Amptec Research 620A SERIES User manual

T.O 33D5-14-52-1
OPERATION / MAINTENANCE MANUAL
AMPTEC RESEARCH MODEL 620A-4
FAILSAFE DIGITAL OHMMETER/
IGNITER CIRCUIT TESTER
NSN 6625-01-460-1499AQ
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Published under authority of the Secretary of the Air Force
18 JULY 2000
ORIGINAL

RESEARCH
AMPTEC
OPERATION / MAINTENANCE MANUAL
Rev. D
AMPTEC RESEARCH
620A SERIES
Failsafe Digital Ohmmeter
10900 Research Blvd., PMB1, Suite # 160 C
AUSTIN TX 78759 USA CAGE CODE 1CRL2
Phone [phone] (from the USA), FAX [phone] (from the USA) or [phone]
website http://www.amptec.com email [email protected]
NSN 6625-01-460-1499NM
20 200 2000 20K OHMS
8mA 1.5mA 150µA 20µA

A MESSAGE FROM THE PRESIDENT
______________________________________________________
We at AMPTEC RESEARCH would like to thank our customers for selecting our
Failsafe Igniter Tester/ Digital Ohmmeter. Over the past 36 years our experienced
engineering staff has designed, manufactured and supplied earlier versions similar to
the AMPTEC 620 and 630 Series Igniter Testers. AMPTEC’s Igniter Tester Safety Approvals include the
U.S. NAVY for the “TOMAHAWK CRUISE MISSILE” and the “STANDARD MISSILE”, the U.S.A.F. for
the AIM-9 “ SIDEWINDER”, AIM-7 “SPARROW” and AIM-120 “AMRAAM”. We value our customers
trust and are looking forward to supporting any new requirement(s) you may have ....
Jim L. Griffith - President AMPTEC RESEARCH
______________________________________________________
AMPTEC RESEARCH certifies that this instrument has been tested and inspected to
meet published specifications as found in this manual on the date stated on the attached
N.I.S.T. Certificate. AMPTEC RESEARCH further certifies that its calibration measurements are
traceable to the U.S. National Institute of Standards and Technology.
________________________________________________________
AMPTEC’s permission and a return authorization (RMA) number must be obtained
directly from our customer service department (via phone, FAX, or email) for repairs
(warranty or otherwise). We must issue you an RMA number so we can keep track of the instrument and it’s
owner (i.e. who to contact).
. AMPTEC RESEARCH will repair or replace the instrument during the warranty period, provided
that it is returned to AMPTEC RESEARCH ( ). No other warranty is expressed or implied.
We are not liable for consequential damages. No liability will be accepted if the product is returned
without such permission.
Some AMPTEC products may have their design frozen, and no changes will be made without prior notice to
the proper authority. Through out this manual, reference is made to the AMPTEC 620 Series Igniter Testers.
Many versions of the 620 Igniter Tester are specifically developed to meet customer requirements. The
specification, operation, drawing and schematic sections of this manual contain the unique detail that define
the 620-xx. Due to continual product refinement, AMPTEC RESEARCH reserves the right on rare
occasions to change any of its products specifications.
U.S. N.I.ST. CALIBRATION CERTIFICATE
AMPTEC 620A IGNITER TESTER WARRANTY
The warranty period for this instrument is 1 year from when it was first
shipped
freight prepaid

TABLE OF CONTENTS
_______________________________________________________________
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 and Revision
Table B-2. Specifications
C-1 Available Accessories and Options
C-2 Test Lead and Probe Sets
D-1 General
D-2 Front Panel
D-3 Rear Panel
D-4 4-Wire Resistance Measurement
D-5 Connections
D-6 Failsafe Operation
D-7 Battery Monitoring Circuitry
Figure D-1. Error Sources in Resistance Measurements
SECTION A: RECEIVING AND INITIAL INSPECTION
SECTION B: 620A 4 IGNITER TESTER /
FAILSAFE OHMMETER SPECIFICATIONS
SECTION C: OPTIONAL ITEMS AND ACCESSORIES
SECTION D: OPERATION AND USE

