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The why and when used of a 129F Condensor

Started by Alex G. Bell, July 29, 2017, 08:02:11 AM

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Alex G. Bell

Quote from: shortrackskater on July 29, 2017, 02:26:36 AM
I'm working on ... a run of the mill WE 302 H1 ... it ... had no audio... nothing. ... the handset wires were ... not connected to anything. Also the black contact tab was on the side, where the white wire was. And I found this small condensor inside, nestled perfectly to make contact with the black and red terminals to the transmitter, in the handset.
So next I just removed the condensor and wired it like my other 302. The phone works fine now - rings, dials out, I have sidetone, etc.
The only thing is that it seems to have just a little more loudness in the CLICK when I depress and release the hookswitch... just a little more.
Otherwise the phone works fine.
Was that condensor to reduce that click? But, if so, why don't my other similar phones have one?
Any difference in the loudness of the click is due to some other cause.  There should be no click at all when the dial or cradle switch is operated. 

If there is a click that indicates either:
(1) there is some DC through the receiver, or
(2) incorrect contact sequence in the cradle switch, or
(3) some resistance in the RD handset cord lead, or
(4) some other mis-wiring. 

One possible reason for DC is in the receiver is leakage in the 2uF capacitor section connected by its RD and BK leads between 101A IND "C" and the BK dial terminal.  You should check for DC across the receiver with a DMM set to the 200mV DC scale.

A long handset cord or one with higher than normal resistance in the RD lead for some other reason will cause the voltage drop produced in the RD conductor as a result of transmitter current, to appear in series with the receiver, causing a click.  The more voltage drop the louder the click.  In the extreme case of RD being open the click will be very loud because all transmitter current will be passing through the receiver.

The GN & BR-BL cradle switch contact must close after YL & BR-YL when releasing the cradle buttons and long enough after for fluctuations in current in the line to have died down, and it must open before YL & BR-YL when depressing the buttons.  If they operate in the wrong sequence there will be a loud click.  If they operate almost simultaneously there may be a weak click.  If depressing and releasing the cradle switch buttons slowly produces no click but letting them pop up does the contacts are operating too close together in time.

Unless this 129F capacitor is shorted it would not interfere with audio.  If shorted it would prevent transmitter operation but not receiver operation.  No audio (not seizing the line) would be due to the transmitter circuit being open due to unconnected leads in the handset and the BK contact "tab" (spring) being in the wrong place.

For reasons no one after decades of puzzling over it so far has been able to explain, 302s sold outside the Bell System to non-Bell companies, which are usually identifiable by having a handset marked F1W with no Bell System marking, were equipped with the 129F capacitor.

Alex G. Bell

Quote from: Jack Ryan on July 29, 2017, 03:49:53 AM
It is a capacitor that is connected across the transmitter to prevent packing or coherence in the presence of radio frequency (RF) signals. The RF may be the result of close proximity to a radio transmitter or sometimes due to sparks or arcing of contacts.
I agree.
Quote
It is not designed to reduce clicking although its use may have that side effect.

Jack
I have to disagree.  Please explain the mechanism by which this can occur.

unbeldi

#2
Quote from: Alex G. Bell on July 29, 2017, 08:02:11 AM
For reasons no one after decades of puzzling over it so far has been able to explain, 302s sold outside the Bell System to non-Bell companies, which are usually identifiable by having a handset marked F1W with no Bell System marking, were equipped with the 129F capacitor.

I believe it may be explained by the possibility that some non-Bell equipment might send dial pulses through the transmitter.  The 302 prevented this with an off-normal shunt in the dial.   Also note that the F3-type handsets also contained the 129F capacitor, presumably because the push-to-talk switch, closing and opening the transmitter circuit, switched DC on and off at a much higher rate than the telephone set would experience during on- and off-hook transitions.

Sharp level transitions caused by switching direct current contain high frequencies that cause the cohering.


Jack Ryan

Quote from: Alex G. Bell on July 29, 2017, 08:04:13 AM
I have to disagree.  Please explain the mechanism by which this can occur.

If there is DC switching sufficient to cause cohering this might cause clicking in the receiver through the action of the induction coil. A capacitor to suppress the the effects of DC switching and therefore the cohering will also suppress clicking in the receiver.

If the cohering is the result of close proximity to a radio transmitter then there is unlikely to be any correlation between the presence of the capacitor and reduced clicking in the receiver.

Jack


unbeldi

#4
Quote from: Jack Ryan on July 29, 2017, 10:12:30 AM
If there is DC switching sufficient to cause cohering this might cause clicking in the receiver through the action of the induction coil. A capacitor to suppress the the effects of DC switching and therefore the cohering will also suppress clicking in the receiver.

If the cohering is the result of close proximity to a radio transmitter then there is unlikely to be any correlation between the presence of the capacitor and reduced clicking in the receiver.

Jack

Cohering occurs from frequencies much above those in the audible spectrum.  The capacitance of the 129F is only 6 nF, so I doubt that the 129F affects anything much at all in the audible range of telephony.   At 1000 Hz, the impedance of a 6 nF cap is about 26 kΩ, and even at 3500 Hz it still is about 8 kΩ.

