234
Frequency Synthesizer Fault Finding
TM8100/TM8200 Service Manual
© Tait Electronics Limited
June 2006
Also, the VCXO input to the mixer (see
noisy and difficult to measure, should be:
6.
Confirm the removal of the fault and go to
. If the repair
failed, replace the board and go to
Task 38 —
TP501 Test Point
If the signal at the
TP
502
test point is correct, check the signal at the
TP
501
test point:
1.
With the oscilloscope probe at the
TP
501
test point
(see
), check the DAC output
CDC
VCXO
MOD
. If a
triangular wave is present, go to
. Otherwise go to
2.
A fault is causing the loop to oscillate. If not already done, remove the
FCL
TOP
can.
3.
Check the waveform at the
TP
500
test point
(see
).
The waveform should be an amplified and inverted version of the
waveform at the
TP
501
test point
.
A4 band:
TP
500
is not marked, but the waveform can be checked at
the via adjacent
C518
.
4.
If the waveform is correct, go to
. If it is not, there is a fault in
the modulator buffer amplifier (
IC502
pins 1 to 3, and associated
components) (see
). Rectify the fault and return to
5.
Connect the
TP
501
test point
to ground by resoldering
R527
in the
position shown in
. The VCXO loop voltage is forced high.
6.
Use the oscilloscope probe to check the VCXO output at
C536
—
probe the via next to C536 (see
). The signal should be:
7.
If the signal is correct, go to
. If it is not, go to
8.
The VCXO circuitry is faulty. If not already done, remove the
VCXO
BOT
can.
9.
Locate and repair the fault in the VCXO circuitry (
Q501
,
Q503
,
XL501
and associated components) (see
10.
Confirm the removal of the fault, and go to
. If the repair
failed, replace the board and go to
VCXO input at
R522
(pin 1 of
IC501
):
VCXO input: sine wave of 20
±
10 mV
pp
A4 band
: VCXO input at
R510
(pin 1 of
IC506
):
VCXO input: sine wave of 20
±
10 mV
pp
VCXO output at C536: sine wave with frequency of 13.017 MHz and
amplitude of 1.1
±
0.2 V
pp
on 1.4
±
0.2 V DC
Содержание TM8235
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