SECTION E: THEORY OF OPERATION
SECTION F: ROUTINE MAINTENANCE
: COMPONENT PARTS LIST
: SCHEMATICS AND DIAGRAMS
: UPDATES, MODIFICATIONS AND ADDENDUMS
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
F-1 General
F-2 Required Test Equipment
F-3 Calibration Procedure
F-4 Battery Replacement Instructions
620-400 620A Ohmmeter (Revision D) Parts/Assembly list
Diagram 620A-600 620A Main PCB Assembly - (Reference Designator Map)
Diagram 620-601-2 Display PCB Assembly
Diagram 620-070 Main PCB Electrical Schematics
(Revision D) 620-070 (DVM1) and 620-070 Current (I source) Source
APPENDIX
SECTION A
SECTION B
SECTION C

A-1. Introduction to the AMPTEC 620A-4
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 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.
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).
Should the rechargeable batteries reach a low
charge level, a minus sign will appear on the
display. The 620A-4 has a
circuit that indicates when to charge the
batteries.
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.
or any of its sales
offices 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, against
loss, be sure to also affix an ID label (contact
info) on the lid of the 620A Igniter Tester with
your phone number, name, address and FAX
number, and email address.
we can often troubleshoot and
identify the problem (i.e. battery loose in the
battery holder), over the phone.
battery monitoring
A-2. Receiving, Unpacking, and Initial
Inspection
DO NOT return equipment
to AMPTEC RESEARCH
By calling
AMPTEC RESEARCH, prior to returning
the 620A,
without first obtaining
as an added precaution

A-3. AC/DC Battery Charger - Power
Requirements
plugs into a standard 115VAC
receptacle
The AMPTEC 620A 4 uses an internal
rechargeable heavy-duty nickel-cadmium
battery pack (4 D cells - 5.7 AHr). Replacement
batteries may be purchased, contact the sale
department at AMPTEC RESEARCH.
The battery charger is an external AC/DC
converter that
. The battery charger provides 6VDC
@ 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 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 620A-4 main power
switch is in the “On” position the rear panel
charging jack is electrically disconnected from
all 620A circuitry.
A fully charged battery pack typically powers
the 620A for approximately 8 hours before
requiring a recharge. AMPTEC installs a quality
set of 4 each Heavy Duty (5700 mAHr) Ni-Cad
batteries. The 620A 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 Igniter Tester. To check the
fuse, remove the cap from the fuseholder. The
620A uses a 2 ampere Slow Blo (3AG size).
Only replace with an identical rated fuse.
A spare or replacement battery charger (option
“DC”) may be purchased if needed, contact the
sales department at AMPTEC RESEARCH.
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 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 in order to fully restore
the charge.
Fuse Holder with Fuse
Charging Jack

Additional note - Main switch Power “On”
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).
. When
the 620A 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
is first turned on), then goes out, the battery levels
are starting to indicate the charge level is starting
to get low.
Once the AMPTEC 620A has had it’s 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 Igniter Tester to
determine that it is still valid, or within its
calibration interval. Avoid exposing the 620A to
extreme temperatures, hot or cold which will affect
accuracy and shorten battery life-span.
The AMPTEC 620A 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.
A-4. Setup and Use
A-5 Serial Number and Revision

