A phone too far ...?

[Telephone Guy]

As always, any enlightenment will be appreciated. We had three phones, all ( I think ) 782's ... the 746-shape DTMF push-button type. They worked ( pretty much ) fine: reception a bit on the quiet side, but extensive involvement from BT says that's purely the distance from the local exchange. More to the point, they would all ring fairly normally and audibly, which is to say, occasionally they might be a little quieter and less strident, but mostly just a normal ring

I've just added a fourth - or not, in fact, because I've now unplugged it again. Again, a 782 ( maybe a little older than the others ... not sure, but if so, not by much, I think ). The result is that all four phones would hardly ring at all; one was normal-ish, one intermittent, one virtually inaudible.

To the best of my knowledge, these all have a REN of 1.0. That is to say: the printing on the underside has faded and been rubbed off over the years, but to my knowledge, these phones usually had a REN of 1.0 anyway ...? Assuming that is the case, it was my understanding that with a maximum total REN of 4.0, all four phones should ring normally.

At one previous address where we had six phones, I seem to recall BT provided some sort of adapter ( not cheaply, as I remember ) to address this problem. This was some years ago, and such as I can remember it at all, it was a headache to fit, and even then it didn't completely 'normalise' the ringing on all of the phones ( bearing in mind that then there were six, now only four ).

So my question is: is there anything I can do within the wiring of the phone(s) that will solve this problem and get all four phones ringing normally ( without causing the other three to 'tinkle' when the receiver is lifted on one of them, which I accidentally managed to do on a previous occasion ), without needing to buy any new equipment ( adapters etc. ) or perform major surgery on the ( largely inaccessible ) wiring to each individual socket?

It occurs to me that this is another question to which the answer is either really simple when you know how, or else there just is no answer ( other than adapters and / or surgery ). At any rate, any help would be appreciated.

[Nickthedentist]

Do all the phones have the 2 x 2000 ohm bell motor?

Older ones, designed for series wiring, were 2 x 500 ohms.

[BRASSBITS]

have you tried plugging them one at a time in the master socket

[Telephone Guy]

Do all the phones have the 2 x 2000 ohm bell motor?

I have no idea ... is this a likely explanation to the point that it's worth me checking all the phones to find out? - If so, what would be the solution?

have you tried plugging them one at a time in the master socket

No: the master socket is in a less-than-easily accessible position. Taking the above into account, again, is this something I should do? I'm reasonably sure each phone will ring normally individually ( at least, they did when they were last in use at my previous address ). Does that tell us anything? - If I were to plug each one of them individually into the master socket and they do ring normally, what does that tell us?

[Dave Moll]

I believe a 782 would have a 1KΩ bell motor (and hence a REN of 4), whereas one with a 4KΩ bell motor (and hence of REN of 1) would have a model number of 8782. The standard workaround for a 1KΩ bell is to remove the strap between T4 and T5 within the 'phone and replace it with a 3.3KΩ resistor.

additional: The easy way to tell which bell motor you have (it may have been upgraded after the phone was built) is to look at the coils. They should each either be marked 500Ω or 2000Ω.

[Telephone Guy]

Thanks for the suggestions. I'll check out the bell motor as soon as time permits. Unfortunately, the stamps on the bottom of the phones have largely worn away, so I can't tell the model numbers for sure: at face value they all look like 782's, except ( as mentioned previously ) that the one that seems to be causing the problem looks slightly older than the rest. Should I check out the bell motors on the other phones as well?

[Dave Moll]

Originally Posted by Telephone Guy

Should I check out the bell motors on the other phones as well?

It's not a bad idea to be sure which version each is. If only one is causing problems, it sounds as though that may be 1K and the others 4K, but no harm in double-checking.

[Telephone Guy]

Well: this has proven to be an interesting exercise. It turns out that all four phones have 2 x 500 ohm bell motors, at least as best I can make out from what's stamped on the plates at the back of the motors. Presumably in theory this means that the phones shouldn't have been ringing even when there were only three of them?

