Ericsson PF 768 Troubleshooting Instructions page 19

PF768, Standard Electrical Repairs
* If the resistance is too low there's a short circuit in one of the circuits fed by
the regulator. Send the phone to the next level.
• If any of the voltages are too high, replace the corresponding regulator.
Check the power reset voltage at C710 (fig. 3.3, >3V).
• If it's too low, replace C710 (class A) and measure again.
* If that doesn't help, replace N703 (class A, fig. 3.3).
Check the VRAD (fig. 3.1, P501 measuring point), VVCO (fig. 3.5) and VRPAD
(fig. 3.5) voltages (all of them ~3.8V).
Note that P501 is a calibration point. It is VRAD you measure but after 2 resistors.
The calibration point has the right voltage as long as the transmitter isn't activated.
Also note that the measuring point for VVCO is under the big radio shielding
(E202) and therefore you have to measure the voltage carefully through one of the
holes with a thin and preferably isolated probe as in fig. 3.6.
• If the VRAD or the VVCO voltage is incorrect, send the phone to the next level.
• If the VRPAD voltage is too low, measure the resistance from it to ground (>25
kohms).
* If the resistance is correct, replace N452 (class A).
* If the resistance is too low there's a short circuit in one of the circuits fed by
the regulator. Send the phone to the next level.
• If the VRPAD voltage is too high, replace N452.
• If all the feed voltages are correct, check the amplitude of the clock frequency at
C680 (class A, >0.6V t-t, fig. 3.3). To check the amplitude you can use an oscil-
loscope, spectrum analyser, frequency counter, diode probe or similar. The D900
circuit usually gets warm when the system clock frequency is missing.
* If the amplitude is too low, send the phone to the next level.
Make sure there aren't any faulty solderings at D600, D610 or D620 (fig. 3.2).
• If they look ok, replace D610 (class A). Do not replace any component if you
haven't established the fact that the amplitude at C680 is correct.
* If that doesn't help, replace D600 (class B) first and then D620 (class A) if
that doesn't help. Try programming the phone in the flash programmer after
every circuit replaced.
If the fault remains, send the phone to the next level.
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