SECTION B - 620A 4 TESTER SPECIFICATIONS
______________________________________________________________
(for 1 year @25°C ± 10°C)
AMPTEC 620A-4 Igniter Tester...........±0.02% of reading ±0.02% of range
(i.e. if you were using a perfect 10.000 Resistance Std.)
- the high limit for the 20.0 Ohm range is a ohm reading
- the low limit for the 20.0 Ohm range is a ohm reading
any readings beyond the limit examples above would be “Out of Specification”
Operating 0°C to 50°C
Storage -10°C to 70°C
±0.002% per °C (from 0°C-15°C and 35°C-50°C)
(Model 620A-4 version) 20 range is <5mA when operating normal, <8mA worst case when faulty
. . (20,000 count) 4½ digit Super Bright Light Emitting Diodes (LED)
. . . ..( or select next higher range) 620A Display flashes
.....Approximately 300ms
....250VDC or AC without damage
.....clamped at ~1.6 volts
....(4"D"5.7AHr Heavy Duty) 1.2V rechargeable nickel-cadmium batteries
provides 6VDC at 100mA nominal
.........9.5" (24cm) W x 11" (27cm) D x 3"(8cm) H
............4lbs net; 6.5 lbs shipping (not counting transit case and accessories)
Table B-2. Specifications
Accuracy:
Temperature Range
Temperature Coefficient
Test Current Level
Instrument Display
Over-Range Indication
Measurement Update Rate
Voltage Protection - Maximum Input
Open Circuit Current Source Compliance Voltage
Power
AC/DC Battery Charger (Option DC)
Dimensions
Weight
W
W
i0.006
9.9994
open circuit condition
peak

SECTION C OPTIONAL ITEMS AND
ACCESSORIES
______________________________________________________________
C-1. Available Accessories and
Options
Listed below are the options available for
use with the AMPTEC 620A Series FailSafe
Ohmmeters.
Option 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. A 220
VAC 50 Hz powered Battery Charger
adapter is also available.
Replacement Batteries
The rechargeable Nicad batteries installed in
the 620A should provide years of trouble-
free operation. Replacement, however, will
eventually be necessary. The 620A uses
four 1.2V cells (5.7 AHr recommended)
installed in a reusable battery box. The
batteries are held in place by reusable tie-
wraps. When ordering replacement
batteries, please specify AMPTEC Stock
#0E-10117, quantity four (4).
Option 100: Carrying Case
Option "100" is a hardshell (impact resistant
plastic) shock absorbing foam lined meter
and accessory carrying case for the 620A
Ohmmeter with extra room for test leads,
battery charger, 4000ITS Connector
Adapter, single pointed probes and operator
manual etc.
Option RACK: Rack Mount Adapter
Option "RACK" includes an adapter tray
that allows the 620A to be installed in a
standard 19" equipment rack.
C-2. Test Lead and Probe Sets
Option “290" Alligator Clip Lead Set
Option "290" is the recommended general
purpose Alligator Clip lead set (red and
black) . 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 (using the 4 wire
method) may have difficulty because both
jaws (both upper and lower) must make
electrical contact with the conductive socket
center.
The AMPTEC 620A-4 ohmmeter as the
NSN 6625-01-460-1499NM with accessory
package includes : a set of Alligator Clip
Leads (option 290), a N.I.S.T. traceable
calibration certificate, battery charger
(option DC), Dual banana to 2 pin circular
ring Plug Adapter (option 430), hardshell
foam lined transit case (option 100) for the
meter and accessories and operator /
maintenance manual shipped as a complete
package.
Option 300: 4-Wire Kelvin Lead Set
Option "300" is a general purpose Kelvin
four wire lead set for the 620A series tester.
Option 300 is a shielded 48" lead set
terminating in ½" opening gold plated
copper Kelvin clips. The option "300" can
clip easily to wires, pins, and medium size
(up to ½" diameter conductors). The dual
banana plugs terminations connect directly
to the 620A's front panel input terminals.
(see Option "290" for a photo of the banana
plug ends)
Option 305: Banana-to-Banana Cable
Option "305" is a 48" shielded cable is term-
inated by 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 401: Handheld Single Probe
Leads
Option "401" is a 620A 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 leads
simply would not work well with.
Option 430 4000-ITS Mating Connector
AMPTEC offers an adapter connector that
provides equivalent circular socket con-
nections found on SIMPSON 4000 ITStm,
and Alinco 101 series Circuit Igniter
Testers.

