GE 26920 Cordless Phone - Battery Does not charge

[MP]

Hi,

I am trying to repair my GE 26920 cordless phone. The problem is
that the battery does not charge and the Charge/In use LED does not
glow on the base unit. Here are my findings

1. Ensured that the contacts are clean
2. Installed a new battery with the same capacity
3. Left it charging overnight.

Finally, decided to open the handset and the base unit to find out if
there is something smokened out.
1. Did not find any parts burnt/dead on the base unit. All
resistors, zener's pass the DMM test
2. On the base unit, A resistor appears blackened due to heat
but the value is shows is 360Ohms and is in series with a zener diode
(13V).
3. The base unit has a Toshiba 87C405 microcontroller. One of
the pin's drive the Charge/In use LED
4. I put a new LED in between the two contact's of the base unit
that would connect to the handset charge contacts. The LED light's
up. I noticed that the 87C405 drive the base of a transistor (1AMH)
which switch's on to provide the charge current ( now for the LED).
Just to be sure, I disconnected this connection and the LED
stopped.

So, why does the battery not charge and why does the LED for charge/
in use does not light up ?

I am trying hard to trace the circuit and understand the operation.
Did a google search for almost a week but could not find any reference
to the schematic board diagram of this instrument or event the service
manual.

Any help/ tips would be appreciated.

Manoj Patil
[email protected]

 

[William Sommerwerck]

You didn't say whether there was voltage on the charging pins. You should
start by tracing backward from the pins.

Also, it is not normal for a properly-specified resistor to be blackened (or
even darkened) by heat. This resistor should be replaced by one of the
correct value and wattage rating.

I realize English is not your first language, so I wanted to note that
plurals do not have apostrophes. For example, pins, not pin's.

 

[MP]

You didn't say whether there was voltage on the charging pins. You should
start by tracing backward from the pins.

Also, it is not normal for a properly-specified resistor to be blackened (or
even darkened) by heat. This resistor should be replaced by one of the
correct value and wattage rating.

I realize English is not your first language, so I wanted to note that
plurals do not have apostrophes. For example, pins, not pin's.

Hi,

1. There is about 8V at the charging pins (not pin's) of the base
unit.
I have googled a ton of websites and one of them mention that
this voltage at the base unit should be varying from 4V to 8V and when
the handset is placed with battery connected, it should drop and
stabilize at about 3.6v. In my case, the voltage is fixed at about
8V and does not vary.

2. I too wanted to replace the resistor. But its badly blackened
with even the color coating layer cracked..... I do not have the
schematic so I don't know what should be the value of the resistor.
Any tips ?

 

[William Sommerwerck]

1. There is about 8V at the charging pins (not pin's) of the base
unit.

<grin>

I have googled a ton of websites and one of them mention that
this voltage at the base unit should be varying from 4V to 8V and when
the handset is placed with battery connected, it should drop and
stabilize at about 3.6v. In my case, the voltage is fixed at about
8V and does not vary.

Hmmm... Interesting.

If the battery is pulling current, then the voltage should drop. That it
does not suggests that (QED) the battery is not connected. Have you checked
all the wiring in the handset, to be certain the battery is properly
connected?

 

[MP]

1.  There is about 8V at the charging pins (not pin's) of the base
unit.

<grin>

I have googled a ton of websites and one of them mention that
this voltage at the base unit should be varying from 4V to 8V and when
the handset is placed with battery connected,  it should drop and
stabilize at about 3.6v. In my case, the voltage is fixed at about
8V and does not vary.

Hmmm... Interesting.

If the battery is pulling current, then the voltage should drop. That it
does not suggests that (QED) the battery is not connected. Have you checked
all the wiring in the handset, to be certain the battery is properly
connected?

For last few hours , been working on this task. Here are some more
findings.

