Author Topic: Another What Is It?  (Read 3452 times)

Offline Adam

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Another What Is It?
« on: June 23, 2011, 01:29:17 PM »
OK.  I got one.  I found this in an (unfortunately) heavily modified Western Electric 6 button set I just received from eBay.  The whatzit is obviously not of Western Electric manufacture.

Upon first seeing it, I thought it might have been a polarity guard.  It can't be that, though, because (a) it was in a rotary phone and (b) a polarity guard would have four connections (2 tip and ring in and 2 tip and ring out) but this whatzit only has two connections.

I have a guess, but I'd like to hear what your guesses are first.  Anybody have a clue?
« Last Edit: June 23, 2011, 01:31:29 PM by Adam »
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Offline MDK

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Re: Another What Is It?
« Reply #1 on: June 23, 2011, 02:11:04 PM »
Looks homemade... 2 diodes, a resistor and a capacitor. A shot of the other side of the board might let us assemble a schematic, and determine the purpose.

Offline Adam

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Re: Another What Is It?
« Reply #2 on: June 23, 2011, 02:36:07 PM »
It's not homemade, the reverse is a very simple printed circuit with a printed designation "EM-3A".

Here's the schematic.
« Last Edit: June 23, 2011, 02:39:04 PM by Adam »
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Offline Bill

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Re: Another What Is It?
« Reply #3 on: June 23, 2011, 04:44:45 PM »
One of the diodes is probably a zener, which is usually used as a voltage limiting device. If you look closely at the diodes, you should be able to see printed numbers/letters. If you can read them to us, we can decipher the type, and maybe the purpose of the circuit. However, a diode in series with a capacitor often doesn't do what the "inventor" intended.

Bill

Offline Adam

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Re: Another What Is It?
« Reply #4 on: June 23, 2011, 05:32:56 PM »
From left to right, the components in my schematic are marked:

Blue Diode: BZX 85(something, could be 3) 4V3
Pink Diode: BZX 853 4V3
Resistor: Brown/Green/Red/Gold (1.5kΩ 5%)
Capacitor: .47K 250V CMC

So, even though the diodes appear to look different, the markings seem to indicate they are the same.  According to a reference I found on the internet, BZX 853 4V3 is a SILICON EPITAXIAL PLANAR ZENER DIODE.
« Last Edit: June 23, 2011, 07:40:45 PM by Adam »
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Offline MDK

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Re: Another What Is It?
« Reply #5 on: June 24, 2011, 08:17:41 AM »
With those both being 4.3V Zener diodes, would this allow AC ringing current through, but clip the voltage?

Offline Adam

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Re: Another What Is It?
« Reply #6 on: June 24, 2011, 09:36:56 AM »
Gee... I thought this would be easier to identify!  :D

When I got it, this set didn't have a ringer.  Also, it didn't have lamps.  I suspect the set was simply used to listen to several audio sources switched via the key.

The only thing phone-like that has two diodes back-to-back is the anti-click varistor that was used on receiver elements.  I'm not sure, I did not really pay close attention as I "decommissioned" this set and parted it out, but I think the whatzit was wired in parallel with the talk circuit of the phone.  Is it possible this was designed to suppress voltage spikes and thereby silence clicks when using the key to switch between sources?
« Last Edit: June 24, 2011, 09:41:20 AM by Adam »
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Offline MDK

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Re: Another What Is It?
« Reply #7 on: June 24, 2011, 09:38:16 AM »
Sounds like a distinct possibility.

Offline dsk

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Re: Another What Is It?
« Reply #8 on: June 24, 2011, 11:33:04 AM »
I would have guessed about 2 possibilities. This unit in series with the ringer, instead of the regular capacitor will help from stealing signal (speed) from a modem on the same line.  The second effect is to reduce or eliminate bell tinkle on a ringer without bias spring.

dsk

Offline Adam

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Re: Another What Is It?
« Reply #9 on: June 24, 2011, 12:02:25 PM »
Electrically, that makes sense.  But the set this was installed in had no ringer.
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Offline Bill

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Re: Another What Is It?
« Reply #10 on: June 24, 2011, 02:09:02 PM »
I'm inclined to go with the anti-click theory.

The ring voltage is (more or less) 90 VAC, which is 125 volts peak. Assuming the device is in series with a ringer, the back-to-back zeners in this circuit would reduce the voltage by about 5 volts (4.3v for the reverse-biased one, plus 0.7 for the forward-biased one) , which doesn't seem like it would accomplish much of anything.

The device can't be in series with the line, since the line carries DC voltage, and the capacitor would block DC.

But in the talk circuit (esp across the earpiece), it would limit the amplitude of the transient voltage spikes that sound like LOUD clicks in the earpiece, while not disturbing the DC path that operates the transmitter.

Or, as you suggested, it could serve as a primitive varistor, switching in a 1.5K load resistor whenever the talk voltage got too loud. However (and correct me if I am wrong), I don't think any of the talk or listen voltages ever get anywhere near the 5 volt level where the zeners would turn on. Meyer 2ndEd suggests that they hit a few hundred millivolts at most.

Bill
« Last Edit: June 24, 2011, 02:12:38 PM by Bill »

Offline Robert C

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Re: Another What Is It?
« Reply #11 on: June 26, 2011, 10:56:01 PM »
Greetings all,

First post.... so be easy <grin>..

I believe that this is an electrical equivalent (sp?) of a mechanical ringer.

We use to see these in SBC interfaces back in the  80's (inside single line jack interfaces).  I believe that tech's were still using brownies and they were all use to seeing a reference 'kick' for a mechanical ringer.

When electronic phone systems and ringers came along.. they were struggling to 'see' a ringer when testing toward a business or residence.  Might have been for the test centers use also... to test before sending out a tech.

Personally, I have never seen one inside of a telephone set... but that is sure similar to what I have seen.  I'll have to look around.. I may have what I am talking about.  The jacks (RJ-25) I think.. had a little grove or notch in them to hold the little circuit board neatly out of the way.

Anyway, my 2 cents.

Regards from Oklahoma,

Robert

Offline Adam

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Re: Another What Is It?
« Reply #12 on: June 27, 2011, 05:56:21 AM »
Wow.  An interesting possibility, and one which makes sense in the context that the set this was installed in had no ringer at all.  Thanks!

Excellent first post!  :)   Welcome to the forum!
Adam Forrest
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Offline Adam

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Re: Another What Is It?
« Reply #13 on: June 27, 2011, 01:51:47 PM »
Robert, the more I think about it, the more I think you must be right.  All the components just make logical sense:

1. A resistance load, like the coils of a ringer
2. in series with a .47mf capacitor, just like a ringer connected to a line would be
3. plus two diodes in series so that DC doesn't pass, to make sure it doesn't interfere with the talk circuit.

A dummy ringer!  Whod've thought?

:)
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Offline AE_Collector

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Re: Another What Is It?
« Reply #14 on: June 27, 2011, 06:08:41 PM »
Would that be what I've heard called a "Half ringer"? Simulates ringers for remote testing but isn't a ringer.

Terry