Input Amplifiers - Fluke PM6690 Service Manual

OFFCTRL
X22 X22
OF F
X23 X23
ON
X24 X24
ONCTRL
Figure 6-6
ON/OFF logic

Input Amplifiers

The input amplifiers for Channel A and Channel B are identi-
cal. A trigger level circuit belongs to each amplifier. The trig-
ger level is adjusted to match the hardware during the voltage
calibration procedure, see Chapter 7. Note that the input am-
plifiers must be adjusted according to Chapter 7 (step re-
sponse, sensitivity etc). The description refers to both chan-
nels (Channel B information within parentheses).
Recall the timer/counter default setting. Select the measure-
ment function Time A-B. Set both input channels to DC, Man
Trig = 0.000 V. No signals connected.
The RF shield must be removed before measuring on the input
amplifiers. It is soldered to two of the shield clips. Don't forget
to put the shield back afterwards and secure it by resoldering.
First measure some DC values. U3 pin 9 (pin 13) should be
near 0.000 V. The same applies to the trigger level, U3 pin 10
(pin 12). The voltage to ground at the point where R171
(R243) and R172 (R244) are connected should be approxi-
mately -0.8 V.
Connect a 1 kHz square wave with amplitude 1 V
Input A (Input B). Measure at the following points (see also
figure cc) and use the ground pads that are distributed over the
PC board:
6-8 Troubleshooting
X50 X50
R449 R449
10K 10K
R584 R584
47 47
2
D
3
CLK
R472 R472
47 47
R478 R478
C389 C389
1M 1M
X51 X51
100NF 100NF
POWER-ON RESET CKT
in 1 MW to
pp
ON H
OFF L
U29A U29A
5
Q R466 R466
6
Q
1K 1K
LVC74A LVC74A
+5VU
R479 R479
220 220
R480 R480
C197 C197 U29C U29C
470 470
100NF 100NF LVC74A LVC74A
Test Points
R140 to R141 (R212 to R213)
R156 to C109 (R229 to C140)
U1 pin 2 (U2 pin 2)
U1 pin 3 (U2 pin 3)
U1 pin 6 (U2 pin 6)
R151 to R157 (R223 to R230)
U3 pin 9 (U3 pin 13)
R309 and R314 (R313 and R315)
R301 both sides (R304 both sides) LVPECL levels 1.6 V and
Test the trigger level by manually setting the following trigger
levels. Check the voltage at X6 (X7) and U3 pin 10 (pin 12).
Set Level
+1 V
+4 V
-4 V
-1 V
Set the timer/counter to default. Select the measuring function
single period. Connect the 1 kHz square wave to channel A
(B). Measure with oscilloscope at X6 (X7). See figure dd for a
typical signal.
If any repair work has been done on the input amplifiers, both
adjustment and voltage calibration must be performed after-
wards. If any repair work has been done on the trigger level
circuits, at least voltage calibration must be performed after-
wards. See Chapter 7.
+5VU
ON L
OFF H
+5VU
R593 R593
1K 1K
R453 R453
1K 1K
R592 R592
1K 1K
Q47 Q47 Q58 Q58
BC847B BC847B BC847B BC847B
A & B are control
signals to PSU
switches and
dummy load.
Approximate Voltage
1.00 V
pp
0.40 V
pp
0.20 V
pp
0.20 V
pp
-1.00 V
dc
0.40 V
pp
0.40 V
pp
ECL levels -1.0 V and -1.7 V
2.6 V
Approximate Voltage
+0.41 V
+1.65 V
-1.65 V
-0.41 V
A
ON H
OFF L
B