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Old 15th Jan 2012, 8:54 pm   #1
JCM3000
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Default Telephone 312 won't ring.

Split from this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=78714

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I have a 312 which won't ring. I've tested continuity of all the wiring etc, the dial contacts are clean and the receiver / transmitter are known to work. It's rewired exactly the same as another 312 which works perfectly and I've removed the 205 and strapped in place of the 3k3, to no avail.
The only thing I can think is wrong is that the bobbins (2 x 500) have 'died'. How can I test this?

Last edited by Station X; 15th Jan 2012 at 9:27 pm. Reason: Thread split.
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Old 15th Jan 2012, 9:17 pm   #2
russell_w_b
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Default Telephone 312 won't ring.

Quote:
Originally Posted by JCM3000 View Post
The only thing I can think is wrong is that the bobbins (2 x 500) have 'died'. How can I test this?
Each bobbin should have a d.c. resistance of 500 Ohms or thereabouts. But don't get hung up if they read 510 Ohms, or 497 Ohms, or whatever. Have you a means of measuring the a.c. ringing voltage at your telephone or incoming line? If so, what is it?

Again, does the polarised armature retain any magnetism? Compare the 'pull' of the central bar-magnet with that of a known good ringer. This polarisation primarily augments sensitivity, but is worth checking.

It won't ring? Voltage at the bell bobbins? Does the L/H bobbin terminal connect to the L/H terminal of the other bobbin? It should, or the field will oppose and cancel out each half cycle.
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Old 15th Jan 2012, 10:00 pm   #3
JCM3000
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Default Re: Telephone 312 won't ring.

Thanks Russell.
They are wired LH to LH. Left bobbin shows resistance of ~510 ohms (difficult to tell using this little multimeter) with max resistance (ie broken circuit) on the right bobbin. I'm currently testing using a tele 704A which is giving ~75V AC according to my multimeter. The arm flicks backward and forward with the slightest manual pressure, as on my good ringers.

Does this mean that there is a break in the coil of the RH bobbin?
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Old 15th Jan 2012, 10:28 pm   #4
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Default Re: Telephone 312 won't ring.

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Originally Posted by JCM3000 View Post
Does this mean that there is a break in the coil of the RH bobbin?
Sounds like it! Check around the terminals, as this is the most likely breaking point (over-enthusiastic soldering, strain, etc...). It may be possible to pick out the fine bell wire and repair it if you haven't a spare bobbin to hand.
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Old 19th Jan 2012, 10:04 pm   #5
JCM3000
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Default Re: Telephone 312 won't ring.

Many thanks - suitable donor located in the loft, transplant carried out and the 'phone now works perfectly!
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