Ericsson PF 768 Troubleshooting Instructions page 15

PF768, Standard Electrical Repairs
Since the measuring point for VVCO is under the big radio shielding (E202) you
have to measure the voltage carefully through one of the holes with a thin and pref-
erably isolated probe as in fig. 3.6.
Fig. 3.6
• If there is voltage at any of the voltages but not all, replace the corresponding
regulator (VDIG – N702, VANA – N700, VDSP – N701, VRAD – N453, VVCO
– N451, VRPAD – N452, all of them class A). Note that the N453 and N451
regulators are under the radio shieldings and can therefore not be replaced
at this level.
• If there is voltage at all of the regulators, measure REGON at V702 pins 2 and 3
(fig. 3.3, ~0V).
* The REGON signal probably comes from one of the regulators or from
V704, V708 or V709 (fig. 3.4) if there's no voltage at V702 pins 2 and 3.
Replace a component at a time and check after every component replaced.
* If there's voltage at both pin 2 and pin 3 or V702 there's probably a short
circuit at the pads for the On/Off key. It's almost always crumbs from the
dome switches that causes this short circuit. Wash it away using alcohol and a
brush.
If the fault remains, send the phone to the next level.
3.4.2 Consumes no current when On/Off key is being
pressed.
Open the phone and check for liquid damages.
Make sure the battery and system connector pads aren't dirty, liquid damaged or
oxidised.
Give the board power and start it up by pressing the On/Off key (in the fixture or in
the back cover with a dummy battery inserted, mount system connector to make the
board lie steady in the back cover).
Check the current consumption.
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