Ericsson PF 768 Troubleshooting Instructions page 24

PF768, Standard Electrical Repairs
Wash the circuit board if needed using alcohol and a brush.
Assemble the phone and test it as in 4.1.
If the fault remains, open the phone. Give the board power and start it up by press-
ing the On/Off key without the system cable connected.
Measure the VDSP (~3.2V, fig. 4.3) feed voltage.
• If the VDSP voltage is too low, replace N701 (class A, fig. 4.3).
• If the VDSP voltage still is too low after replacing the N701 circuit there is prob-
ably a short circuit in one of the components fed by the VDSP voltage regulator.
If that's the case, send the phone to the next level.
Measure the voltage at both sides of R601 and at D600 pin 70 (~5V, fig. 4.3 and 4.2,
you follow the PHFI signal).
Measure the voltage at both sides of R605 and at D600 pin 67 (~5V, fig. 4.5 and 4.2,
you follow the EXTAUDI signal).
• If the voltage at only one side of R601 or R605 is too low, replace the corre-
sponding resistor (fig. 4.3 and 4.5, both of class A).
• If the voltage is too low at both sides of R601 or R605, check the VSIMPAD
voltage at marked sides of R635 and R636 (~5V, fig. 4.4 and 4.5).
* If there's no voltage there, check the resistance from the marked side of
R635 or R636 to N705 pin 3 (~0 ohms, fig. 4.5/4.4, 4.2).
* If the resistance is too high there's a foil damage and the phone is to be
discarded.
* If the resistance is correct, proceed to section 6.3.
* If the VSIMPAD voltage is correct, measure the resistances of R635 and
R636 (~22 kohms, fig. 4.4 or 4.5).
* If any of the resistances are incorrect, replace the corresponding resistor.
Measure the resistance from X602 pad 5 to ground (>100 kohms, fig. 4.6). Make
sure there's no dirt or oxide between the components below the dome switches (the
components in fig. 4.5).
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