Jack Ryan

Quote from: unbeldi on July 29, 2017, 10:40:46 AM
Cohering occurs from frequencies much above those in the audible spectrum.  The capacitance of the 129F is only 6 nF, so I doubt that the 129F affects anything much at all in the audible range.   At 1000 Hz, the impedance of a 6 nF cap is about 26 kΩ, and even at 3500 Hz it still is about 8 kΩ.


I know how coherance occurs, I already explained that.

I said that the capacitor was not designed to reduce clicking but its presence *may* affect clicking. And it *may*, but not in any significant way.

I'll leave you with it.

Jack

Alex G. Bell

Quote from: unbeldi on July 29, 2017, 09:44:53 AM
I believe it may be explained by the possibility that some non-Bell equipment might send dial pulses through the transmitter.  The 302 prevented this with an off-normal shunt in the dial.   Also note that the F3-type handsets also contained the 129F capacitor, presumably because the push-to-talk switch, closing and opening the transmitter circuit, switched DC on and off at a much higher rate than the telephone set would experience during on- and off-hook transitions.

Sharp level transitions caused by switching direct current contain high frequencies that cause the cohering.
How would non-Bell equipment send dial pulses through the transmitter of a 302?  If you were talking about an F1 handset sold separately or even a 250 set which might be connected to an unknown subset, I could see some possibility, but I don't see it for a complete combined telephone set.

For that matter, WE SXS equipment, identical to AE's in this respect and not substantially different in the most important respects, interrupted loop current and produced a loud "thwack" in the receiver during switching through from each Selector switch to the next stage.  That's a sharp transition from the switching of DC current.  In fact, for years the Bell System purchased SXS switching equipment from AE and much of it was still in service into the 1970s.  So I don't believe the conditions of use were significantly different.

unbeldi

#7
Quote from: Alex G. Bell on July 29, 2017, 02:23:49 PM
How would non-Bell equipment send dial pulses through the transmitter of a 302?
F1W handsets were not only used for 302s.  Handsets were bought by many makers of communications equipment, including incl. radio applications.  I can see how the small addition of a capacitor made a potentially more attractive product for the general market.  The independent makers of telephone equipment used much more dubious, less visible arguments often to promote their products.

In addition, transmitters used within the Bell System were always returned to the service centers that had the equipment to test and reload transmitters whenever needed.  Outside the Bell System that probably was not the case very often, so a small increase in projected life time for the transmitter might have been desirable.

poplar1

 BSP states that whenever an F1A or E1E handset is used with a common battery sidetone set, then a 129-type condenser is required. Not required for anti-sidetone sets except where needed for RF.
"C'est pas une restauration, c'est une rénovation."--François Martin.

Alex G. Bell

Quote from: poplar1 on July 29, 2017, 04:12:23 PM
BSP states that whenever an F1A or E1E handset is used with a common battery sidetone set, then a 129-type condenser is required. Not required for anti-sidetone sets except where needed for RF.
Thanks.  I remember now that being cited in the past.  Which leads back to the original question as to why it would be provided in a 302 sold to non-Bell buyers.

unbeldi

Quote from: Alex G. Bell on July 29, 2017, 04:17:33 PM
Thanks.  I remember now that being cited in the past.  Which leads back to the original question as to why it would be provided in a 302 sold to non-Bell buyers.
Because it extends the lifetime of the transmitter.

Alex G. Bell

Quote from: unbeldi on July 29, 2017, 05:47:27 PM
Because it extends the lifetime of the transmitter.
Then the Bell System would have been at least equally concerned about their own maintenance costs and done the same on 302s used within the Bell System.  I don't see that the conditions of use which differed enough to call for their use only on non-Bell System 302s has been identified.

unbeldi

Quote from: Alex G. Bell on July 29, 2017, 05:57:48 PM
Then the Bell System would have been at least equally concerned about their own maintenance costs and done the same on 302s used within the Bell System.  I don't see that the conditions of use which differed enough to call for their use only on non-Bell System 302s has been identified.
I think I addressed that previously.

unbeldi

#13
Quote from: Alex G. Bell on July 29, 2017, 05:57:48 PM
Then the Bell System would have been at least equally concerned about their own maintenance costs and done the same on 302s used within the Bell System.  I don't see that the conditions of use which differed enough to call for their use only on non-Bell System 302s has been identified.

Within the economics of the Bell System, it may not have been necessary to extend the lifetime of the transmitters for the most used equipment.  The Bell System had very efficient testing, repair, and refurbishing cycles. The Bell System had locked-in customer base that did not need to be convinced of quality aspects.

But the independent market was different, and indeed needed to be convinced of superior quality and longevity of equipment, especially since every independent maker proclaimed superior equipment over WECo's at a better price, and the Bell System was enemy #1.

The primary reason for selling into the independent market was to set national standards for technology, equipment, methods, with the goal of universal telephone service monopolized by AT&T.  This was the vision of T. Vail when he took the leadership over the system in 1907.  He wanted to assure that AT&T was the company central to that goal, and the only one. So, he permitted WECo to sell to independents to get them to adopt the standards set by AT&T.   I see no reason to believe that this reasoning changed even until the breakup of the system.

I think, the small panel of technically knowledgeable people in the collector clubs today can evaluate quite properly the purely technical reasons for including or not the 129F in the handset. What is very hard today is the proper evaluation of the economic, market-place aspects of decades past in the Bell System. I think that is where the reasons for many decisions like this may be found.