the value of old test and repair equipment

[tony duell]

> > Since a Bell modem would cause a lot of problems on an old
> > UK phone line,
> 
> Why is that? Are the electrical characteristics of the phone line
> actually that different from US lines?

Electrically the lines are almost identical (there are differences in things
like ringing voltage and frequency, but you can plug a US telephone
into a UK line with no technical problems).

The problem is that one of the 300 baud Bell tones is effectively the
same as (I think) the long-distance clearing signal. Something like that
anyway. If you connect your 300 baud US modem over here it will 
effectively cause the exchange to hang up.

Modems sold in the UK have/had to be incapable of sending the
Bell tones, for that reason. One famous one based on the AM7910
chip had a pin in the mode selector switch that prevented it being
set to the positions for the Bell tones. This pin could be removed 
without even opening the case. I did that to mine, as I use it on a line
simulator (not the public network) for testing US modems, etc.

-tony