ERICSSON GF786, GF788, GF788E Troubleshooting guide

4/00021-2/FEA 209 544/6 C Troubleshooting guide

Troubleshooting guide.

GF 788/768/788e Standard.

Contents page

Component Classes 2
Abbreviations 3
Pin placements 4

1. RTC 6

2. Keyboard 8

3. Display 13

4. Charging 18

5. Illumination and buzzer 23

6. Doesn’t start 28

7. SIM fault 37

8. No serv 41

9. Audio 45

10. Spare part list 53

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Troubleshooting guide 4/00021-2/FEA 209 544/6 C

Component classes.

All the components in the phone are divided into classes and after every component in the troubleshooting guide
you have a class written. The components are dividedintofourclasses:A,B,CandD.Theclassofthecomponent
depends on how much of the phone’s performance is affected when replacing it.

Class A and B: A test call towards the “real” net (not only towards a GSM test instrument) and run it through

the normal tests is enough to verify the functionality since the performance of the phone is only
slightly affected.

Class C: Since the tolerances of the component are so great it can substantially affect the performance of

the phone you need to calibrate it at station level after replacing the component.

Class D: Class D components need to be calibrated at board level using very advanced equipment and

may therefore not be replaced.

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Abbreviations.

A: The power module at some phones.
B: Crystal.
C: Capacitor.
D: Digital circuit.
F: Over voltage protection.
G: VCO.
H: Buzzer, LED, pads for display.
J: Connector.
L: Coil.
N: Analogue circuit.
R: Resistor.
S: Keyboard pads.
U: BALUN. A circuit that converts a signal from balanced to unbalanced or the opposite.
V: Transistor or diode.
X: Contact surface at the circuit board.
Z: Filter.

AGND: Ground for analogue signals.
DCIO: DC voltage used for charging the battery through the system connector.
DCON: Logical signal from the processor that keeps the phone running after you’ve released the On/Off

key.

EXTAUD: Input signal at the system connector that the processor uses to determine if there’s any external

audio equipment attached.

EXTAUDI: The same signal as the EXTAUD signal but at the processor side.
GND: Ground.
LED3K: Logical signal used to activate the background illumination.
ONSRQ: Voltage from the On/Off key that starts the phone.
PORTHF: Input signal at the system connector that the processor uses to determine if there’s any handsfree

equipment attached.

PHF1: The same signal as PORTHF but at the processor side.
REGON: Logical signal that activates the voltage regulators.
RTC: Real time clock. The clock that keeps track of time and date.
SIMCLK: Clock signal from the processor used for communications with the SIM.
SIMDAT: Data signal from the processor used for communications with the SIM.
SIMRST: Reset signal from the processor used for communications with the SIM.
SIMVCC: Feed voltage for the SIM.
SWDC: Switched VBATT.
VANA: DC voltage for the analogue part of the logic (N800).
VBATT: Battery voltage.
VDIG: DC voltage for the processor and memory.
VDSP: DC voltage for the DSP (Digital Signal Processor).
VLCD: DC voltage for the display that controls the contrast.
VRAD: DC voltage for the radio part except the synthesizer.
VRPAD: DC voltage for the radio part in D600 (also used for the top diode and the buzzer).
VRTC: DC voltage for the real time clock.
VSIMPAD: VDIG voltage that has been switched up to 5V used for SIM.
VVCO: DC voltage for the synthesizer.

I2C: Two line serial communications standard using one clock and one data line.
LO: Local oscillator.
PWM: Pulse width modulation.

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Pin placements

Single diode (PIN diode). Electrolytic capacitor.

Double diode or single trans-

stor.

Eight pin circuit. 688 VCO circuit 788 VCO circuit

Sixteen pin circuit Twenty pin circuit Crystal

Five pin circuit (usually volt-

age regulator).

Double transistor.

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N800 D900

D600

D620 D610

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1. RTC

1.1 Find the fault.

Start the phone with a SIM-card and a fully charged battery inserted. Set the correct time. Remove the battery and
reinsert it after a minute.

• If the clock says 00:00, go to 1.2.
Compare to the correct time.

• If the clock is speeding or if it is halted, go to 1.3.

The component side of the circuit board is shown in fig. 1.1.

Fig. 1.1

1.2 The phone shows 00:00 after removing and reinserting the battery.

Open the phone and make sure that the backup capacitor, C720 (class A, fig. 1.2), is correctly soldered.

