Tait TB7100, Service Manual

Page: 153 / 370

TB7100 Service Manual Frequency Synthesizer Fault Finding 155
© Tait Electronics Limited October 2005
11.3 Phase-locked Loop
Introduction
If there is no fault with the power supplies, check the critical output from,
and inputs to, the PLL:
■
Task 7 : supply for charge pump
■
Task 8 : reference frequency input
■
Task 9 :
DIG SYN EN
line input
■
Task 10 :
SYN LOCK
line output
The measurement points for diagnosing faults concerning the PLL inputs
and output are summarized in
Figure 11.5 .
Task 7 —
Supply for
Charge Pump
First check the supply for the charge pump of the PLL.
1. Measure the supply for the charge pump at pin 16 of IC503
(see
Figure 11.2 ).
2. If the voltage is correct, go to Task 8 . If it is not, go to Step 3 .
3. Check the C-multiplier Q512 (see Figure 11.2 ) and check IC503
itself; if necessary, replace the transistor or IC.
4. If there is a fault, repair the circuit, confirm the removal of the fault,
and go to
“Final Tasks” on page 123 . If the repair failed or no fault
could be found, replace the main-board assembly and go to “Final
Tasks” on page 123 .
pin 16 of IC503: 5.0
±
0.3 V DC