Anyone have the schematic for a Supra Modem 2400 ? Or fix for
an flakey off-hook condition?

23 responses total.

Modem energizes with the off-hook light dimly on at all times. On
power-up, the relay usually stays on-hook. Sometimes on power up,
the relay energizes and stays energized.
 
modem follows ath1 and ath0 commands correctly (it thinks), returning
OK for each. OH light goes from dim to bright when it is supposed to
go on/off hook. 
  
modem will NOT break dialtone for pulse dialing (dunno about dtmf).
 
After the relay is energized (either on power up, or ath1 command) it
stays that way until power off.
  
Any clues? Anyone have a schematic?

Anyone home?

Nope!

Lets see if anyone is at home for this one:

My 14.4 Supra FAX Modem 144LC wouldn't connect this morning. Checking it
out, I found the line-to-phone link was dead. I opened it up and found
what appeared to be a fuse on one side of the line connector. A continuity
check showed it was open. [It was plugged into the line last night during
the thunder storms....] It is identified as a BUSS SFT 375 ma 125V Z027.
It has the right dimensions for a surface mount fuse (0.285"x.170"). I'll
check out the modem by jumping this fuse and, if the modem works OK,
replace it. I was going to place an order with Digi-Key anyway, and they
have a LittleFuse equivalent ("Pico SMD..Fast Acting"). My questions: 

1. Does anyone have an equivalent fuse they would sell?
2. Does anyone *want* a couple (they sell them 5/$7.51)?
3. What's available in the way of an external phone line fuse-box or
   surge suppressor? 
4. What words of wisdom might you have?

frankly, i've not seen such a fuse in any of the modems i've examined, but
that doesn't mean much, i've only played with a few (well, several).

otoh, i've seen lots of phone-line surge supressors from small ones that go
in the modular jack on the wall, to computer power surge supressors with modem
jacks in them.  i've never used these, and don't have any knowledge of their
worth.

   My 144LC was fried in a thunderstorm a couple years ago.  I bought a GVC 
later that week and must admit, I'm glad the Supra died :)  While it was 
a shame to lose the money, the LC was really unstable when it came to 
connecting with old 2400s or dying modems.

   Anyway, I grabbed a phone surge protector from CCS for a few bucks.  I 
haven't fried the GVC yet, so I can't really say if it works but it 
hasn't failed yet.

I've never had a bit of trouble with my 144LC, connecting to anything
(though I haven't tested all of its functions). I'm rather fond of it as
it is so compact - its my portable, to go with a Powerbook. Maybe your
144LC just needs its fuse changed? I have some available at only $10 ea. 8^}

My 144LC is now naked, leaning against some books, with an alligator clip
shorting out the fuse. Interestingly, the sound from the speaker is much
muted, when its not in its case. 

Good thing it has a fuse there.  I don't think most of the modems I've seen
(even internals, where the modem getting hit in a thunderstorm could possibly
do things to the motherboard as well) haven't had them.  I run my modem line
through a surge protector that's part of the power surge protector I got a
year or so ago.  Lots of the power surge protectors have them built in these
days.

I've seen that combination listed...though it would be handy to have just
a little inline suppressor, or just fuses, for the phone cord. I'm surprised
they aren't a standard item. Well, I ordered 5 0.375 A SM fuses. Does
anyone know of any dead Supras I might buy for $5.... ?

You're really lucky something more expensive didn't fry to protect the
fuse.

Heh...I *was* thinking of choosing a 0.5 A fuse as a replacement, thinking
that the fuse really wore out, rather than was cooked, but I've gotten
good service from the first fuse (and the reaplcement will be trivial).
I don't know why they don't put a fuse holder on the board instead, though.

I've always had to chuckle at the phrase "fuse protected", as I've replaced
many a transistor that croaked to protect the fuse....

The deed is done, and my modem is back in its case, humming along.

I know why its called SM technology.... 8^{. What's a good way to hold the
thingy on its pads while soldering? Also, I learned I need a Variac for my
25 W iron - oxidation of the solder on the tip isn't a real problem when
one is applying a lot of solder and flux, but with this titchy stuff I see
I need a cooler iron. 

So, I have 4 more SM fuses, looking for good homes. Anyone have a Supra
modem with a blown fuse they'd like to sell to me? For more SM practice,
of course. 

I meant to mention that Radio Shack has a phone line surge suppressor
for ca. $15. I'm tempted (but until then, I'll just unplug the phone line).

Actually, you can use a regular lamp dimmer for your soldering iron.  An older
and less controllable technique I've also used is a diode, in line with the
power cord.  Cuts power in half.

Lamp dimmer is a good idea. I have a small Variac, so I thought of it
first. The only "lamp dimmers" I have in my house and use are X-10
modules.  What's a good criterion for setting the iron temperature? I
know, it melts the solder...but the upper choice? 

"Your mileage may vary" is about as good a rule as I can do.  I used to have
the diode on a 2 way switch, and I'd switch back and forth as the iron got
too hot or too cool.  Naturally, that means you'll have to know how to tell
hot/cold.  It's a pure resistive load, like a light bulb, so percent of the
knob rotation might be a good start.  Or plug in a light bulb to the same
dimmer and work from brightness.

Ultimately, you'll have to find a setting that solders but doesn't get too
hot.

There is an article in the QST just received by a SMT beginner that made
an amplifier. I learned from that that there probably *isn't* a swift
way to hold the component on the board for soldering - he said he used
a "plastic tuning tool" to hold the chip while soldering..presumably one
hand holding the chip on the board, the second applying the iron, and
the third the solder? Hmmm...would a SMT item be of any interest here? 
I'd be interested in learning technques, but are there enough experienced
practitioners interested also?

When I mount a SMT device I tin one side of the device and one of the pads
on the PC board.  Then, using tweezers to hold the device in place, I 
tack-solder the tined side of the part to the tined pad.  It's easy from
there.
 
In industry SMT devices are glued to the board with special glues before
being soldered.
 
I have also used hot air soldering techniques to solder parts using solder
paste.  Either way do do it, it's not easy though.

I laid a weight on the chip, and used a jeweler's loupe (from about 2
inches), so I had the board, soldering iron, solder, and my nose, all
together there. I still shifted the chip, so had to reheat and fiddle. I'm
used to making jigs for woodwork, so next time would rig up a piece of
springy metal to press the chip to the board more strongly. 

Careful about those solder fumes when you get that close!  :)

Did you know that they also make SMT fuse holders for SMT fuses?

No. However I see that in Digi-Key now. The fuse is shorter than the
one I replaced, but the block is much larger - it would not have been an
option.

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