As a matter of interest - referring to the 3.3k resistor workaround - all of the phones appear to have terminals 4, 5 & 6 strapped together; unstrapping T4 from T5 to replace with a resistor would, as far as I can tell, involve either cutting the strap or replacing the T5 / T6 part of it with a piece of wire. All of this is do-able, as far as I can tell; 3.3k resistors don't seem to be hard to come by. However: in light of the fact that all four phones seem to have the same bell motors and be configured the same way, would the general school of thought be that I should perform the operation on all four, or just the one that's caused offence, as it were?

Two final questions, if I might crave your further indulgence: firstly, if I perform the workaround on any or all of the phones, is it likely to result in 'bell tinkle' when the receiver is lifted on one of the phones? - and secondly ( although I suspect the answer is no ), does any of this have any effect on transmission or reception quality or volume?

As always, any and all thoughts gratefully received.

[ThePillenwerfer]

T4, T5 and T6 will be connected with two separate straps so you only have to remove the one between T4 and T5 to replace it with a resistor — no cutting or bits of wire needed.

For the money and effort involved you may as well do the lot. It will have no effect on sound quality; the bells will be a bit quieter but it'll be hard to tell.

As for what something that shouldn't work having done so, that's the difference between theory and practise. REN is supposed to be 4 but can be more depending how far you are from the exchange.

[Telephone Guy]

Thank you for that ... I'll buy four resistors, then.

Don't mind the fact of the bells being quieter, especially if it's only slightly, as long as they all ring. No 'bell tinkle' on other phones when you pick the receiver up on one of them, you think? As for the strapping: after checking all four phones, I'd have sworn that T4 / T5 / T6 were strapped together with a single three-way strap on all of them, but perhaps I was mistaken. Worst-case-scenario ,presumably even if that is the case, it's only a matter of doing what I previously thought with cutting the strap and replacing T5 / 6 with a wire? Any other observations appreciated, but in the meantime I'll Google 3.3k resistors.

As an afterthought, as I'm doing said Googling, it seems you can get various wattages of 3.3k resistors. Does the wattage matter, and if so, what would be recommended?

[Dave Moll]

Originally Posted by ThePillenwerfer

As for what something that shouldn't work having done so, that's the difference between theory and practise. REN is supposed to be 4 but can be more depending how far you are from the exchange.

In the days before I installed a PABX, I had certainly reached more than four 'phones without problems by the time I installed a ringing booster - and I'm over three miles from the exchange.

[ThePillenwerfer]

There shouldn't be any tinkle and bog standard ¼W resistors will be fine, though a higher rating would be fine too of course.

If it turns out to be a triple link, just bend the offending end up.

...and found this on eBay https://www.ebay.co.uk/itm/Resistors...gAAOSwv2le1h-~

SHould last a while!

[Telephone Guy]

So: an update then, if a slightly weird one.

Firstly, thanks for the link to the resistors; I'd already ordered them by that point, but I appreciate the effort nonetheless!

Secondly, apologies to those who rightly corrected me about the triple strap / double strap confusion; although, in my defence, it looked for all the world as though the straps were a single three-way strap, they were in fact two double straps side by side as Pillenwerfer said they would be. My bad.

So this being said, I got the resistors and set about replacing the T4 / T5 straps, my intention being, as advised, to do all four phones as a precaution ( it being the case that if I've understood correctly, theoretically at least, the four phones as they stand should have a combined REN of around 16...! ). As mentioned, as far as I can tell all the phones are identical, or more or less so; they're all either 782s or 8782s. The bell motors appear to all be 2 x 500 ohm, and the strapping is the same on all four ( i.e. T4 / T5 / T6 strapped together ). For simplicity's sake I'll call them A, B. C & D. Phone A is the most-recently-added one, which started this in the first place; although, again if I've understood correctly, the existing three phones should still have had a combined REN of 12! ).

So I 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.