Fig. 1.2

If it is, replace it. Assemble the phone, start it up and set the correct time. Wait a few minutes for the capacitor to
charge. Remove the battery and reinsert it after a minute. Check that the fault is fixed (the backup capacitor needs

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4/00021-2/FEA 209 544/6 C Troubleshooting guide

a few hours of charging to get fully charged). Compare to the correct time. If the clock is speeding or if it is
halted, go to 1.3.

1.3 The clock is speeding or is halted.

Open the phone and make sure the RTCcrystal, B600, is correctly soldered. If it is, replace B600, C690 and C691
(all of them class A, fig. 1.3). Assemble the phone and compare to the correct time.

Fig. 1.3

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2. Keyboard

2.1 Kind of keyboard fault.

Insert a SIM-card and a fully charged battery into the phone. Start the phone using the On/Off button. If it doesn’t
start at all, go to chapter 6.
Press all the keys (including the volume keys) and verify which of them that doesn’t work. Verification is most
easily done like this:

1. Applies for GF 788/788e/788c:
Change the menu size to extended.
Go to “Settings/Key sound” in the menus and change to “click”.

Applies for GF 768/768c:
Go to “Key sound” in the menus and change to “click”.

2. Press the keys 1, 2, 3…*, 0, #. A clicking sound should be heard and the corresponding sign should be
shown in the display at every key pressed. Then press Yes, No, CLR, “<” and “>”. When pressing Yes the
phone should try to place a call, when pressing No it should disconnect it. Pressing CLR should make it
clear the display and “<” and “>” should make it scroll through the menus.

3. Press the volume keys. At every press there should be a clicking sound.

• If only the volume keys are non-functional, go to 2.2.

• If one or more keys are non-functional, go to 2.3

• If one or both of the volume keys and one or more of the other keys are non-functional, read both 2.2 and 2.3.

2.2 The volume keys are non-functional.

Open the phone and check for water damage, especially at the microswitches J820 and J821 (fig. 2.1).

Fig. 2.1

Make sure the microswitches J820 and J821 (both of them class A) are intact and correctly soldered.
Measure the resistance of the microswitch that is broken. You measure from leg 1 or 3 to leg 2 or 4 (fig. 2.2). The
resistance should be more than 100 kohms when not pressed and ~0 ohms when pressed.

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Fig. 2.2

• If the resistance is high all the time, replace the corresponding microswitch.

• If the resistance is low all the time, replace the corresponding microswitch. If that doesn’t help, check the sol-

derings at D600 pads 1, 123, 124 and 128. If the solderings are correct, replace D600 (class B, fig. 2.3).

Fig. 2.3

• If the switches work, replace the volume buttons and try again.

• If that doesn’t help, check the solderings at D600 pads 1, 123, 124 and 128. If the solderings are correct,

replace D600 (class B, fig. 2.3).

2.3 One or more keys non-functional.

Open the phone and check for water damage. Remove the dome foil and check for water damage and mechanical
damage, especially at the non-functional keys.
Clean the keypads thoroughly.Mount a new dome foil and check the keyboard for damages. Assemble the phone
and check the kayboard function as described in 2.1.
If that doesn’t help, open the phone, give it power and start it up (using the On/Off key or by setting DCIO high).
Functional schematics are shown in fig. 2.4.

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Fig. 2.4

Measure VDIG (fig. 2.5, ~3.2V).

• If the voltage is incorrect, go to chapter 6.

• If it’s correct, check if there’s any voltage (~3.2V) at marked sides of R630, R631, R632, R633 and R634 (all

in fig. 2.5).

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Fig. 2.5

• If the voltage is missing at one or more of the resistors, measure the resistance from marked side of C600 to

the marked side of the resistor (~0 ohms). If the resistance is too high there’s a foil damage and the phone is to
be discarded.

Measure the voltage at the opposite side of the resistance (~3.2V).

• If the voltage is too low at one or more of the resistors, check the resistance of the corresponding resistor (all

of them 100 kohms and class A). If the resistances are correct, check the solderings at D600 pads 120, 122­128 and 1. If the solderings are correct, replace D600 (class B, fig. 2.1).

If the voltages are correct at both sides of R630-R634 but the keyboard doesn’t work, check the solderings at
D600 pads 120, 122-128 and 1. If the solderings are correct, replace D600 (class B, fig. 2.1). If it doesn’t help,
remove the dome foil again. Check for voltage (~3.2V) at marked sides of the key pads in the row of the non-func­tional keys (fig. 2.6). Observe that there is VBATT voltage (~4.8V) at the marked side of the “No”-key. If the
VBATT voltage at the “No” key is missing there’s a foil damage and the phone is to be discarded.

