Ericsson LBI-31511B, Maintenance Manual, Page: 7 / 18

TEST PROCEDURE
These Test Procedures are designed to help you to service
a receiver that is operating -- but not properly. The problems
encountered could be low power, poor sensitivity, distortion,
limiter not operating properly and low gain. By following the
sequence of test steps starting with Step 1, the defect can be
quickly localized. Once the defective stage is pin-pointed,
refer to the "Service Check" listed to correct the problem.
Additional corrective measures are included in the Trou-
bleshooting Procedure. Before starting with the Receiver Test
Procedures, be sure the receiver is tuned and aligned to the
proper operating frequency.
TEST EQUIPMENT REQUIRED
•
Distortion Analyzer
•
Signal Generator
•
6 dB attenuation pad
•
Audio Isolation Transformer
•
4 ohm resistor (15 watt minimum)
PRELIMINARY ADJUSTMENTS
1. Unsquelch the receiver.
STEP 1
AUDIO POWER OUTPUT
AND DISTORTION
TEST PROCEDURE
Measure Audio Power Output as follows:
A. Apply a 1000 microvolt, on-frequency test signal
modulated by 1,000 Hz with
±
3.0 kHz deviation to
antenna jack J2.
B. With 12 Watt Speaker
Disconnect speaker lead pins from J1A-36 and 37
on rear of control unit. Connect a 4.0 ohm, 15 watt
load resistor across system metering jack J602-6
and 7 on the TRS board.
Connect the isolation transformer input across the
resistor. Connect the isolation transformer output to
the Distortion Analyzer (See Figure 5).
C. Adjust the VOLUME control for 12 watts output
6.93 VRMS using the Distortion Analyzer as a volt-
meter.
D. Make distortion measurements according to manu-
facturer’s instructions. Reading should be less than
3%. If the receiver sensitivity is to be measured, leave
all controls and equipment as they are.
SERVICE CHECK
If the distortion is more than 3%, or maximum
audio output is less than 12 watts, make the follow-
ing checks:
E. Battery and regulator voltage --- low voltage will
cause distortion. (Refer to Receiver Schematic Dia-
gram for voltages.)
F. Audio Gain (Refer to Receiver Troubleshooting Pro-
cedure.)
G. FM Detector Alignment (Refer to Receiver Align-
ment.)
STEP 2
USABLE SENSITIVITY
(12 dB SINAD)
If STEP 1 checks out properly, measure the receiver
sensitivity as follows:
A. Apply a 1000 microvolt, on-frequency signal modu-
lated by 1000 Hz with 3.0 kHz deviation to J601.
B. Place the RANGE switch on the Distortion analyzer in
the 200 to 2000 Hz distortion range position (1000 Hz
filter in the circuit). Tune the filter for minimum reading
or null on the lowest possible scale (100%, 30%, etc.)
C. Place the RANGE switch SET LEVEL position (filter
out of the circuit) and adjust the input LEVEL control
for a +2 dB reading on a mid range (30%).
D. Set signal generator output to 0.3
µ
V. Switch the
RANGE control from SET LEVEL to the distortion
range Readjust Distortion Analyzer SET LEVEL as
required until a 12 dB difference (+2 dB to -10 dB) is
obtained between the SET LEVEL and distortion range
positions (filter out and filter in).
E. The 12 dB difference (Signal plus Noise and Distortion
to noise plus distortion ratio) is the "usable" sensitivity
level. The sensitivity should be less than rated 12 dB
SINAD specifications with an audio output of at least
6 watts (49 Volts RMS across 4.0 ohm receiver load
using the Distortion Analyzer as a Voltmeter).
F. Leave all controls as they are and all equipment con-
nected if the Modulation Acceptance Bandwidth test is
to be performed.
SERVICE CHECK
If the sensitivity level is more than the rated 12 dB
SINAD, check the alignment of the RF stages as
directed in the Alignment Procedure.
STEP 3
MODULATION ACCEPTANCE BAND-
WIDTH (IF BANDWIDTH)
If STEPS 1 and 2 check out properly, measure the
bandwidth as follows:
A. Set audio to 10% of rated output.
B. Set the Signal Generator output for twice the microvolt
reading obtained in the 12 dB SINAD measurement.
C. Set the Range control on the Distortion Analyzer in the
SET LEVEL position (1000 Hz filter out of the circuit),
and adjust the input LEVEL control for a +2 dB reading
on the 30% range.
D. While increasing the deviation of the Signal Generator,
switch the RANGE control from SET LEVEL to dis-
tortion range until a 12 dB difference is obtained be-
tween the SET LEVEL and distortion range readings
(from +2 dB to -10 dB).
E. The deviation control reading for the 12 dB difference
is the Modulation Acceptance Bandwidth of the re-
ceiver. It should be more than
±
7.0 kHz.
SERVICE CHECK
If the Modulation Acceptance Bandwidth test does not
indicate the proper width, check the synthesizer fre-
quency and then refer to the Alternate IF Alignment
Procedure.
RECEIVER TEST PROCEDURE
NOTE
These procedures are written around the Heathkit
Distortion Analyzer. If a Distortion Analyzer other
than the Heath IM-12 is used, measure the sensitivity
and modulation acceptance bandwidth in accordance
with manufacturer’s instructions.
LBI-31511 LBI-31511
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