Service Model 8903B
7. Connect the voltmeter to pin 6 of U12. The voltmeter should read between -2.5 to -2.0 Vdc.
Hint: The emitter
of
Q2B should be approximately -640 mVdc.
Hint: At this point, operation of the Filter Amplifier has been verified except for QlB, CR14, U23, U24, and the
filtering capability of the circuit. Check U23 and U24 by performing the
Output RMSIAveragelQuasi-peak
Detector General Check.
@
SINAD Meter Circuits Check
1.
On the Audio Analyzer, key in 41.0 SPCL
to
initialize the instrument. Set the INPUT and OUTPUT switches
to ground. Key in AMPTD
1
V. Key in
1.11
SPCL to set the input gain
to
0
dB.
Key in 3.1 SPCL to set the
output gain to
0
dB.
Key in 45.1 SPCL to enable the SINAD meter. Connect the HIGH OUTPUT to the HIGH
INPUT.
2. Connect a dc voltmeter to A3TP15
(RMSB).
On the Audio Analyzer, fine adjust the SOURCE amplitude for a
reading of +1.00 Vdc on the voltmeter.
Hint: The voltage at A3TP15 should read approximately
1
Vdc before adjustment.
If
it
does not, see Service Sheet
3 and check the Pre-Notch RMS Detector.
3. Connect the voltmeter to A4TP1
(DC
OUT). The voltage should read between +0.99 and
+1.01
Vdc.
Hint: If the voltage
is
out of limits, perform the
Output RMSIAveragelQuasi-peak Detector General Check.
4. Insert a 31.6 kR resistor in parallel with
R56.
This simulates a 6
dB
SINAD.
5. Connect the voltmeter to pin 14 of U17C. The voltage should be between +320
and
+360 mVdc.
Hint: The voltage at pins 3
and
5 of Q4 should be between
-500
and
-480 mVdc.
6.
Connect the voltmeter to
pin
5
of U17B. The voltage should be the same
as
in
step 5 except that the loading
of a
10
MR
voltmeter input will cause the signal level to be 0.3% low. The SINAD meter should
read
between 5.5
and 6.5
dB.
Hint: The drains of Q5 and Q6 should read the same as pin
5
of U17B. Q5 and Q6 should be
OB.
Pins 4 and
6
of
U19 should be a TTL high.
Hint:
If
the meter reads only slightly out of limits, perform the
SINAD Meter Adjustment,
paragraph 5-14.
7. On the Audio Analyzer, key in 0.528 SPCL
to
switch the
SINAD
meter
off.
The voltage
at
pin 5 of U17B
should read
0
to
+10 mVdc. The SINAD meter should read
0.
Hint: Q6 should be on. Pin 4 of U19A should be
a
TTL low. Pin 6 of U19B should be a TTL high.
8.
On the Audio Analyzer, key in 0.524 SPCL
to
peg the meter. The voltage at
pin
5 of U17B should read between
+1.5 and $2.5 Vdc. The SINAD meter should read slightly above full scale.
Hink
Q5
should be on. Pin 6 of U19B should be a TTL low. Pin 4 of U19A should be a TTL high.
(J5)
Overload Detector Check
1.
On the Audio Analyzer, key in 41.0 SPCL
to
initialize the instrument. Set the INPUT
and
OUTPUT switches
to ground. Key in AMPTD 4 V. Key
in
1.11
SPCL
to
set the input gain
to
0
dB.
Key in
3.1
SPCL to set the
output gain
to
0
dB.
Connect the HIGH OUTPUT to the HIGH INPUT.
2. Connect
a
high-impedance, dc coupled oscilloscope to the drain
of
Q7. The level should be a TTL high.
Hint: Pin 13 of U19D should be between -15.2 and -14.7 Vdc.
3. On the Audio Analyzer, key in AMPTD 4.2 V. The level should be a TTL low.
Hint: Pin
13
of U19D should be between
0
and
+ZOO
mVdc.
8B-48
rev.20JUN87
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