Then I insert the resistor in place of the T4 / T5 strap in phone B. Now, phone A still rings normally, phone B doesn't ring at all, and the other two do ring but sound like they're struggling. My expectation, given that the object of the exercise is to bring the combined REN as near to the prescribed maximum of 4 as possible, was that the overall situation would improve as it went along, if anything; but now, phone B wasn't ringing at all, so I removed the resistor and replaced the strap ( having left phone A with the resistor in, as this seemed to work ). I haven't tried using the resistors in either of the other phones; since they're a lot smaller and more fragile than when I last used them ( many many years ago ), it's a lot more fiddly job than I'd expected, and I've no desire to do it if it's going to worsen the situation rather than improve it.

I'm very mindful of Pillenwerfer's quite accurate observation concerning the difference between theory and reality, although this seems to be a lot bigger difference than I'd expect. If anyone feels it has any bearing, I believe we are something like four miles from the exchange ( not 100% certain ).

If anyone has any thoughts or observations on any of the above, I'd be delighted to hear them!!

[ThePillenwerfer]

That's dashed odd as putting a resistor on in one 'phone shouldn't effect the others.

It COULD be that 'phone B's bells (or Gongs to use the correct GPO term) aren't set-up properly. If you push the striker over towards a gong, do it from the PCB end, it should just be clear of the gong itself. The screws on top of the gongs can be loosened and the gong rotated to be closer or further away from the striker and the the screw re-tightened.

[ThePillenwerfer]

Just in case it helps, here's what the circuit board should look like.

[Dave Moll]

Originally Posted by ThePillenwerfer

here's what the circuit board should look like.

I would have expected to see a strap between T16 (one end of bell) and T17/T18/T19 ("A" leg of line).

edit: Looking more closely, I see the bell wire has been moved to T17.

[russell_w_b]

I would be inclined to set up each telephone individually, including 3k3 resistor to get the ring satisfactory, even if that means adjusting the gongs to suit. As Pillenwerfer suggests, clap the bell by moving the armature across the magnets smartly with your finger. The hammer should hit the gong and bounce back or else the hammer will mute the sound.

With a 3k3 resistor in series with the bell and assuming a 75V sinusoidal ringing supply and neglecting line loop resistance, the individual bell currents for each of the four telephones will be about 5.7mA, when fed from a master LJU with the one capacitor in cct. This is quite low, but will still work. By contrast, a single telephone on the same supply will draw about 14mA if a resistor is fitted and about 35mA with a link instead of a resistor.

The difference in loudness is not proportional to the different currents drawn, but the clap may be erratic. Another factor is whether the permanent magnet in the 59A bell movement is still cutting the mustard. If it has lost some of its magnetism it won't be as good.

[Telephone Guy]

That's dashed odd as putting a resistor on in one 'phone shouldn't effect the others.

No, I wouldn't have thought so; and yet it apparently did, or at least, nothing else was changed, so I presume that's what happened.

I'll certainly try re-setting the bells in the phone that stopped ringing, although I'm constrained to point out that no matter how they're set, they worked well enough before putting the resistor in and again after removing it. Still, worth a try. The circuit board, as far as I can tell, does look like the one in the picture, except a couple of the wire colours are different.

Russell: with the best will in the world, I think you may be crediting me with far more technical knowledge than I actually possess. I understood your first paragraph and will do as you suggest and report back the results. Your second paragraph ground to a halt in my head immediately after '75v sinusoidal ringing supply'. I understand the terminology - which is to say, I know what a sinusoidal ringing supply is, I know ( more or less ) what a capacitor is, etc. - but the significance of the information in this context is lost on me, I'm afraid. If I tell you that the majority of my engineering knowledge was gained from using a Philips Electronic Engineer kit some fifty years ago, that might give you a clearer idea of the level of comprehension you're dealing with here. The possibility of a defective magnet is an interesting one, but again, I must point out that the bell worked perfectly ( well: not perfectly, to be fair, but adequately ) before the resistor was inserted and again when it was removed.

As time permits, I'll try setting up each phone individually as you suggest and see what happens. Notwithstanding whatever the results may be, I'm still baffled as to how phone B wouldn't ring at all when the resistor was in, but will when it isn't.