Resistor and ring strength on a 332

[worcesterdave]

Over the last couple of years I have refurbished a dozen or so 300 series Bakelite telephones. I always add a 3.3K resistor in order to reduce the REN number of the telephone but I have noticed that it sometimes significantly reduces the strength of the ring with a rather reluctant clapper on the gongs.

The telephone I'm currently refurbishing has a really weak ring but if I bypass the 3.3k resistor it rings loud and clear. I'm reluctant to leave out the resistor but wondered whether it was worth experimenting with a lower value e.g. 1K?

[Tim]

You could but it will affect what is called your R.E.N(ringer equivalence number) in other words the number of 2000 ohm BELLS(remote bells and stand alone/plug in ringers included) that your line can support. This is usually quoted as 4 for an average BT line but might be more if you live near the exchange. The phone will probably work with a 1k bell resistor but you won't be able to uses as many other phones as you would if you kept to a 3000 ohm bell resistor. The bell gongs are off centre, and can be rotated to adjust. Have you tried this?

[val33vo]

These type of phones were never meant to be operated on a plug and socket system and the bells were all designed to be connected in series rather than parallel as nowadays, when any phone was lifted off a set of contacts in the phone disconnected the total bell circuit for all the phones in the house, The coils were off much lower impedance than a modern phones electronic ringer and were resonant at the exchange ringer frequency so did not require a lot of voltage across any one

As another respondant to your e-mail has pointed out you can adjust the sensitivity of the bells by moving the gongs as they are mounted eccentrically

My only worry would be that you will affect your broadband performance if you have a few of these connected in your house ( due to the relatively high frequencies involved it is prone to standing waves off of any stubs which can seriously reduce or even stop it working altogether, I know of this through wiring up a remote terminal for my work computer when I used to work for B.T in a telephone exchange )

Whatever dont call B.T as you will find it a rather expensive event

regards

val33vo

[tele919er]

I suppose the first thing you could try is a 3k resistor. It's not a preferred value but they are available from Maplins. 3.3k is always quoted due to it being a preferred value but your really after a total impedance of 4k.

That would reduce the impedance by 300 ohms and give a little more voltage to the bell coils but still keeping it to REN1.

If that doesn't work then you could go a bit lower still, say 2.7 or 2.2K, and I think without too much detriment to your REN.

Failing all this, perhaps you should try adjusting the bell. The striker should be such that when the armature is operated fully one way by hand the striker should be as close as possible to the gong without actually touching it. Adjust by rotating the gongs. Do the same the other way. Also check that the gap between the armature and the pole faces is between 15 to 25 mils, you can experiment with this until you get a nice ring.

[AndiiT]

Hi,
Post #8 here and this thread might make interesting/useful reading.

Regards
Andrew

[worcesterdave]

Hello,

Thanks chaps,

I'll go to Maplins and buy resistors of various values between 1K-3K and experiment. I do sometimes adjust the positioning of the gongs but in this case it hasn't really helped. A 2.7k or 3K resistor might be just enough.

I don't always have this problem but I suppose that the resistance of the bell coils are going vary subtly between telephones?

[AndiiT]

Hi,
There is a lot more going on than just the resistance of the bell coils; Bell motors (as they are sometimes referred to) are current driven from the telephone exchange and being an inductive device have a nominal impedance of either 1000 Ohms or 4000 Ohms depending on whether they were designed/fitted before the advent of the "new" (now 31 or so years old so hardly all that new) plug and socket system.

The exchange provides ringing current of 75 Volts AC at 25 Hertz, I believe that the 1.8uF "bell" capacitor, in either the telephone or the master socket, and the combination of the bell coil impedance have a good reactance at this frequency, which should provide decent results from the bell motor; this has been discussed elsewhere in these forums some time ago but I can't find the appropriate thread.

It may be that one of the coils in your telephone bell has developed shorting turns, or perhaps the coils have not been wired in a "series aiding" configuration.

The other types of ringing device connected together with the telephone bell can also contribute to poor/no ringing, especially if a combination of capacitive (such as a tone caller) and inductive (such as your bell) ringers are in use at the same time as they may act as a notch filter at the ringing frequency. This scenario isn't common in practice but a lot of BT instruction books used to mention it just to be on the safe side.

Regards
Andrew

[russell_w_b]

Originally Posted by AndiiT

Hi,
There is a lot more going on than just the resistance of the bell coils; Bell motors (as they are sometimes referred to) are current driven from the telephone exchange and being an inductive device have a nominal impedance of either 1000 Ohms or 4000 Ohms depending on whether they were designed/fitted before the advent of the "new" (now 31 or so years old so hardly all that new) plug and socket system.

The impedance of a 1000R (at d.c.) bell-motor is typically 1550 + j2031, with the extra 550 'resistance' being made up of a small proximity effect and greater magnetic losses. It is not simply 1000 Ohms, not at ringing current, only at d.c.

The bell-motor will part resonate at 25Hz with the 1u8 capacitor, which, neglecting leakage, will have an impedance of -j3537. The bell alone, at an impedance (25Hz) of 2555 Z will draw 29.4mA at 75V. But when it is in series with the capacitor, the combined impedance will be 1550 - j1506 (i.e: capacitive overall) or 2161 Z, and so the current drawn will be greater, at 34.7mA.

When you connect other telephones up, such that the bell-motors are all in parallel and in series with the one capacitor, the load will vary as follows:

Total Z for two instruments: 2637
Total Z for three instruments: 2906
Total Z for four instruments: 3054

When one considers line impedance as well, it is obvious that the available current at your house will be shared out amongst other instruments:

One telephone will have 88.6V @ 34.7mA dropped across the bell motor (assuming that the full 75V a.c. is available at your house, of course, which is not always the case...)

Two telephones will have 36.3V each bell, @ 14.2mA
Three will have 21.2V @ 8.6mA
Four, 15.7V @ 6.14mA

More current equals more ting!

Does your bell polarising magnet still retain any magnetism? I was thinking one of your bobbins might have the wiring reversed, but this would result in silence; I've tried it.

[Station X]

Thread hijacking post and reply to it moved to a new thread here:-

https://www.vintage-radio.net/forum/...ad.php?t=78768

[worcesterdave]

I've managed to get a stronger ring by using a 2.2K resistor and be adjusting the positioning of the gongs. It's still not as strong as without the resistor but it doesn't sound so hesitant now and is perfectly acceptable. Many thanks for the help.

[russell_w_b]

Another option is to obtain a pair of 2K bobbins (making a 4K ringer) from a redundant 87** instrument, and fit them to your bell-motor. You can then do away with your resistor.

If your current refurb job is to be used as the sole instrument, I would just forget the resistor. If it works, it'll be OK. Out of interest, what sort of ringing current are you drawing?