>> Many years ago the standard was somewhat different, and the yellow
>> wire was sometimes used as a ground. Then, for a time, I think the
>> yellow wire was used to power the lights on princess phones. Almost
>> certainly the yellow wire is either dead or shorted to one of the
>> other wires. Check this with a volt meter.
> The yellow wire was indeed used for ground, required for the generally
> used type of party-line ringing, and also for calling party
> identification when DDD came along.
> Two wires were required, as for all electrical circuits, for the
> lights on Princess and Trimline phones. They were normally on
> yellow-black. Usually a wall war was used, but there were also
> separate plug-in transformers with binding post terminals that could
> be put in an inconspicuous location and multipled (normally on the
> yellow-black) to several Princess or Trimline phones.
Yep -- I remember that setup well. There were different wall warts depending on the configuration being set up. Our house had one but the current was higher because there were two Trimlines and one Princess in use.
Later examples of Trimline phones got the power for the lights from
> the phone pairs (another task for the C.O. battery).
Yep, got one of those on my desk right now. A yellow Trimline to be specific. Has green LED's to provide the lighting.
[TELECOM Digest Editor's Note: One thing I have noticed which is different between the older Trimline phones (with a separate power supply) and the newer units is that the newer units (with an LED to light the buttons) run from telco battery instead of an external power supply. The old units would stay lighted all the time unless you flipped the switch to turn off the light bulb. The newer units (with an LED powered from telco battery) are dark when the phone is on hook and also go dark for a few seconds as the buttons are pressed. That is unfortunate, because the older units also made very nice 'night lights' in a darkened room. With the newer (telco battery powered LED) units, you can not see the phone in a dark room until you have already found it and have it off hook. PAT]