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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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18th Jul 2020, 8:24 am | #21 |
Dekatron
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Re: A phone too far ...?
Telephone Guy...
Don't worry about the technical blurb. Just be aware that the impedance of a bell movement is 'nominal' and actually varies. Of the small sample I've analysed (all 59A standard bell movements) they vary from impedances of 2460 ohms Z to 2628 Ohms Z. It may be that your telephones represent a wide sample of impedances and when you have that little current driving them, any differences will show. Your line loop might be a considerable resistance too, limiting the current even further. The resistor (3k3) drops approximately half the voltage available to ring the bell and is a considerable loss. Trouble is, leaving them strapped out may cause a 'ring-trip' overload at the exchange. If, say, four telephones with resistors each draw 5.7mA, then strapping each resistor will allow them to draw 6.2mA. Not a vast increase, but an increase nonetheless. It sounds like you're 'on the edge', current-wise!
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23rd Jul 2020, 2:24 pm | #22 |
Tetrode
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Re: A phone too far ...?
I'd like to call this a progress report, except that that implies there's been progress.
To briefly recap the setup: four phones, all 782s or possibly 8782s. Originally only three ( phones B, C & D ), all of which worked fine. Added phone A to the equation: none of the phones would now ring properly ( either intermittent or not at all ). Since then I've established that all four phones have 2 x 500ohm bell motors. Each phone works fine when plugged in individually, either to the master socket or any extension socket. All phones originally strapped the same, i.e. T4 / T5 / T6 strapped together. Checked and set all bell positions correctly as per instructions in previous posts just for good measure, although as I say, each phone works perfectly when plugged in on it's own. Inserted the resistor in place of the T4 / T5 strap in phone A. Phone A now rings loud and clear. The other phones do ring: phone B sounds like it's struggling to ring, the other two ring slightly quieter but audibly. After that, leaving the resistor in phone A, if I insert a resistor in phones B, C or D ( singly - haven't tried them all together for obvious reasons ), the phone with the resistor in it doesn't ring at all but the others all do; with varying 'intensity', but I'm assuming that's just the differences between individual phones. The point is that they do all ring audibly but the one with the resistor in doesn't, at all - except for the most-recently-added phone A, of course. I'm half tempted to leave the resistor in phone A and leave the rest as they are, inasmuch as they do at least all ring that way; but B, C & D don't ring 'properly', and sometimes seem to struggle to ring at all. Plus, of course, by my very rough arithmetic, this must still mean that I've got a total REN of something in the order of 13 on the circuit as a whole, which can't be good. Interested to hear any and all thoughts! |
23rd Jul 2020, 3:12 pm | #23 |
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Re: A phone too far ...?
It's probably worth inserting resistors into all of them and seeing how well they all ring with the ringing current balanced between them. You can always revert to your present situation if that makes things worse.
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23rd Jul 2020, 6:15 pm | #24 | |
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Re: A phone too far ...?
Quote:
Given that a magneto bell makes a reasonable sound as low as 6mA (if set up right) you could easily be OK if all telephones are fitted with resistors.
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25th Jul 2020, 3:34 pm | #25 |
Tetrode
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Re: A phone too far ...?
It worries me that once again we seem to have hit a difference between the theory and the reality.
All four phones now have resistors in; all strapped identically as per instructions and all bells also set as per instructions. Phone A - the new addition - rings as healthy as a horse. Phones B, C and D struggle to varying degrees; they do ring, just about, but very intermittently. In other words, exactly as per the fifteenth post on page 1, when there was a resistor in phone A but none in phones B, C or D. As a matter of interest, if I unplug phone A altogether ( i.e. now I'm back to where I was in the first place, except all the remaining phones have resistors in whereas they didn't previously ), the other three phones all ring more-or-less normally ( more or less inasmuch as phone D is very slightly intermittent, but not noticeably more than it ever was ). Inasmuch as it doesn't seem to have made any material difference, I'm not sure whether to leave the resistors in phones B, C & D or remove them. In view of the fact that neither my fingers nor my eyesight are ideally made for the job, coupled with the fact that two of the four sockets aren't that easy to reach ( hence having taken about an hour and a half to do the job in the first place ), I plan to leave them in place for the time being, at least until anyone has any other ideas. Speaking of which ... does anyone have any other ideas, or as barmy as it seems, is it just the case that we're going to have to accept one phone ringing normally and the others struggling to ring at all? |
25th Jul 2020, 4:07 pm | #26 |
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Re: A phone too far ...?
