Added a jack to a modem. Having problems.

[Oakton]

I've been breadboarding and building electronic
devices for many years. No matter how experienced
you are, there are times when help is needed. This
is one of those occasions.

I've got an elderly and chronically ill relative.
She can use a computer, but I've always
helped her out when she's having problems.
Her old V.90 modem needed to be replaced.
Please, don't tell me that dial-up is dead.
I've offered to pay for any broadband service
she might like to try. She's online for about
45 minutes each day and she only visits a
few web sites. Like millions of others, she
doesn't need broadband.

I decided to buy the best 56K modem I could find.
I wanted a true hardware-controller based internal
PCI modem. I selected a MultiTech MultiModem
ZPX, which has a flashable BIOS. A lot of modem
manufacturers use words like "controller based,"
buy they actually rely on a DSP.

The moment I removed the modem from the
shipping carton I knew I had made a mistake.
This relative has significant hearing loss. I
plugged a pair of amplified speakers into a jack on
her old modem so she could hear the dialtone
and handshaking. This high end MultiModem
didn't have any jacks.

I didn't care about voiding the warranty. I wanted
to get the thing installed in her PC as fast as
possible. I used a short length of narrow gauge
coax audio cable and connected it across the
two pins on that micro sized piezo speaker
element found on every 56K modem. I installed
a monaural jack through one of the retaining
brackets in an empty expansion slot. Since all
speakers are configured for stereo, I used a
stereo-to-monaural adapter plug.

I coupled the center conductor from the audio
coax through a 1 uf capacitor. The braid went
to ground.

When I prompted the modem to connect all I
heard was a low level oscillating noise. I tried
pulling to piezo element from the card, but
it didn't make any difference. Strangely enough,
if I break the ground connection I can hear a
barely audible dialtone and handshaking. Turning
the volume control all the way up doesn't help.

Is there a schematic I could look at that would
help me resolve this problem? Even better would
be some small add-on kit that I could interface
with this modem.

 

[kchriste]

Is the "speaker" on the modem really a piezo device? Ohm meter it to be sure. I suspect it really has a small voice coil of 100 ohms or so.
Does one side of the "speaker" on the modem go to ground on the card or is it driven in a bridge configuration? Again, ohm meter it to be sure.

Since all speakers are configured for stereo, I used a stereo-to-monaural adapter plug.

I have a great distrust of these type of adapters. I don't know how many times I've seen them "short out" a signal path. Put a stereo jack in the retaining bracket of the empty expansion slot and wire the left and right channels in parallel. Or first, just plug the amplified speakers in directly. You'll only get one side, but you'll eliminate any possible problems with the stereo-to-monaural adapter plug.

 

[audioguru]

The telephone line and the modem speaker are balanced. Your amplified speakers are unbalanced.
Add a 1:1 transformer to keep the telephone line parts balanced then the secondary can be unbalanced to the amplified speakers without problems.

 

[Oakton]

Thank you kchriste and audioguru.

I apologize for my rambling and lengthy
post. When it comes to designing and
building circuits a schematic, block
diagram, or picture is worth a thousand,
no, more like a million words!

I don't think I've ever seen a schematic
of a 56K modem. I know the basics of
how they work, but I've never studied
one in detail.

Thanks again!

Oakton