1. The burnt resistor was showing 330Ohms on my DMM. I shunted it
with another 140 Ohms resistor. Put a LED across the handset
charge terminals of the base unit. Bingo !!! the base unit Charge/
In-use LED light's up......... so basically it may be that the
original value of the resistance was around 150Ohms but due to heat,
this got bumped to 330Ohms ( wild guess )
The LED was drawing about 35mA of current


2. Now, instead of the LED, I wired the handset, connected the
battery ( 3.6V, 300mAH) to the battery terminals on the handset
PCB.
The charge/in-use LED of the base unit lit up........ but after
about a minute, the LED of the base unit is no more on...
Removed the battery, measured the terminal voltage and it show's
3.6V but a little bit usage ( by pressing talk button on the
handset) drops it to 2.2V
The battery pack is brand new and not a old one so I cannot doubt the
battery.


3. I repeated the step 1 and the LED of base unit is on again...

4. So basically, I still have not fixed this instrument. How can
the battery voltage jump to 3.6V in just a minute ? Should it not be
gradually increasing ( sorry, its been a while i have handled
batteries so a bit rustic). why does the base unit LED turns off in
a minute ? When this instrument was brand new, i remember this used
to remain on as long as the handset was kept on the base unit.

5. I am going to buy one more brand new battery set and try the
above steps again just to be sure that the battery is not the culprit.


Note: Yes, English is not my mother-tongue but its the language I
have used in my profession for last 20 years. Any more grammar errors
are welcome

 

[Bob Shuman]

You need to know the proper current limiting resistor value. It sounds to
me as though 150 ohms is still possibly too high. If you measure the
voltage drop across that resistor, you can determine the charging current.
Compare that to the suggested charge rate for the 3.6V battery pack and then
adjust the resistor accordingly. I do not believe it is a defective battery
pack, so would not purchase another replacement till you determine that it
is receiving the correct charging current.

It is possible that the original resistor could have burned up over time if
you had a dead shorted battery pack. I'd do with a higher calculated
maximum wattage (assuming the full DC supply voltage gets dropped across
that resistor) just to be sure this does not happen next time around.

Good luck. Your use of the English language is very good.

Bob



For last few hours , been working on this task. Here are some more
findings.

1. The burnt resistor was showing 330Ohms on my DMM. I shunted it
with another 140 Ohms resistor. Put a LED across the handset
charge terminals of the base unit. Bingo !!! the base unit Charge/
In-use LED light's up......... so basically it may be that the
original value of the resistance was around 150Ohms but due to heat,
this got bumped to 330Ohms ( wild guess )
The LED was drawing about 35mA of current


2. Now, instead of the LED, I wired the handset, connected the
battery ( 3.6V, 300mAH) to the battery terminals on the handset
PCB.
The charge/in-use LED of the base unit lit up........ but after
about a minute, the LED of the base unit is no more on...
Removed the battery, measured the terminal voltage and it show's
3.6V but a little bit usage ( by pressing talk button on the
handset) drops it to 2.2V
The battery pack is brand new and not a old one so I cannot doubt the
battery.


3. I repeated the step 1 and the LED of base unit is on again...

4. So basically, I still have not fixed this instrument. How can
the battery voltage jump to 3.6V in just a minute ? Should it not be
gradually increasing ( sorry, its been a while i have handled
batteries so a bit rustic). why does the base unit LED turns off in
a minute ? When this instrument was brand new, i remember this used
to remain on as long as the handset was kept on the base unit.

5. I am going to buy one more brand new battery set and try the
above steps again just to be sure that the battery is not the culprit.


Note: Yes, English is not my mother-tongue but its the language I
have used in my profession for last 20 years. Any more grammar errors
are welcome

 

[William Sommerwerck]

2. Now, instead of the LED, I wired the handset, connected the
battery ( 3.6V, 300mAH) to the battery terminals on the handset
PCB.
The charge/in-use LED of the base unit lit up... but after
about a minute, the LED of the base unit is no more on...
Removed the battery, measured the terminal voltage and it
shows 3.6V but a little bit usage ( by pressing talk button on the
handset) drops it to 2.2V
The battery pack is brand new and not a old one so I cannot doubt the
battery.

You're assuming that the light going out means the battery has completely
charging. This might not be true. Let it sit longer.


So basically, I still have not fixed this instrument. How can
the battery voltage jump to 3.6V in just a minute ?

It's normal for nickel-cadmium batteries to return to "full" (or near-full)
voltage after a short charge -- but the battery _isn't_ fully charged. Be
patient.

