B5.14
T855 Fault Finding
M850-00
31/09/98 Copyright TEL
Programming and
serial bus OK?
Is reference
clock present
at IC740 pin 20
(12.8MHz >1.5Vpp)?
Check programming.
Check serial bus
when programming
i.e. clock/data/enable
activity (IC740 pins
18, 19 & 17).
N
N
Check loop filter components
R775, C774, C776, R510,
C505, IC750. Is the supply
to IC750 +20V +4, -2V?
Check regulator circuitry.
Check C772, R774, R772,
C770, R763 & Q790.
Check Q760, Q780,
R766 & C765.
Check charge pump components
(Q775 & Q780) and Q790.
Check R762 & C762.
Check the signal path from
the VCO to synthesiser.
Measure the frequency from
the VCO at IC740 pin 11 using
a frequency counter.
Is the level >-10dBm?
Does the frequency adjust
as the VCO trimmer is
adjusted (out of lock)?
Check VCO.
Y
Y
Y
Signal
Y
Y
Y
No signal
Replace synthesiser
(IC740).
Y
N Y
Y
N
N
N
N
Check phase detector output
pins (IC740 pins 3 & 4).
Are there very narrow pulses
(~50ns) at the reference frequency?
(i.e. 5kHz : 200µs period
6.25kHz : 160µs period)
*See Note below.
Is the charge pump bias
voltage at the base of
Q760 & Q780 between
3.5 and 5V? (5V when locked)
N
Is the voltage on C762,
R760, and R758 9V ±0.2V?
Is the loop filter reference
voltage at C772 (+) 6.5V ±0.2V?
N
Check divider buffer
circuit (Q795) and bias
voltages on Q795 emitter
(1.3V ±0.2V).
*Note:
When locked, both of these
are very narrow (~50ns).
If unlocked, one will be
very narrow (~50ns),
and the other wide (>15µs).
Check microcontroller
section (IC810).
Check TCXO (IC700)
and buffers (IC710).
Replace synthesiser
(IC740).
Continued from previous page
B
Is the charge pump voltage
on Q790 gate between
3 and 8.5V? (6.5V ±0.2V locked)
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