Broken modem - what keeps it off-hook all the time?

[espresso]

Hi,

I have this problem with my 56k PCI dialup modem. Whenever I plug in the phone line cable in it, the line becomes busy because the modem keeps it off-hook all the time.
It can dial numbers but it won't connect because something keeps it off-hook.

I wanted to see if this was a hardware problem and apperently it is. Because it occurs even if the modem is out of the PC.

Is there any particular component that I should pay attention to? Tested all the resististors and diodes so far.

 

[brevor]

Look at the input components near the phone line jack. There is usually an optoisolator. these frequently become damaged from lightning hits.

 

[VSMVDD]

there is sometimes a switching transistor
used to pick the line up when a ring is detected

if its online all the time perhaps this has gone open collector{always open line}
or short emiter collector {always closed}

also check for shorted pin diodes in the same area

can you still send the modem an AT&T and get a reply???

this will test coms to its software / hardware codec
use hyperterminal and send the command to its port

if it replys its the ring detector circuit fault
if it doesnt throw it away

a final test is to check coms even thou its open all the time

if its just this circuit
then it will still com with another modem you can use another computer to test it
by plugging in a cable between the modem sockets
{american standard has a small plug both ends only uk has a non standard plug}

again use hyperterminal and set it to monitor for rings

 

[banjo]

Hi,

Lots of good suggestions already. What I would add is to remove the PCI card from the PC and connect it only to the phone line. If it still keeps the line off-hook, then it must be a component directly connected to the phone line that is low impedance.

If the line remains on-hook and is functioning normally, then perhaps the modem has a problem where the circuitry is being told to attach to the phone line. In this case, look for the relay that connects to the phone line and check its driver transistor for a short.

--- Steve

 

[espresso]

Guys thank you for your answers.
The problem is that I can't locate the switching transistor. How can I tell which one it is?

can you still send the modem an AT&T and get a reply???

Yes, it responds to AT commands, I tried it.

What I would add is to remove the PCI card from the PC and connect it only to the phone line.

I tried this too and I wrote it in my first post. It keeps it online all the time.

I have tested the transistors and diodes with a multimeter (diode testing position) without removing them from the board. And they seem to be OK. Should I remove the transistors and then test?

Here is a picture of the unfortunate modem (simple PCI modem):

http://img254.imageshack.us/my.php?image=070119003930mo7.jpg

 

[sinip]

Very blurred picture, but it should be the black one in upper left corner of the board. Three pins on the right side and one bigger pin on the left side.

 

[espresso]

Thanks for the reply!
It's BCP53. The datasheet says:

APPLICATIONS
-MEDIUM VOLTAGE LOAD SWITCH
TRANSISTORS

-OUTPUT STAGE FOR AUDIO AMPLIFIERS
CIRCUITS

-AUTOMOTIVE POST-VOLTAGE
REGULATION

I was suspicious about it too. Maybe because it's the biggest one around
I will have to unsolder it and then test it. I was hoping to get a reply first since I had no idea which of the transistors it could be.

P.S.
Just tested, the transistor is working. It's gotta be another one. Hard to tell, I have to this myself by testing each one on the board. Pozdrav.

 

[VSMVDD]

there will be a transistor that drives thisone you checked usualy smd

the best way is to use a motherboard cpu memory and os hdd mounted outside the pc
and also then mount the pci modem

then you can use a volt meter to measure
the devices various voltages on there base pin connections


HOWEVER !!!

if it keeps it open all the time
i suspect a bent or broken pin in the socket
as for a line to be open it has to be an open circuit
normaly the two pins will short over mostly {except the ring circuit}

if it is a dual socket type check this area
there is also a large fuse inline as a protector
on some types
so take a better picture i cant see that one clearly
if you look just under the transistor {at the top back of the card}
there is a green flat resistor
check this also


i notice on a modem i have here some smd parts
that are marked FB1 FB2 etc and some resistor smd's marked 000

these are fuseable breakage points and zero ohm resistors {same thing sort of}

a week part
they are in series with the incomming line

and worth checking they are low resistances and not open circuit

 

[espresso]

there is also a large fuse inline as a protector

Yes! The varistor was causing the problem. But it wasn't shorted. I don't know how...
I simply removed it.
Thank you for this info very much. And thank to all people that replied in this topic.

So, what is the typical value of this varistor as I intend to get a new one?

 

[VSMVDD]

no idea looking at this modem it says 330 on it
this isnt 330 R this is 33 and 0 zeros so 33R

 

[sinip]

there is also a large fuse inline as a protector

Yes! The varistor was causing the problem. But it wasn't shorted. I don't know how...
I simply removed it.
Thank you for this info very much. And thank to all people that replied in this topic.

So, what is the typical value of this varistor as I intend to get a new one?

It doesn't have to be shorted completely to cause off-hook state. If its resistance is lower than certain level, the switchboard will see it as off-hook. You should ask for telephone line protective varistor, and if you can choose only by voltage, 150V should do. On-hook voltage is usually 48 to 60V, depending on switchboard and ringing can go as high as 120V 25Hz AC, so 150V should give you safe margin.