Franc Zabkar је напиÑ?ао
On 21 Jun 2006 12:04:18 -0700, put finger to
keyboard and composed:


Franc Zabkar wrote:
On 19 Jun 2006 08:44:34 -0700, put finger to
keyboard and composed:

Looked at several Motorola SM56 modems an all have the same fault-non
infinite resistance at line jack which causes them to take over the
line once the line is plugged, so none of the telephones can be used!
Checked the MOV and it is ok and there is no relay on modem !(at least
not a box type, just few too many smd transistors doing smth?)

There is a diode bridge so the line interface is a 'dry transformer'
type.

If there is a transformer, then the DAA is probably dry, as you say.
Otherwise there would usually be two chips, one on the line side and
one on the host side, with two or three ceramic caps providing the
isolation.

Once you disconnect the line and plug it back in, it goes free for
few moments...

The symptom you are describing is that of stuck contacts in the hook
relay. But then you say that there is none ...

What do you suggest I check, other than the optocoupler?

The optocoupler is there to detect the ring voltage and to isolate it
from the host side electronics. There should be a 1uF 250V cap in
series with its input, as well as a ~10K resistor.

( because I
think it has to be ok since the modem is actually working if I manage
to hook to provider in few seconds before it takes over the line)

There has to be some way for the host side electronics to control the
off-hook/on-hook state of the DAA. Look for a hook relay, possibly
solid state. Otherwise look for any device that bridges the gap
between the host and line sides.

Can you upload a photo to your web space?

- Franc Zabkar

here are the photos
http://putfile.com/pic.php?pic=6/17113372064.jpg&s=x2
http://putfile.com/pic.php?pic=6/17113412857.jpg&s=x2

the big lower left transistor is equiv of 2n5550 so I guess replaces
the box relay,
I wonder if somehow it could be replaced with it

It appears that you may have a "Kobian Mercury HPI56MII Data/Fax PCI"
modem:
http://www.modem-help.co.uk/mfc/kobian.html#hpi56mii

The two 16-pin chips, and/or possibly the 14-pin chip, appear to be
the line side and host side pair/trio of a silicon DAA. The two
ceramic caps provide the data path and isolation barrier. There is a
protective gap in the PCB underneath them. The line side IC would
incorporate the function of the hook relay and signal transformer. The
host side IC would, among other things, provide the off-hook/on-hook
control signals. In many conventional transformer-based modems, the
loop current passes through, and is stabilised by, a Darlington
transistor. This may be the function of the 2N5550 (???)
thanks for taking a look
One confirmation the chips you mention- mot6269352/4 are making up the
DAA
is the diode attached to its pins probably as flyback for the internal
relay.
I doubt the diode bridge is relevant since it should be located behind
the relay in front of (incorporated) transformer. The resistance at
modems line jack is few Kohms(Varistor desoldered) either right away or
in case of several other modems only after the modem(out of the PC)
grabs the line! ATZ/ATH/ATH1 report ok but do not release the line. I
guess somehow smth energizes the relay .. - Franc Zabkar
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