On Feb 6, 2016, Jim Wilkins wrote
(in article ):

"Joseph wrote in message
s.com...
On Feb 6, 2016, Jim Wilkins wrote
(in article ):

idwrote in
message ...
On 2016-02-06, Jim wrote:

I often look at all that stuff, and I find extreme ingenuity with
which people overcame limitations of their current technologies,
it
is
fascinating.

i

Tbe one that impresses me most is the old analog telephone which
does
everything over two wires without active electronics, only one very
clever transformer, speaker and carbon mike. I couldn't quickly
find a
circuit description and should return to fixing my fallen TV
antenna
that had me abseiling down the snow-covered roof.
Here is the core circuit of the pre-transistor circuit:

.https://en.wikipedia.org/wiki/Telephone_hybrid

Joe Gwinn

Instead of the phone circuit it shows the Central Office interface
between a two-wire local subscriber line and 4-wire long distance
lines with amplifiers for each direction.

The hybrid combines or separates the mouth and ear signals so
effectively that a little of the user's voice has to be leaked back
into their ear as "sidetone" to keep them from shouting. It "knows"
the difference between the transmit and receive signals in the single
wire pair. We learned the possibly bad habit of blowing gently and
silently into the microphone to make a sound that only appears in the
speaker.

Yeah, I was focused on the hybrid, which is used at both ends.

Somewhere I have complete circuit for a POTS phone, but a modern unit with
touchtone dial (the original, with pot cores and a germanium transistor). I
recall it came from an old issue of BSTJ.

I have and use a bunch of telephones from that era. After 40 years, the tone
frequencies had drifted a bit, and I had to retune the touchtone pads.


Joe Gwinn


This covers the circuit pretty well.
http://www.epanorama.net/circuits/teleinterface.html

The memory aid we learned for the functions a telephone switch must
perform is BORSCHT.
https://en.wikipedia.org/wiki/BORSCHT

Transistors enabled many improvements but 1950's rotary dial phones
with no active electronics still work on the system.

-jsw