Power Supply; External Reference Input - Fluke PM6690 Service Manual

n
Adjustment
– Connect the probe tip to the center hole of J7.The sur-
rounding holes are suitable ground connections. See Fig-
ure 7-2.
– Adjust R196 until the pulses on the oscilloscope look
the same. See Figure 7-3. Use a screwdriver type of ad-
justment tool.
– Check Step Response X1, repeat Step Response X1 and
Linearity several times if necessary.
Repeat all steps above for Input B on the timer/counter. Con-
nect the probe tip to J9. Adjust R268.
Sensitivity
n
Setup
– Keep the settings on the counter from Linearity.
n
Adjustment
– Connect the DMM to the test points X34 (+) and X35
(-). See Figure 7-2.
– Adjust R197 until the DMM reads +17 ± 0.2 mV. Use a
screwdriver type of adjustment tool.
Don't forget to resolder the RF shield.

Power Supply

AC/DC BOARD
+5 V adj.
X9
J17
X26
X21
Figure 7-4
Adjusting +5 V and checking +15 V and -15 V.
n
Setup
– Do as described under Preparations above.
– Select default settings by keying in
®
Save/Recall Recall Setup
GND
RF SHIELD
®
USER OPT
®
Default
n
Adjustment
– Connect the DMM to test point X9 and ground (metal
shield lid over input amplifiers). See Figure 6-4 and
Figure 7-4.
– Adjust the +5 V trimmer on the AC/DC power supply
board with a screwdriver type of adjustment tool until
the DMM reads +5.10 ± 0.005 V.
– Check that the voltage from the AC/DC power supply at
X26 is +15 ± 0.7 V.
– Check that the voltage from the AC/DC power supply at
X21 is -15 ± 0.7 V.
Note: Adjusting the +5 V does not affect other settings in the
instrument.

External Reference Input

n
Setup
– Do as described under Preparations above.
– Select default settings by keying in
®
Save/Recall Recall Setup
– Connect a 1 MHz sinewave signal with amplitude 0.1
) into 50 W from an LF synthesizer to the
V (0.28 V
rms pp
external reference input. Use a 50 W termination on the
input.
– Set an oscilloscope with X10 probe to 10 ms/div,
100 mV/div (including probe) and AC coupling.
n
Adjustment
– Connect the probe to X19. There are two ground pads in
the vicinity. See Figure 7-5.
– Adjust C442 to maximum amplitude. Use a screwdriver
type of adjustment tool.
J22
AC/DC BOARD
Figure 7-5
Adjusting the external reference input.
®
USER OPT
®
Default
J25
J23 J26 J21
C442
GND
GND
X19
Power Supply 7-13