Clearly, for some reason 'phone A is taking the lion's share of the ringing current, but as to why this is so, I am completely stumped. When the 'phones are on-hook, the only components in circuit are the bells and any series resistors. If all 'phones have 1KΩ bell motors in series with 3.3KΩ resistors, they should each take one quarter of the current. There has to be something reducing the overall impedance of 'phone A's ringer for it to sap the current from the others.
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25th Jul 2020, 4:39 pm | #27 |
Tetrode
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Re: A phone too far ...?
Even without any greater technical knowledge than I possess, I do follow your logic; like you, however, I cannot fathom why this should be the case. Clearly I'm not out to sabotage my own phone system, so I can attest to the fact that all four phones have identical bell motors ( unless we're really going out on a limb and entertaining the notion that one of them could be wrongly labelled, but hardly seems likely ), all are strapped identically, all have 3.3k resistors between T4 and T5, all the bells are set up as per instructions, and that's how they're ringing - or not.
Unfortunately I don't have a test meter, or I could ( presumably ) establish categorically how much current each phone is drawing. I wonder whether another way of looking at the problem is: instead of trying to fathom why phone A is seemingly drawing more current than the other three, simply acknowledge that it is, and consider what can be done about it? Now the other three phones all have resistors in, maybe remove the one from phone A? Probably way too simplistic, to be fair, but just a thought. More than happy to hear from anyone else more knowledgeable than I ( so, all of you, more or less ) who may want to weigh in. |
25th Jul 2020, 4:49 pm | #28 |
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Re: A phone too far ...?
The effect of doing so should be to exacerbate the problem by causing 'phone A to draw even more current!
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25th Jul 2020, 4:54 pm | #29 |
Tetrode
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Re: A phone too far ...?
Fair point. I was just having a dopey five minutes. Starting to lose it a bit, I think ...!
... although ... I'm almost afraid to ask, but ... given that I've only got the 3.3k resistors, so no others to choose from, maybe add a second resistor in phone A ( series or parallel, if it makes any sense at all? - that's where my memory of the Philips Electronic Engineer kit starts to go a bit hazy )? |
25th Jul 2020, 5:14 pm | #30 |
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Re: A phone too far ...?
Putting a second resistor in 'phone A in series* is worth a try, just to see what effect it has. The easiest way to achieve this would be to replace the strap between T16 and T17 with a second resistor, giving you one each side of the ringer.
*With resistors, parallel reduces the resistance, whereas with capacitors, parallel increases the capacitance.
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25th Jul 2020, 5:32 pm | #31 |
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Re: A phone too far ...?
I'd start by removing the resistor from 'phone A and connecting nothing in its place. This should open the bell circuit of that telephone. Now see what rings now. What _should_ happen is 'phone A would be silent and the other telephones should ring as if 'phone A is not there.
If not, then there is some other fault, probably in 'phone A, which needs to be sorted out |
25th Jul 2020, 5:43 pm | #32 | |
Tetrode
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Re: A phone too far ...?
Quote:
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25th Jul 2020, 5:44 pm | #33 |
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Re: A phone too far ...?
Ringer impedance does vary, but only to the extent of a few fractions of milliamperes drawn. They're all roughly the same although a really long line may pick out the weaker ones.
