Fluke PM6690 Service Manual page 49

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.
blue black
PCB PIN CONNECTOR J17
X9
6
X21
-15 V GND
Figure 6-2
Dummy load connection.
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 6-2. 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 Fig-
ure 6-2. Measure on the connector +5.1 ± 0.05 V, +15 ± 1 V
and -15 ± 1 V.
ON/OFF
LOGIC
Q46, Q47,
Q58, U29
J17:2,3
+5 VU
J17:1
+15 VU
J17:6
-15 VU
-5.2 VA
U124
LINEAR
Figure 6-1
Power distribution
black brown brown red
DUMMY LOAD
47 W
1
X26
GND +5 V +5 V +15 V
X22
DUMMY
+3.3 VA
LOAD
U126
X24
Q56, Q57
LINEAR
+15 VD
Q49, Q50
X15
SWITCH
+12 VU
X17
U130
LINEAR
mPROC.
-12 VA
X8
U123
X10
LINEAR
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 Fig-
ure 6-2. In a timer/counter with all capacitors uncharged, +15
V and -15 V should be at least MW.
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 6-2. The currents drawn
from the different supply voltages depend on options in-
stalled. Before making this measurement, you should remove
any prescaler option.
A timer/counter without options gives the following typical
results:
+5 V 0.7 A
+15 V 0.25 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 6-1. All secondary voltages are switched off in standby
mode except +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 6-6.
X11
X12
+5 VD +3.3 VD
U127 U128
SWITCH DC/DC
+12 VA X16
U129
LINEAR
J28
FAN
Q48, U30
-5.2 VI
-2.1 V
Q51, U35
Q17, U6
X62
LINEAR
LINEAR
X14 X2
+1.8 V +2.5 V
U118 U117
DC/DC DC/DC
+5 VA X13
X25
Troubleshooting 6-5
X1