U3 Receiver Hums While Disconnected

[unbeldi]

I just tested a 1961 U3 receiver with a green varistor. Mine has a DC resistance of 33 ohms across both terminals.  I connected it to an oscillator tuned to 60 Hz frequency output, with an output impedance of 50 ohms.  Listening to it with my ear, it's clear the efficiency of the receiver drops rapidly below 100 Hz, well below 300 or 400 Hz already.

It's hard for me to hear it anymore at 60 Hz when the RMS voltage across the element is much below 100 mV and at about 40 or 50 mV I essentially have to guess that it's still there.  A younger person undoubtedly would do better.

Now, for something to induce a 50 mV potential across only a 33 ohm coil when that coil is mostly enclosed in a metal case like the U3, it would have to be an enormously powerful external field.  So... I think that is not very likely in a household.  The more likely scenario would be that there is a high-frequency radiation produced by something that has the 60 Hz line-frequency modulated on top of it, and that the U3 picks up the RF, and possibly the varistor/coil combo works as a demodulator producing the audio.  But this should stop when the contacts are bridged.  Perhaps the coil is damaged internally.
 

[JmanA9]

I spent some more time playing with the receiver.  When I remove the receiver from the handset and walk around my apartment with my ear pressed against it, the volume of the hum changes very little, if at all.  When I short the varistor and walk around the apartment, the receiver hums when I am near fluorescent lights and electronic equipment, but there are parts of the apartment where the the receiver is hum-free.  When I shorted out the varistor earlier, I must not have been far away from electronic equipment.  It seems to me that the varistor is to blame.

In response to cutting one end of the varistor, I'm in the middle of a move and this 2500D is the only telephone I brought to my new apartment.  I don't want to do anything that could potentially take it out of service since I don't plan to move my other telephones for a few more days.  From what I've read, the receiver should still function without the varistor, correct?  I'm not opposed to cutting one or both ends once I have another receiver on hand.

unbeldi, my ears are 22 years old.

[Adam]

You know why your receiver hums?

Because it doesn't know the words!

[Sorry, somebody had to say it.]

[DavePEI]

You know why your receiver hums?

Because it doesn't know the words!

[Sorry, somebody had to say it.]

Bad, Adam, bad !

[poplar1]

The U3 will work without a varistor. There will be a louder click when you pick up the handset or hang up.

[trainman]

I have that same effect with one of my phones. I have a Kellogg ringer box that someone made into a wall phone by adding the hook switch and a G3 receiver. I noticed the hum as well. The phone isn't connected to the line. I went so far as to disconenct the receiver from the phone. Hum remains.

Things in the area that may be picked up the hum. CFL light in the hall where the phone is. Photocell exterior lights that induce hash on my Ac wiring. It makes trying to listen to AM radios almost impollsible when the lights turn on.

So, the OP isn't going crazy.

[DavePEI]

No, its easy to joke about it, but, no, he isn't going crazy. Strange things happen.  All of us  here in the Asylum sympathize with him greatly!

Spock calling Captain Kirk, Hummmmmm! Something wrong with this darn communicator! Commander Data, can you fix it? I think it is the fault of the Tribbles! Hello........

Dave

[trainman]

Tried an experiment. Putting the receiver next to a light switch makes it hum. Moving it away from the light switch, the hum goes away. My wall phone hums because it is mounted on a wall where there is a light switch on the other side. I also have electronic dimmer switches in my house.

So, like others have said, it appears it is picked up AC hum from the building wires.

[Bill]

What t I find to be interesting is that even when you shorted out the varistor you continued to hear the hum. When you shorted out the varistor, you automatically shorted the receiver's coil since it is bridged across it.

The coil itself picks up the hum signal by magnetic induction, as Dave mentioned. And since the coil is a magnetic device, it is the current that energizes the coil, and makes the diaphragm vibrate. But no current can flow through the coil unless there is an external path to carry it.

Think of a power transformer. You have a primary winding, which generates a magnetic field. The secondary
winding picks up this field by magnetic induction, but no current flows in the secondary unless you add an external connection (load) across the secondary terminals.

In this receiver, the varistor can provide the path for that current. If you put a hard short across the varistor (and therefore across the coil), you have simply provided a better path for the hum current. Like putting a short across the power transformer secondary, a lot of current flows.

So I'm still thinking that if you open the varistor, the hum will stop.

Bill

[poplar1]

Maybe Edison had the right idea when he suggested that commercial power should be DC instead of AC. And maybe if they would get rid of the florescent lights in classrooms, the teachers could stop prescibing Ritalin for the hyperactive pupils.

[dsk]

May be a little off topic now 

Sometimes I really would like to try some of those medicament's on the pupils, and observe what happens.  I guess practicing a wise bedtime and a good breakfast is at least half of what they need, a little exercise  would do the rest.  Regarding the fluorescent lights, its even worse at 50 Hz, but thay actually works at dc too (slightly different circuit).   

dsk