps (see
points A and B in the diagram below) at the joint
where the handles meet the case. Once the recessed
circular end caps have been removed (i.e. pop out with
screwdriver), you should be able to locate the spring-
loaded assembly with the access screws. Care should
be taken when unscrewing the handle access screws as
they hold back the spring-loaded lock-notch assembly.
Once the screws have been removed the spring-loaded
handle lock assembly will come apart (take note of the
parts and sequence of disassembly). Remove the
broken handle and replace with the new handle.
Return the spring-loaded lock-notch assembly to the
center of the rotating joint (points A and B below).
Replace the retaining screws back into the center of
the spring-loaded lock-notch assembly. Replace the
black end caps. The repair is complete.
Rotating the AMPTEC 620A-4 Instrument
Case and Handle Position
The AMPTEC 620A-4 handles position can be
easily adjusted or rotated with the simple
maneuver described below. There is a lock
notch rotating joint at points A and B below,
where the handles meet the case. Never force
the handles to rotate as this will damage the
lock notch rotating joint and break the handle.
By pulling outward firmly but not too hard, you
can release the lock rotating notch mechanism
inside the handle pivot assembly. While pulling
out at points A and B, rotate the handle position
to the desired location. If the handles do not
freely pivot/rotate, then slightly more pulling
force at points A and B may be required (to
unlock the notch mechanism). Once you have
rotated the handles to the desired angle simply
release the outward pull at points A and B. You
should then feel or hear a "click" as the handle
locks/notches into the new position.
A
B

SECTION E - GENERAL OPERATION ANDDESIGN
E-1. General
The AMPTEC RESEARCH 620A-4 Igniter
Tester is shown in block diagram form in Figure
E-1. All diagrams and information disclosed in
this chapter is proprietary and is included in
order to make troubleshooting to component level
possible.
The AMPTEC 620A-4 series Igniter Tester
uses modern solid-state semiconductors
exclusively and digital CMOS circuits
extensively to minimize power requirements
and make battery operation useful and
practical. AMPTEC also maintains a spare
parts inventory of all components found in the
620A-4 Tester and its customer service
department can also provide additional
assistance in the trouble shooting process.
E-2. Troubleshooting
Since the 620A-4 Tester is used to test potential
deadly explosive force detonators and warheads
of missiles etc., personnel that are not qualified
to make such electrical repairs on the 620A-4
Tester should not even attempt to remove the
calibration access screws or open the main
panel or effect any repair whatsoever.
Apparent 620A-4 Tester malfunctions can
sometimes be the result of bad test
lead/connection wiring, wrong connections,
misinterpretation of specifications, low battery
levels, and in rare cases due to an incomplete
understanding of the instrument and how to use it.
A thorough review of the operating instructions
for this instrument is recommended prior to any
component replacement. Check to be sure that
cables and other test equipment are in good
working order before attempting to troubleshoot
the 620A-4 series igniter tester.
If you turn on the AMPTEC 620A-4, and the
display does not come on, it may indicate the
batteries need charging, or fuse needs replacing.
If the 620A-4 exhibits problems that cannot be
eliminated by reviewing Chapters B and D, the
following guidelines have been established to
help solve the problem.
E-2-1. Localizing the Problem
Chapter D-2 discusses how to use the
Functional Test Section of the 620A-4 Tester to
help localize the problem. The key to successful
troubleshooting is to localize the problem to a
general electronic parameter as much as possible
before trying to pin the problem down to a
specific component. Certain questions should
be asked such as "Does the problem occur on all
ranges or on a specific range only?". If the
620A-4 Tester does not come on when powered
up, did you check the front panel fuse. The
power supplies for both the current source and
the digital voltmeter electronics are also one of
the first things that should be tested.
As it is not possible to anticipate all failure
modes of the 620A-4 series igniter tester,
servicing personnel should become familiar with
this section to gain a complete understanding of
the internal workings of the ohmmeter.
E-2-2. Component Replacement
If the malfunction is a faulty component, the
accuracy of the 620A-4 series igniter tester can
be maintained only if the 620A-4 is re-
calibrated following the component replacement
and the following precautions are taken:
Use only the specified component or its exact
equivalent. Spare parts can be ordered from
your nearest AMPTEC RESEARCH Service
Center or directly from the factory by referring
to the AMPTEC Stock Number listed in the
Parts Lists section at the back of this manual.
The highest quality 63/37 grade rosin core
electronic grade solder with a 50W or lower
maximum power soldering iron should be used.
Never use an acid core solder as corrosion of
components leads and PCB etch loss can occur.

