[Hallicrafters] Phone tips

[Mike Peron]

Waldo,
Your problem is very unusual. I used to work as a tech 35 years ago in the phone business. The phone company system has been as stable as AM/FM standards over many decades. Not sure if that has changed in these days of digital internet phones. From the days of the first dial phone has been a two wire system (tip & ring they used to call it). Taking the phone "off hook" drops the open circuit voltage of 50 vdc (provided by a huge set of batteries at the central office) down to around 10 to 15 vdc (essentially the voltage drop from the CO down to the transformer in the phone itself). While "on hook" the phone rings by the application of 90 vac superimposed on the 50 vdc (used to be done with a clever set of mechanical cams at the CO that simply switched the AC on and off at the desired rep rate). The ringer mechanism (electromagnetic at first then progressing to solid state) is therefore DC isolated. There should be a large non-polarized isolation capacitor hooked to the ringer. Check it for leakage or shorts. If OK then hang a scope or AC voltmeter on your phone line and have someone give you a ring and see what you read. Perhaps (although I doubt) the phone company has modified their interface spec and forgot to tell the rest of us. Adding a couple 100 volt zeners back to back (or perhaps a transzorb) would be a nice safeguard.

Hopes this helps

Mike
WB6UTW