I wouldn't get hung up on REN, or 3k3 resistors. As well as trying an extra series resistor in Phone A, you could parallel a couple of 3k3 resistors-per-phone so you have 1k65 in series instead of 3k3. The definition of REN = 1 is based upon the impedance (not the d.c. resistance) of a type 59D bell, according to ETSI standards for telecommunications. The impedance of a 59D bell, the one with the 4K bell motor is around 10960 Ohms ∟49° or 7185.6 + j8275.4. The d.c. resistance figures are just given as a guide and assumes that electromagnetic bells with indictance, as well as resistance, are deployed. A lot of electronic phones claim a REN of 1 but in reality are much less than this. So you could probably get away with it. However, if you try Dave's suggestion first and it works, then I'd leave it. Just by the way... You have disconnected all straps T8 - T9 haven't you? or T7 - T8? Or even both? Just eliminating the chance of a capacitor being inadvertently being wired into cct!
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Regds, Russell W. B. G4YLI. Last edited by russell_w_b; 25th Jul 2020 at 5:50 pm. Reason: Further details about REN added. |
25th Jul 2020, 5:47 pm | #34 |
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Re: A phone too far ...?
Yes. I have left phones with no resistor in circuit so they don't ring - just a break - when I can hear the remaining bells on the system, just to keep the ringer loading down.
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25th Jul 2020, 5:54 pm | #35 | ||
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Re: A phone too far ...?
Quote:
That's what I was trying to eliminate. If 'phone A continues to ring without the resistor or if the other 'phones misbehave when 'phone A (without the resistor) is plugged in then there is some current path in 'phone A that shouldn't be there. |
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27th Jul 2020, 2:10 pm | #36 | |
Tetrode
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Re: A phone too far ...?
Due to time constraints, not yet had time to try the various permutations, but I intend to work through them in running order as and when I can.
One thing, though: Quote:
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27th Jul 2020, 2:19 pm | #37 | |
Tetrode
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Re: A phone too far ...?
This just occurred to me as an afterthought.
Quote:
I've got a horrible feeling I'm making a rod for my own back by asking this question, but to me it seems to make sense ... not that it gets me any nearer solving the overall problem, unless it's of any diagnostic value. |
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27th Jul 2020, 2:27 pm | #38 |
Octode
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Re: A phone too far ...?
T7 and T9 connect to the telephone's internal ringing capacitor. That isn't needed these days as there's one in the Master Socket. It's leaving it connected that will have "Unwanted side effects." T8 should still have the red wire in the line cord and the orange one going to the dial on it.
When converting a telephone to work on plugs and sockets it's USUAL to remove the T6-T7 strap but removing the T8-T9 one works just as well, as does removing both. The trouble is that none of us can actually see your telephones so don't know how they've been set-up in the past. There are many non-standard variations that will work, or work in some circumstances. Last edited by ThePillenwerfer; 27th Jul 2020 at 2:41 pm. |
27th Jul 2020, 3:01 pm | #39 | |
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Re: A phone too far ...?
Quote:
Current through telephone A with 3k3 resistor in series with bell will be about 0.488 x current through telephones B, C, D and 0.142 x line current. Current through telephones B, C, D will each be 0.291 x line current (rounding values). So the telephone with the resistor in series with the bell is drawing nealy HALF the current of each of the other instruments. Whatever it is you've done, I would patent it!
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27th Jul 2020, 3:13 pm | #40 |
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Re: A phone too far ...?
Incidentally, for my bell-movement I'm assuming an impedance of 1549 + j2051, or 2570.2 Ohms Z at a phase-angle of 53 degrees. This is a fair representation of my samples and the figure was obtained by the 'three-voltmeter and cosine rule' method to represent an impedance drawing 'real' current (supplied by an ex-PABX sinusoidal ringing generator with an open-cct voltage of 90V at 25Hz).
Note that the impedance will change as the current changes because of in-phase eddy current and hysteresis losses, so this figure is not constant and depends on the voltage applied to the bell movement: a measurement on an impedance analyser will give a different result to that using 'real' volts and amperes. This may have an effect on your ringing.
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