sturdy old desktop phone


My 1980's model Southwestern Bell Freedom Phone desktop phone was made before quality and durability were completely forgotten by designers. It has a nice accessible hookswitch with contacts visible for inspection, and a really nice touchtone keypad that feels solid and looks like it will last a long time.
This phone has three wires connected to the input cable, red green and black, while all my other phones have only two wires connected to the input cable.
I haven't used this phone in a few years and it seems to be totally dead. There is no dial tone or static or line noise, no dialing will work, no tones, nothing.
So far I've checked the input resistance with the hookswitch open and closed and it gets a finite but very large resistance (8.5 Meg-OHM, I think) with the phone off hook, between the red and green wires, and something similar with the other pairs. The black wire is actually connected to the hookswitch contacts and also to a metal plate on the dialing keypad.
The piece you hold in your hand (handset?) will work when connected to another phone.
What should I do next to diagnose the problem? I have perused this group's topics, but I'm afraid I have little experience with phone repair beyond cleaning the contacts on the hookswitch.
Thanks, Guy
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wrote:

I'm 99% sure that is way too high. What is the resistance of the handset. The handset uses all 4 wires, 2 for the microphone and 2 for the speaker. I don't remember which are which, but the two for the mike should be electrically separate from the two for the speaker when the handset is not plugged in.

So it must be good. so is the 8megohms something in the handset or something that gets inserted by the phone? After you measure you'll know. If the resistance is in the phone, my first guess would be that it's because one of the switches aren't closing right when you take the handset off the hook. You can take a piece of wire and short out the switches that should be closed and see if there is a switch that isn't making contact like it should. A wire with an alligator clip on at least one end is good for something like this.

I'd bypass them before cleaning them, because it's probably easier and you can get a clear yes/no where you go back and forth. Once you clean them you can't make them dirty again so you might reassemble everything only to find out it was an intermittent problem that is back again. OTOH, in this case, I gather it is not intermittent. So it probablydoesn't matter.

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badgerboy wrote:

Some of the earlier touch tone phones were polarity sensitive.
Just for shits and grins try reversing the green and red leads. It can't do any damage and it might just solve the problem.
I'm puzzled by the extremely high resistance too, but there just might be a series diode in that phone used to avoid damaging it on reversed polarity and since you didn't mention measuring the resistance "in both directions" with whatever meter you used, that could be it.
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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