1950 500 no dial tone - please help!

[RotarDad]

Hello forum - I have just completed a restoration on my 12/50 500 (425A network and 311A equalizer).  But..... It doesn't work....    Here are the symptoms:

1) No dial tone with the handset is lifted (I do hear a low volume "white noise" and can hear static when I connect the line wires).  No ability to dial out.

2) It does ring nicely on incoming calls.  However, when handset is lifted, the ringer stops, but I can still hear electrical noise in the earpiece that matches the ringer signal.  No ability to take the incoming call, and the ringer resumes ringing when I hang up.

3) At one point I did get a dial tone ( without any wire change) but it was not very loud, and I could induce static by tapping on any component in the base.

I've carefully compared with a known-good 2/51 500 and the wiring and dial pulse cam are all good as far as I can see.  I carefully cleaned the switch hook contacts using narrow strips of paper pulled through the closed contacts, but no effect.  What am I missing???  Thank you!!

[rdelius]

It is possible that the equilizer is bad

[RotarDad]

I also eliminated the dial from the circuit and wired the net like a manual set (moved brown stripe wire from "F" to "RR" terminal on network).  No change.

Rdelius - would a bad equalizer prevent dial tone?  I could unsolder the equalizer leads and connect the handset like a 500T ( which had a terminal board instead of the equalizer).  I'd like to avoid that unless I'm more certain about the source of the issue.

[rdelius]

I  believe that the equiliser had a resistance lamp to regulate current and the the lamp could burn out

[Dan/Panther]

I would not unsolder any components until you have absolutely eliminated all possible causes for the problem first.

D/P

[RotarDad]

Thanks Dan.  I too consider unsoldering things to be a last resort.  I'm using an Xlink Bluetooth device to test the phone as well as my Panasonic 616.  I don't have a POTS line anymore.  Not sure if that would make any difference.  Again my 2/51 500 works fine with either of these phone systems. 

I'd greatly appreciate more input on this issue.....  Thanks!

[RotarDad]

Got it fixed!!!!!

The issue turned out to be the center contact for the T-1 transmitter.  See the pic where the center contact in the black G-1 handset is too low and was not making electrical contact....  I started by replacing the entire handset and cord and the set worked.  Then a process of elimination until I noticed the low contact.

I'll have to review the schematic in more detail, but the T-1 circuit is required for dial tone...

[Dan/Panther]

Great find. Sometimes a problem like that can drive you nuts, especially if the contact is only slightly low.

D/P

[andre_janew]

Never thought something like that would cause problems in a 500 set.  Of course, all my 500 sets have a 425B or newer network and will still have dial tone even with the transmitter removed.

[RotarDad]

Andre - Good point about the 425b setup still providing dial tone without the T-1 transmitter.  I hadn't realized that.  We know that taking a phone off-hook creates a DC signal path across the wire pair, which is detected as loop current back at the central office, which then provides dial tone.  On the 425A/311A set-up, the T-1 must be part of a single circuit path that is interrupted when removed.  This explains why my phone kept ringing when the handset was lifted - no signal path.  With the 425B, there must be a another complete signal path even without the transmitter, so the central office "sees" the phone.  I wonder if this was done to expedite repairs - an open transmitter circuit wouldn't completely disable the phone so trouble-shooting is easier??

[RotarDad]

Dan - Thanks for sharing in my repair "victory"...   I was very bummed when that phone didn't work after all my restoration effort.   I can appreciate an aircraft in a museum restored to "static display" level, but it bugs me that it isn't complete.  I much prefer a restoration to "flying condition" even if the plane is never flown again.