 

[MP]

2.   Now,  instead of the LED,   I wired the handset,  connected the
battery ( 3.6V,  300mAH) to the battery terminals on the handset
PCB.
The charge/in-use LED of the base unit lit up... but after
about a minute,  the LED of the base unit is no more on...
Removed the battery, measured the terminal voltage and it
shows 3.6V but a little bit usage ( by pressing talk button on the
handset) drops it to 2.2V
The battery pack is brand new and not a old one  so I cannot doubt the
battery.

You're assuming that the light going out means the battery has completely
charging. This might not be true. Let it sit longer.

So basically, I still have not fixed this instrument. How can
the battery voltage jump to 3.6V in just a minute ?

It's normal for nickel-cadmium batteries to return to "full" (or near-full)
voltage after a short charge -- but the battery _isn't_ fully charged. Be
patient.

-----------------------------------

More updates.

1. Finally, I think I have managed to figure out the tentatively
right value of the resistor. I have put a 82Ohms resistor and the
voltages now appear correct. There is about 3.4V across the ni-cd
battery terminals connector.
There is zener diode across these terminals and using current meter, I
have noticed that this zener diode is always conducting and there is
about 25mA flowing through it
I left the battery connected to the connector for more than 12hrs and
it does not get charged. Using a current meter, I see that the
meter oscillates between 0mA and 4mA continuously.
Is this behaviour normal ? This time, I even tried with a BRAND
NEW battery pack and there is no current flowing through it too..


2. There is a microcontroller chip on the handset too. To get 3.6V
across the battery connector, I have isolated the microcontroller
and the circuitry surrounding it by disconnecting one of the jumper
providing supply.
In fact this jumper is directly connected via the copper tracks to the
cathode of the zener diode. Interestingly, I notice that
everytime I connect back this jumper, the voltage across the zener
( and battery connector) falls to 1.5V

3. There are few SMD transistor like devices on the handset PCB.
It has M6 written on it and a google search indicates that this can be
a Jfet or a NPN transistor. I did following check on the terminals,
and these are my results
The M6 is a three terminal device and it has 1 pin on the right side
and 2 pins on the left side. I guess this is SOT23 but not very sure
( a bit rustic on that now)
Lets say we name the pin on the left bottom as 3, left top as 2 and
right side as 1 and using a DMM in th diode testing mode/setting,
below is the table of results
3 2 1 DMM Reading
R B 1
R B 690
R B 1
B R 1
B R 680
B R 1650
Now, I am finding it difficult to decide on what exactly is this
device ? There are 3 to 4 of them on the board and all of them
show the same readings.

Any tips to find the faulty component causing the voltage to drop to
1.5v from 3.6v ?

 

[William Sommerwerck]

If there is voltage at the charging terminals, but the battery doesn't
charge, does that not suggest the battery is not making contact?

 

[Bob Shuman]

Depends on if the measured voltage at the battery charging terminals was
while the phone was on the charging cradle or not. Also, if the phone was
indeed on the cradle at the time, then I'd suspect the circuit inside the
phone itself may be open circuit due to a number of possible causes.

 

[William Sommerwerck]

Which was exactly my point. (I should have said "connection" rather than
"contact", but...)

 

[MP]

Which was exactly my point. (I should have said "connection" rather than
"contact", but...)






- Show quoted text -

Thanks for your response. Here are my inputs

1. The handset is not kept on the base cradle. I have physically
wired them so that the contact issue is ruled out.

2. I am sure there is no issue with the contacts/connection with
battery and the terminal connector on the PCB. Also if you note my
description above, I have actually measured the current flowing into
the battery and it oscillates between 0mA and 4mA. This shows there
is no connection issue. Moreover, there is 22mA flowing through the
zener all the times. In this situation, I am wondering if there
would be any significant current flowing into the battery when the
zener is conducting. Would the zener be not the least resistance
path to the current instead of the battery directly in parallel ?
Typically, as per design, should the zener be always be conducting
across the battery ?

3. I am trying to find out the details of the SMD M6. Any clues ?

Its been more than a week and this instrument is still not fixed.....