[VoIP] Oslo phone

[Steph Kerman]

Russ Price wrote:
> Steph Kerman wrote:
>> If there is leakage you will be able to see that by placing a DMM set 
>> to DC mA in series during ringing. The DMM should register zero 
>> during the silent intervals. In theory it should register zero during 
>> the ringing intervals too but some DMMs do not behave well below 60 
>> Hz so it may thrash around, which would be a false indication. If it 
>> registers zero during ringing that would be a reliable indication 
>> that there is no leakage problem.
> I'll need to get my meter - I left it at work.
These days you can get amazingly functional DMMs for around $10 at many 
auto parts stores.  Like phones, DMMs good enough for "everyday use" 
have reached the level of being disposable.  I keep a few of these 
around to supplement my better DMMs because for many tests it's 
sometimes necessary to make multiple readings at once and moving leads 
around gets old quickly when taking a few readings.
>> Most likely if it does have too low an impedance for the channel bank 
>> you can put a resistor equal to the DC resistance of the ringer in 
>> series with it and it will still ring adequately without tripping 
>> ringing. What is the resistance of the ringer coil and the series
>> capacitance value?
> The markings on the ringer are too faded to discern. The capacitor is 
> marked as 0.5 microfarads, and it appears to be a double unit with 
> three wires going in, two soldered to individual terminals and one 
> soldered to two terminals via a soldered jumper.
If that is indeed the ringer capacitor it is a low value.  However 
that's an uncharacteristically low value for an old phone and one which 
the channel card should be able to cope with without difficulty since 
the capacitance value and coil resistance are usually inversely 
related.  Smaller cap. values go with higher resistance ringers.
> The wires to the coil, ringer, and capacitor are bundled together with 
> twine into an ostentatiously neat cable, so it's difficult to tell which
> wire goes where. I'll have to open it up again - the terminals have 
> various markings (but I don't understand Norwegian).
>> What model Oslo phone is it? There are modern ones and very antique 
>> ones such as were used on the first WECo Rotary offices. These have 
>> the 7001 type dial with 0 and 1 holes at the bottom and a number 
>> plate which rotates with the finger wheel.
> This one is an Elektrisk Bureau phone (based on the EB logo marked on 
> the capacitor), model number unknown, shaped like a rectangular box 
> with a cradle on top, and the dial is mounted on the cradle. The 
> dial's not a 7001; most likely it's an early Ericsson dial with the 
> Oslo layout. The zero is at 6 o'clock like an AE or BPO dial, there's 
> a very faint winding sound as I pull the dial back to the fingerstop, 
> and when the dial returns I can hear clicketyclicketyclick much like 
> early WE dials, with the governor making a slight buzz like an AE dial.
If this is a black metal box with decals and metal cradle the phone 
sounds old enough that I would be skeptical that the ringer capacitance 
is anything lower than 1uF, in which case the ringer resistance (and 
hence impedance) could well be pretty low.  0.5uF capacitors were 
usually used with much later phones having higher resistance ringers.  
In addition, Europe generally used lower ringing voltage than was used 
in the US, necessitating lower ringer resistances and at the same time 
making the addition of a resistor less likely to cause unsatisfactory 
ringing.

Lacking instruments at the moment as you are, if I were you I would try 
placing 1000, 2000 ohms and higher values in series, raising the 
resistance until (1) the problem goes away and (2) then further until 
the ring becomes unsatisfactory and picking a value in between, assuming 
the tripping problem disappears at a lower resistance than the one at 
which ringing become unsatisfactory.  The resistor probably needs to be 
rated for at least 1 watt but I have not calculated the real requirement 
and can only guess at it without more information.

Steph