The diagram below shows that the backside
of the OP430 connector adapter has a PVC
plastic tube filled with an insulating casting
resin. The hardened tube provides additional
mechanical support while handling and
connecting the mating connector.
The Safety Board (U.S.A.F. Non-Nuclear
Munitions ) approved adapter (OP430) has
dual banana leads which provide a 4 wire
Kelvin connection from the adapter to the
620A Ohmmeter (No lead resistance errors).
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 mates with the SIMPSON
4000ITS or Alinco 101 series Igniter
Tester(s). The “OP430" Adapter mimics the
older 4000ITS Ohmmeter (pin into socket)
connection.
The “OP430” enables the AMPTEC 620A
Ohmmeter to act as a 2- wire ohmmeter
from the OP430 connector out to the
resistance under test, just as the 4000ITS
Ohmmeter or Alinco 101 series Igniter
Testers would perform.
For the most recent information regarding
this adapter, related probes and accessories
please refer to the accessories section of
AMPTEC’s website found at
“http://www.amptec.com” .
Option 500 - Isolated Analog Output
This rear panel mounted isolated analog
output provides an isolated analog DC
Voltage output signal that is directly
proportional to the 620A displayed
measurement (i.e. 0.5 VDC for 5.000 ohms,
1 VDC for 10.000 Ohms). Option 500 is
useful for tracking 620A Igniter Tester
measurements for datalogging purposes with
an external device (i.e. chart recorder, or
system DMM with GPIB). The isolation
(OP500) provided is rated @2000 Volts
peak, to insure that any datalogging
instrument connected to OP500 interface has
virtually no chance of adversely effecting or
impacting the 620A Igniter Tester’s safety.
AMPTEC welcomes new requirements.
You may contact the sales department at
AMPTEC RESEARCH if you need a special
probe, adapter, or option not listed in this
manual.

SECTION D: OPERATION AND USE
_____________________________________________________________
D-1. General
This section contains complete operating
instructions for the AMPTEC 620A Series
Failsafe Ohmmeters. A description of the
front panel controls, connection instructions,
and the theory behind 4-wire resistance
measurement is discussed in this section.
D-2. Front Panel
» Power Switch
When the front panel power switch is placed
in the OFF/CHARGE position, all power is
removed from the output terminals. While in
the “OFF” switch position, the battery pack
is connected to the rear panel charging jack
(see diagram next column).
When the front panel power switch is placed
in the ON position, the battery pack is
disconnected (isolated) from the rear panel
charging jack. The possibility of a voltage
between the device under test and AC Power
ground is therefore eliminated. The operator
need not be concerned if the battery charging
adapter is plugged in while making
resistance measurements.
Range Switch
The AMPTEC 620A input range is selected
by depressing the desired button on a gold
plated multi-station interlocking pushbutton
array located on the right-hand side of the
front panel. The pushbutton for the lowest
resistance range is nearest to the POWER
switch.
D-3. Rear Panel
Fuseholder
The fuseholder is mounted on the rear panel
and contains a 2 amp in-line fuse. This fuse
is designed to protect the battery pack from
excessive charging currents. On some
620A versions a fuseholder is mounted on