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Fig. 2.6

If voltage is missing at some keys in a row (se functional scheme fig. 2.4) there’s a foil damage and the phone is to
be discarded. This is verified by measuring the resistance from the marked side of a working pad to a non-working
(~0 ohms). The foil damage is usually caused by water damage. If, for instance, there’s voltage at pads 4 and 5 but
not at 6 and “volume up” there’s a foil damage between keys 5 and 6 and the phone is to be discarded.

If a column or part of it is non-functional, check the solderings at D600 pads 1 and 120-128. If all of them are cor­rect, replace D6000 (class B).

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3. Display.

3.1 Type of fault.

Insert a fully charged battery into the phone and start it up.

• If it doesn’t start, go to chapter 6.

• If the display is missing one or more segments, go to 3.2.

• If the display is totally empty, go to 3.3.

• If the display is completely black (all the segments are “lit” and sometimes you can make out what is dis-

played), go to 3.4.

3.2 Segments are missing.

Open the phone and check for water damage. Replace the display.

3.3 The display is totally empty.

Insert a dummy battery into the phone, start it up and check the current consumption. You can also disassemble
the phone, give the board power, start it up with the display mounted and check the current consumption that way.

• If the phone consumes more than 200mA, make sure the display isn’t mechanically damaged. If it isn’t,

replace the elastomer and try again. If that doesn’t help, replace the display and try again. If the current con­sumption still is too high, go to chapter 6.

• If the phone consumes less than 200mA, open the phone and check for water damage.

Give the board power and start it up without the display mounted.
The component side of the board is shown in fig. 3.1.

Fig. 3.1

3.3.1 Applies for GF 788/768/788e phones (2/ROA 119 3283/7 and 2/ROA 117 3748/2 boards).

Measure the voltages at the V608 (class A) diode. Compare the result to the values in fig. 3.2.

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Fig. 3.2

All the values are 0.2V.
If any of the voltages are incorrect, measure the resistance of C633 (class A, >100 kohms, fig. 3.2) and C824

(class A, >25 kohms, fig. 3.2).

• If any of the resistances are too low, replace the corresponding capacitor.

• If the voltages and VRPAD are correct, replace the diode. If VRPAD is incorrect, go to chapter 6.

• If the voltages at the diode (except VRPAD) are incorrect after replacing it, check the soldering at D600 pad

96.

• Replace C632 (class A, fig. 3.2) and measure the voltages at V608 again. If the voltages still are incorrect,

replace D600 (class B, fig. 3.1).

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Fig. 3.3

Fig. 3.4

If the voltages at V608 are correct but the display is totally empty when phone is on, measure the voltages at the
display pads, H622 (fig. 3.4):

Pad # Name Voltage

1
2

2

I

C-CLOCK

2

C-DATA

I

~3.2V
~3.2V

3 VDIG ~3.2V
4 VDIG ~3.2V
5 GND 0V
6 VLCD ~5.2V

Table 3.1

• If there isn’t VLCD voltage at pad 6, measure the resistances from the pad to V608 pin 1 (~0 ohms) and from

pad 5 to GND (~0 ohms). If any of the resistances are too high there’s a foil damage and the phone is to be dis-

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carded.

If I2C-DATA or I2C-CLOCK voltage is missing, check VDIG at C600 (~3.2V, fig. 3.2).

• If VDIG is incorrect, go to chapter 6.

• If VDIG is correct, measure the resistance of R619 (class A, 2.2 kohms, fig. 3.2), R620 (class A, 2.2 kohms,

fig 3.2), R615 (class A, 10 kohms, fig. 3.4) and R616 (class A, 10 kohms, fig. 3.4). Make sure the solderings at
D600 pads 3 and 4 (fig. 3.1) are correct.

• If the resistances and the solderings are correct, replace D600 (class B).

3.3.2 Applies for GF788c/768c (2/ROA 117 3632/1 boards).

Measure the voltages at the V608 (class A) and V611 (class A) diodes. Compare the result to the values in fig. 3.5.

Fig. 3.5

All the values are 0.2V.
If any of the voltages are incorrect, measure the resistance of C633 (class A, >100 kohms, fig. 3.5) and C824

(class A, >25 kohms, fig. 3.5).

• If any of the resistances are too low, replace the corresponding capacitor.

• If the voltages are correct, replace both the diodes.

• If the voltages at the diodes are incorrect after replacing it, check the soldering at D600 pad 95 and 96.

• Replace C634 and C632 (both class A, fig. 3.5) and measure the voltages at V608 and V611 again. If the volt-

ages still are incorrect, replace D600 (class B, fig. 3.1).

If the voltages at V608 and V611 are correct but the display is totally empty when phone is on, measure the volt­ages at the display pads, H622 (fig. 3.6):

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