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LB phone line resistance max value?

Started by RB, April 18, 2020, 10:06:04 PM

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RB

I seem to have lost some notes...
I am comparing CB phone resistance to LB phone resistance.
I know with CB phones, CO equipment is 400 ohms max??
CB lines should not exceed 1300 ohms total.
CB phones, should be between 200-400 ohms.
But what is the general resistance of an LB phone? ???
Regards

dsk

That is interesting, my highest resistance of a CB trunk line is actually 470 ohms, and it works.   The only reason for this resistance on that trunk is to seize the line.

On LB lines it is more difficult, but as far as I remember 16 magneto phones in parallel on a long line in northern Norway  was the highest load I can remember to have heard about.  The phones had 2-3 kilo ohms ringers across the line.

The dc resistance of a phone in off hook state was approx 70 ohms. (130 on the older ones) No capacitors at all.
dsk

Jack Ryan

There isn't really a requirement for the resistance of a LB telephone to be anything specific and the off hook resistance varies with circuit configuration. Somewhere between about 70 Ohms and infinity.

A 317 might have the receiver (80 Ohms) in series with the IC secondary (another 80 Ohms) making 160 Ohms total. That 160 Ohms is in parallel with the ringer (say 1000 Ohms, often higher) which make 138 Ohms.

If a capacitor is used, the resistance becomes 1000 Ohms (capacitor in series with the receiver) or 160 Ohms (capacitor in series with the ringer). In other configurations where the ringer is switched out of circuit when the receiver is lifted, the off hook resistance (with capacitor in series with the receiver) will be infinite.

So, in answer to your question, the maximum off hook loop resistance is infinity.

Regards
Jack


RB