Fluke PM6690, Service Manual, Page: 48 / 200

Troubleshooting
General
The '90' is a highly integrated Timer/Counter in which a dedi-
cated FPGA counter circuit handles the signal processing, and
a microcontroller does the postprocessing and supervising
jobs. A number of additional functional units support these
basic tasks, for instance power supply, reference oscillator,
wideband input amplifiers, comparators, memory (RAM &
ROM), digital/analog converters, etc.
The units are treated from a troubleshooting point of view in
this chapter, which means that units described earlier should
be considered faultless before troubleshooting on units de-
scribed later.
WARNING: Live parts and accessible termi-
nals which can be dangerous to life are
always exposed inside the unit when it is
connected to the line power. Use extreme
caution when handling, testing or adjust-
ing the counter.
Where to Start
After reading the safety instructions, continue with this
chapter for troubleshooting and repair instructions. When you
have fixed the instrument, always do the Safety Inspection
and Test after Repair, as described later in this Chapter. Then
do the checks in Chapter 2, Performance Check. Recalibrate if
required by following the adjustment instructions in Chapter
7, Calibration Adjustments.
Logic Levels
The '90' contains logic of different families. The levels of the
standard families are listed in Table 6-1. In addition to these
families there is also low-level logic requiring lower supply
voltages, e.g. +3.3 V, +2.5 V and +1.8 V.
Required Test Equipment
To be able to test the instrument properly using this manual
you will need the equipment listed in Table 2-1. The list con-
tains the critical parameter specifications.
Operating Conditions
Power voltage must be between 90 V
AC
and 260 V
AC
. The in-
strument adapts automatically to the applied voltage.
Basic Functional Units
These are the units that are described in this chapter with ref-
erence to the page where the section starts:
•
Power Supply (p. 6-4)
•
Input Amplifiers (p. 6-8)
•
Timebase Reference Circuits (p. 6-12)
•
Prescalers (p.6-15)
•
Microprocessor & Memories (p. 6-15)
•
Microprocessor Bus & Interfaces (p. 6-21)
•
Measurement Logic (p. 6-27)
Power Supply
The DC voltages needed in the instrument are generated from
the three main voltages of the AC/DC module.
The instrument has only a secondary power switch, i.e. the
AC/DC module is always operating if line power is applied.
The three output DC voltages (+5 V, +15 V, -15 V) from the
module are present on the main circuit board. When switched
off, the instrument is in the standby mode. In this mode only
6-4 Troubleshooting
Positive
ECL
Negative
ECL
CMOS TTL
Supply voltage
+5 V -5 V +5 V +5 V
Signal ground
0 V 0 V 0 V 0 V
Input voltage
High, VIH
>+3.9 V >-1.1 V >+4 V >+2 V
Low, VIL
<+3.5 V <-1.5 V <+1 V <+0.8 V
Output voltage
High, VOH
>+4 V >-1 V >+4.9 V >+2.7 V
Low, VOL
<+3.3 V <-1.7 V <+0.05 V <+0.4 V
Bias ref. voltage,
VBB
+3.7 V -1.3 V - -
Table 6-1
Logic levels.