Fluke PM6690, Service Manual, Page: 77 / 200

the ON/OFF circuitry and the optional oven oscillator are
powered.
The AC/DC module should not be repaired. Not even the
built-in fuse should be replaced. Built-in circuits protect
against external overloads, so a blown fuse indicates that a se-
vere internal fault has occurred. Replace the complete module
instead.
Test the AC/DC module by measuring the three DC voltages
in TP9 (+5.1 ± 0.05 V), TP21 (-15 ± 1 V) and TP26
(+15 ± 1 V) on the main circuit board. See Figure 6-55. Note
that there is a +5 V trimmer on the module. If the +5 V is not
correct, disconnect the cable to J17 on the main circuit board.
Put a 47
W
1 W resistor on the connector according to Figure
6-55. Measure on the con5.1 ± 0.05 V, +15 ± 1 V and
-15 ± 1 V.
It can be difficult to measure the resistance in the supply con-
nector J17 on the main circuit board, because charges are kept
by capacitors some time after line power is removed. Remove
the cable from the AC/DC module. The resistance between
+5 V and ground should be approximately 700
W
. See Figure
6-55. In a timer/counter with all capacitors uncharged, +15 V
and -15 V should be >1 M
W
.
Another way to test J17 is to connect 3 DC voltages from a
separate bench power supply directly to J17 (suitable connec-
tor MOLEX 09-91-0600). See Figure 6-55. The currents
drawn from the different supply voltages depend on options
installed. Before making this measurement, you should re-
move any prescaler option.
A timer/counter without options gives the following typical
results:
+5 V 0.7 A
+15 V 0.3 A
- 15 V 0.4 A
The oven oscillator increases the + 15 V current between
0.35 A (cold) and 0.1 A (warm).
A block diagram of the secondary power supply is shown in
Figure 6-54. All secondary voltages are switched off in
standby mode 12 VU for the optional oven oscillator.
The ON/OFF circuitry controls the ON and OFF of the sec-
ondary voltages. Its own supply voltage is always ON, as long
as the instrument is connected to line power. See Figure 6-59.
Troubleshooting 6-33
X2
+1.8 V
U118
DC/DC
+3.3 VD
U128
DC/DC
+5 VD
U127
SWITCH
X14
X11
+3.3 VA
U126
LINEAR
X12
DUMMY
LOAD
Q56, Q57
ON/OFF
LOGIC
Q46, Q47,
Q58, U29
X22
X24
+12 VA
U129
LINEAR
+15 VD
Q49, Q50
SWITCH
X15
+12 VU
U130
LINEAR
X17
FAN
Q48, U30
+15 VU
+5 VU
J28
X16
m
PROC.
X10
-12 VA
U123
LINEAR
-5.2 VA
U124
LINEAR
X8
X62
X25
-15 VU
J17:2,3
J17:1
J17:6
FROM
AC/DC
M
ODULE
+5 VA X13
m
PROC.
KEY
-5.2 VI
Q51, U35
LINEAR
-2.1 V
Q17, U6
LINEAR
Figure 6-54
Power distribution
47
W
red brown brown black black blue
AC/DC
M
ODULE
CABLE
ASSEMBL
Y
PCB PIN CONNECTOR J17
X26
X9
X21
+15 V +5 V +5 V GND GND -15 V
DUMMY LOAD
1
6
Figure 6-55
Dummy load connection.