Off-topic informational anti-spam anecdotal

From: Philip.Belben_at_powertech.co.uk <(Philip.Belben_at_powertech.co.uk)>
Date: Fri Jul 17 03:59:11 1998

Tony Duell quoted me as having written:

>> battery - 12V was all that was left after voltage drop when the 30mA or
>
> Don't forget the few volts (probably 10V or so) dropped across the line relay
> coils at the exchange to sense the off-hook condition. The system is
> (obviously) designed so that a short on the line (no matter how short the
> line is) won't cause damage. Applying 'battery' straight to a line wouldn't
> allow for this.

Good point.

>> had to go the several miles to your house and back. May still be like
>> that, come to think of it.
>
> AFAIK it is. The specs of 'phone lines rarely change, and there are plenty
> of 706's and 746's still in use. Maybe even the odd 300-series.

I still use a tone-dial 746 as one of my phones and a rotary dial
300-series wall mount version (don't know the number) as my other. The
only two reasons I don't use my type 332 are 1) the induction coil is
dead and 2) I have now borrowed microphone and speaker diaphragm to keep
the wall mount one going.

> Note for US types. UK 'phone wiring then was even stranger than it is

Another note for US types. Type 330 and its derivatives = pre-war
design of phone. My 332 actually has cloth insulation on internal
wires, although the (much rarer) wall mounted one has plastic
insulation. Late 1950s (?), much lighter design, PCB based, introduced,
called 706. As components got smaller, the number and size of PCBs
reduced, and the 746 has all the components, including the line drop
compensator (resistors, diodes, light bulbs) on a single PCB. This
design remained in use for around 20 years - PCB from phone circa 1970
interchangeable with PCB from phone circa 1990, the main difference
being the size of the capacitors and (if you're lucky) the provision of
a few extra screw terminals. (dial, bell, handset etc. all terminated
with ring or fork (spade) terminals to screw onto these).

I keep several type 746 phones in stock - people still occasionally want
rotary dial phones...

>> the 1960s (? What date is the 706 anyway ?), many UK phones had the
>> "line drop compensator" on a plug-in module, presumably so you could
>> swap it for one with different resistors...
>
> AFAIK the 'line drop compensator' had a constant-current barretter lamp
> on it, and there was only one version. Certainly I've only ever seen one
> version, and that's the one shown in all schematics. Alas my copy of

I've only ever seen one version, but then again, I've never seen a
schematic that actually gives the resistor values.

> You could plug the 'line drop compensator in upside-down, whereupon it
> was out of circuit and it bridged the appropriate terminals to use the
> 'phone without it. I beleive you did this at the end of very long lines.

Could be what I was thinking of.

Philip.
Received on Fri Jul 17 1998 - 03:59:11 BST