Re: Recovery through motorbike vacation

On 2010-06-15, JDB <jd-asr@xxxxxxxxxxxx> wrote:
LP <usenet@xxxxxxxx> wrote:
http://paulseward.com/photos/2010/june-2010/DSC_2071_ATE_10-2_medium.jpg.php

Ooh, shiny!

It will be once I'm done with it - I can't leave the rust holes in the
case as they are so it'll be a new panel or two, then shotblast the rest
of the case and repaint it.

Obviously I'll get it working first. Function before Form and all that!

It's a baby really, it's only got 45 relays and 4 uniselectors. Although
they all have lots of spring sets to clean and tension so it's bound to be
full of all sorts of interesting intermittent faults.

It's been a while since I worked on an analog P{A,}BX, but those look more
like what nowadays would be considered power relays than the typical sealed
types rated for low-current switching.

I'm not 100% sure what current most of them will be switching, but as the
uniselector magnets generally require at least 500mA (Or in some cases a
lot more) to operate and most of the relays are used to drive the
uniselectors.

They're pretty standard relays for this class of kit (at least in the UK,
I've not had much exposure to other countries systems)

I say "standard", the relays themselves are modular, so by swapping around
contact sets, coils, armatures and slugs you can pretty much have any kind
of operate/release characteristics you want.

Some of the relays in the voice path have 3 windings, and act as audio
transformers for injecting the dialtone etc into the circuit - proper
clever stuff.

Before I started working with this kit, I thought relays were pretty
straight forward things with a coil and some contacts. I was rather wrong
on that one!

Since this isn't alt.sparky.recovery
it'll hardly be UI that relay/switching contacts typically have a *minimum*
current rating, required to burn away oxides and contaminants.

This is why the voice current is superimposed on top of a 50V DC offset,
to "wet" the contacts and burn off the spiders (That and with all relays
operating at 50V it makes things so much easier!)

The relay contacts themselves are all bifurcated, so that if one contact
point gets a little mucky the other one will make contact until the crud
has burnt off the dirty one.

Typical
telecom-rated relays go down to 10uA, vs upwards of tens of milliAmps for
more general purpose types. This is typically a source for snaps, crackles &
pops and other intermittent goodness.

I love the smell of contact arcs in the morning...

The contacts are silver plated and separated by paxolin. Over time the
silver migrates through the paxolin (in a nice invisible manner obviously)
and shorts out two of the pins on the relay set.

Much like the unintended consequences of the recent legislation/regulations
meant to Make The World A Better Place by eliminating dangerous chemicals
from industrial/consumer electronics, then?

Pretty much, yes!

You think they'd learn... on no. Actually, you probably wouldn't.

-Paul
--
http://paulseward.com
.