the front panel. Replace blown fuses with the
same type and rating only!
The battery charging jack is a barrel type and is
located on the 620A 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 "DC".
The four-terminal configuration of the 620A
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 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 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.
Charging Jack
D-4. 4-Wire Resistance Measurement
As a
side note, with the AMPTEC 620A-5, the
charging jack is mounted on the unit’s front
panel.
current loop
Figure D-1. Error Sources in Resistance
Measurements eliminated with Four Wire
Kelvin 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
. This ensures that the
current is carried in the largest conductor and
that the voltage input is shielded. The 620A
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 five way input jacks allow
for clean, simple connections to bare wires,
cables terminated with spade lugs, and banana
jacks.
D-5. Connections
negative (-) side of the
banana jack is plugged into the current
terminals of the 620A
620A CLOSEUP - INPUT JACKS (4 wire connection)
*contact resistance
4.8 VDC
BATTERY
DC CURRENT
LIMITED
CONSTANT
CURRENT
SOURCE
VOLTAGE
LIMITING
RESISTOR
IGNITER
Current Loop
LEAD RESISTANCE
LEAD RESISTANCE
FAIL-SAFE
OHMMETER
DIGITAL VOLTMETER
Vhi
Ihi
I
low
Vlow
V high I high
V low I low
*
*

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 VHI
and IHI 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 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 front panel. Please refer to section 5-6
for a technical description of the failsafe
circuitry.
Every 620A 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).
As a further precaution the 620A is isolated
from the AC line whenever the POWER
switch is in the ON position.
The 620A receives its power from an
internal rechargeable battery pack. The
620A must be in the OFF/CHARGING
position in order 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
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 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.

SECTION E THEORY OF OPERATION
_____________________________________________________________
E-1. General
The AMPTEC RESEARCH 620A Fail Safe
Ohmmeter is shown in block diagram form
in Figure E-1. All information disclosed in
this section is proprietary and is included in
order to aid in troubleshooting to component
level possible.
The AMPTEC 620A uses solid-state
semiconductors exclusively and CMOS
circuits extensively to minimize power
requirements and make battery operation
practical.
E-2. Troubleshooting
Malfunctions are often the result of
misinterpretation of specifications or due to
an incomplete understanding of the
instrument. 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. If the 620A exhibits
problems that cannot be eliminated by
reviewing Sections B and D, the following
guidelines have been established to help
solve the problem.
E-2-1. Localizing the Problem
The key to successful troubleshooting is to
localize the problem 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?". The power supplies are also one of
the first things that should be checked.
Since it is not possible to anticipate all
failure modes of the 620A, 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 problem has been identified as a faulty
component, the accuracy of the 620A can be
maintained only if 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.
Refer to the AMPTEC Components
Parts List Stock Number described in
the Appendix Section A pages in the
back of this manual.
* Use only 63/37 grade rosin core
electronic grade solder with a 50W
or lower maximum power soldering
iron.
* When soldering, heat the terminal of
the component, not the solder.
Apply solder smoothly and evenly.
Do not move the component until the
solder has cooled. Bad solder joints
can cause more problems!
* Static sensitive parts require special
handling procedures. Always treat
an unknown part as if it were static
sensitive.

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 Converter
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.

Detailed Description
Analog Section
Figures E2 diagrams A thru D shows the
equivalent circuit of the analog section in
three 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 section 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, to charge the Auto-Zero
capacitor until the integrator output dose 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 umbalanced 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 referance
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 drives the seven
segment displays. The BCD data is
converted to seven segment format by IC4.
When the 620A 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 on the next page and described
in Section E-5-2 (below). The amplifier of
IC201-12 is an invertor, 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-
4 are shorted, and 1 volt is applied to Ein so
that Ihi is positive. To equalize 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 RLincreases the voltage at
the input to IC201. A balanced condition
will be reached when the output of 1C201 is
equal to the non-inverting input of 1C202.
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 RLis 10 milliamperes, just
as it was through the jumper when the
output terminals were shorted.
E-6. Fail-safe Design
Reference to the Model 620A schematic will
show that the output of IC202-6 is actually
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. 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
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.
E-7. Power Supplies
The Model 620A is powered by a
rechargeable internal battery pack and
cannot be operated directly from the battery
charging adapter. This is to prevent the
possibility of a short to the AC line. When
the POWER switch is in the OFF or
CHARGING position, the batteries are
connected to the rear panel charging jack to
allow for recharge. 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
620A.