Thanks for the responses.
The classrooms are in a mobile unit install over a year ago.
The two cables where install from the main office to the
Mobile unit with no splices and are aprox. 450 feet long.
The cable is four conductor with shield. The white and shield
Are connect to the normally open call button and the black and red
are connected to the speaker, 70V, and lowest wattage tap.
I have used a meter to check for any shorts or grounds there are none.
Both cables test the same. In one room the system works, the other
room does not.
I have swapped the speaker and used different combinations of wires
including the green that is a spare. The results are the same.
Iswapped connects at the head end to a known good input and the same
results. The office gets the call from the classroom
and answers. The teacher in the classroom can hear the secretary but
the secretary cannot hear the classroom, no background noise, nothing.
All calls are made and answered with the receiver off the hook.
Thanks again for the help.
RBS
Well, that is a tough one. You've done all the right things
to troubleshoot, and it sounds as if the rooms have been
added correctly.
My hunch is that one of the audio pair to that trailer
(red/green) is shorted to Earth, NOT necessarily to shield.
It has probably occurred where the cable enters the skin of
the trailer, but could be in the main building as well.
Since the trailers are probably in close proximity, get
ahold of any two-conductor (even an extension cord would
work temporarily) and with some jumpers, connect the "bad"
speaker to the audio pair of the "good" speaker in the other
trailer. If you get two-way audio, you'll know you need to
pull new 357 to the "bad" trailer.
Another alternative, since this is configured as a three-
wire system, is to substitute the green wire at BOTH ends
for the red, then the black, and see if that resolves the
issue. If it works, you may be able to leave it that way if
the noise level isn't too high, as the green is unshielded.
.....
Just had another thought. You said you checked for shorts,
but did you check for opens? Disconnect both red/green at
both the speaker and the head end. Read across them from
either end, you should get infinite Ohms (if not, they're
shorted somewhere). Now twist one end together and read
across them from the other end, you should get something
less than 10 Ohms. If not, obviously an open conductor.
The most likely place will be at the splice in the trailer,
which should be either at the switch or at the speaker.
And yes, it is still possible to get some one-way audio on a
system such as this with one conductor open.
--
Mark
The truth as I perceive it to be.
Your perception may be different.
Triple Z is spam control.