When soldering, heat the PCB pad and the lead
of the component, not the solder. After several
seconds of the component lead in contact with
the hot soldering iron apply solder smoothly and
evenly onto the PCB pad and component lead
not the soldering iron. Do not touch or move
the replacement part until the solder has cooled.
Cold solder and bad solder joints can cause
more problems.
Use the chassis ground (connect to the common
terminal of the functional test section)
connection - i.e. connect to an earth ground to
avoid a static discharge to a static sensitive
component. Handle all 620A-4 internal
components as if they are static sensitive if you
are not sure.
See Next Page for Start of 620 Circuit Descriptions and Functional Diagrams

Figure E1 - 620 Conversion Timing Diagram
AFTER ZERO CROSSING,
ANALOG SECTION WILL BE
INAUTOZEROCONFIGURATION
NUMBER OF COUNTS TO ZERO CROSSING
PROPORTIONAL TO VIN
INTERNAL
LATCH
BUSY
OUTPUT
CLOCK
AZ
DEINT PHASE ll
INT PHASE ll
AZ PHASE l
ZERO CROSSING
DETECTED
INTEGRATOR
OUTPUT
ZERO CROSSING
OCCURS
POLARITY
DETECTED
E-3. Circuit Descriptions
The circuit descriptions which follow are
referenced to Figures E-1, E-2, E-3 and the
schematic diagrams at the back of this manual.
In the following descriptions, references to
integrated circuits are given in the form "IC201-
1", which refers to Integrated Circuit 201, pin 1.
E-4. Analog to Digital Conversion
The A to D conversion is done with a ICL8068
/ICL71C03 chip set. The ICL8068 takes care of
the analog part and the ICL71C03 takes care of
the digital part of the 4 ½ digit 20,000 count
dual slope conversion.
MODEL 620A-4 OHMMETER BLOCK DIAGRAM
COUNTS
PHASE I
PHASE II
PHASE III
4 ½ DIGIT
I0.00 I
I0.000
20.00I
ZERO
CROSSING
DETECTOR
COMPARATOR
CONTROL
LOGIC
COUNTERS
INTEG
BUFFER
INPUT
SWITCH
MULTIPLEXER
UUT
DISPLAY
DC CONSTANT
CURRENT SOURCE
RANGE
SWITCH

FIGURE E-2B. PHASE ll INTEGRATE INPUT
POLARITY
FF
3
CSTRAY
CREF
6
1
VIN
ZERO CROSSING
DETECTOR
COMPARATOR
_
A3
+
INTEGRATOR
_
A2
+CAZ
BUFFER
_
A1
+
1 µF
4
2
5
RINT CINT
FIGURE E-2C. PHASE lll AND DE-INTEGRATE
POLARITY
FF
3
CSTRAY
CREF
6
1
VIN
ZERO CROSSING
DETECTOR
COMPARATOR
_
A3
+
INTEGRATOR
_
A2
+CAZ
BUFFER
_
A1
+
1 µF
4
2
5
RINT CINT
FIGURE E-2D. PHASE lll AND DE-INTEGRATE
POLARITY
FF
3
CSTRAY
CREF
6
1
VIN
ZERO CROSSING
DETECTOR
COMPARATOR
_
A3
+
INTEGRATOR
_
A2
+CAZ
BUFFER
_
A1
+
1 µF
4
2
5
RINT CINT
Figures E2. Main Analog Section of DVM Circuit - IC1 and IC2
3
FIGURE E-2A. PHASE l AUTO-ZERO
6
1
VIN
CSTRAY
CREF
ZERO CROSSING
DETECTOR
COMPARATOR
_
A3
+
INTEGRATOR
_
A2
+CAZ
BUFFER
_
A1
+
1 µF
4
2
5
RINT CINT

Detailed Description
Analog Section
Figures E2 diagrams A thru D shows the
equivalent circuit of the analog section in 3
different phases of operation. The system will
perform conversions at a rate determined by the
clock frequency 40,002 clock periods per cycle.
(see Figure E1B shown earlier in this chapter for
details of conversion timing).
Auto-Zero Phase I (Figure E2A)
During the Auto-Zero, the input of the buffer is
connected to V REF through switch 2, and switch
3 closes a loop around the integrator and
comparator, the purpose of which is to charge the
Auto-Zero capacitor until the integrator output
does not change with time. Also, switches 1 and 2
recharge the reference capacitor to V REF.
Input Integrate Phase II (Figure E2B)
During Input Integrate the Auto-Zero loop is
opened and the Analog Input is connected to the
Buffer Input through switch 4 and C REF if the
input signal is zero, the buffer, integrator and
comparator will see the same voltage that existed
in the previous state (Auto-Zero). Thus, the
integrator output will not change but will remain
stationary during the entire input integrate cycle.
If V IN is not equal to zero, and an unbalanced
condition exists compared to the Auto-Zero
Phase, the integrator will generate a ramp whose
slope is proportional to V IN.
Deintegrate Phase II (Figures E2C and Figures
E2D)
During the Deintegrate phase, switch 5 is closed
and a voltage which is V REF more positive than
during Auto-Zero is impressed on the BUFFER
INPUT. Thus, the reference capacitor stores the
equivalent voltage. This returns the output of the
integrator to the zero-crossing point established in
Phase I. The time, or number of counts, required
to do this is proportional to the input voltage.
E-4-1. Reference Voltage
The precision reference voltage required to do
the A/D conversion is developed by IC201. The
zener voltage is attenuated to approximately -
0.5V. This voltage is applied to IC2-7.
E-4-2. LED Display
The output format from IC2 is in Binary Coded
Decimal (BCD) format. Each digit is scanned
for 10 clock pulses. The scan sequence is D5 D4
D3 D2 D1. This drives Q1 thru Q5, which in
turn drivers the seven segment displays. The
BCD data is converted to seven segment format
by IC4. When the 620A-4 electronics are in
open circuit or over-range mode the display
flashes “0000". IC5 is a 1 MHz oscillator
which is divided by 10 by IC6. The 100 KHz
clock output then goes to IC2.
E-5. Ohms-To-DC Converter
The ohms-to-DC converter generates a constant
current which is passed through the device under
test to develop the voltage measured by the A/D
converter.
E-5-1. Constant Current Source
The constant current source is composed of
IC201, IC202, Q202, D203 and their associated
components. The input to the constant current
source is approximately +1.05 volts, developed
at IC201-7 and connected to IC201-13 through
R209 and R210. The heart of the constant
current source is the voltage-to-current
converter. A simplified schematic of this circuit
is shown in Figure E-4 and described in Section
E-5-2. The amplifier of IC201-12 is an inverter,
and its output is applied to IC201-9. The
amplifier of IC201-8 has unity gain due to the
feedback through R213. Its output is applied
to the inverting input of IC202-3. The output of
IC202-6 provides feedback to the non-inverting
input of IC201-10. This circuit operates to
maintain the inverting input at IC202-3 and the
non-inverting input at IC202-2 at the same
potential.

E-5-2 Constant Current Circuit Operation
Assume that terminals Ihi and Ilo of Figure E-3
are shorted, and 1 volt is applied to Ein so that Ihi
is positive. Toequalize the 1 volt applied to Ein
, the inputs of IC202, IC201 must be driven to
zero. This condition occurs only when the
voltage drops across R212 and R222 are equal
to the drops across R213 and R221. For these
voltage drops to be equal, the output of IC202
must be at +1 volt. Since the output of IC201-8
must be zero, the drop across R213 is 0.5 volts,
making the inverting input 0.5 volts. The drops
across R212, R221 and R222 will also be 0.5
volts. Since the inputs to IC201 are essentially
equal, its output is zero (offset by the few
microvolts required to drive IC202 to +1 volt).
Under these conditions the sum of the voltages
across R212, R213, R221, and R222 equals the
sum of Ein plus the output of IC202.
Consider now that the short is removed from the
Ihi and Ilo terminals and a 100-ohm resistor (RL)
is connected in its place. The current through RL
increases the voltage at the input to IC201. A
balanced condition will be reached when the
output of IC201 is equal to the non-inverting
input of IC202. Again, this condition occurs
when the voltage drops across R212 and R222
are equal to the voltage drops across R213 and
R221. At this time the output of IC202 is 1.1
volts. The voltage drop across the range resistor
is 1 volt, just as it was when the output terminals
were shorted. The current through RL is 10
milliamperes, just as it was through the jumper
when the output terminals were shorted.
E-6. Failsafe Design
Reference to the AMPTEC 620A-4 Tester
Igniter Tester schematic will show that the
output of IC202-6 is applied to the base of
transistor Q202, which acts as a current limiter.
The worst-case component failure that could
occur in this circuit would be a Q202 short,
which would effectively connect the -5 volt
supply directly across R218, D202, the range
resistor and RL.
igniter could not exceed safe limits, because the
-5 volt and +5V supplies includes inherent
current limiting. Because of the design of both
supply isolation transformers T101 and T102,
the ±5 volt supplies can only deliver 20 to 25
milliamperes before the DC/DC converter
disengages, dropping the -5 volt output to zero.
See Section D-7.
The AMPTEC 620A-4 Tester is powered by a
rechargeable internal battery pack and cannot be
operated directly from the battery charging
adapter. This is to eliminate the possibility of
an electrical short to/from the AC line. Only
when the 620 POWER switch is in the “OFF/
CHARGING” position are the batteries
connected to front panel charging jack. When
the POWER switch is in the ON position, the
batteries are disconnected from the battery
charger and connected to the internal circuits of
the AMPTEC 620 Igniter Tester.
D203, however, acts as a 1.6 volt zener diode,
limiting the voltage that can appear across these
components. Even if every component in the
amplifier circuit shorted, the current through the