Electric door bell wiring question

[crt]

We have inherited with an old house an equally old GEC door bell and indicator system - 240v comes into a transformer and 4v-12v comes out the other side and then supplies an old fashioned bell and the indicator system which puts a flag up depending whether its the front or back door bell being rung.

It seems to work only erratically (and not at all at the moment). I've tested the 240v in which is fine, and there seems to be 12v coming out. All wires to bell and indicators seem tightly connected.

One oddity is that there are two 5mm x 20mm 500ma 12v fuses on the 12v side of the transformer, one on each of the wires coming out. (I have assumed the 240v side is fused by whichever lighting circuit is feeding it).

Does anyone have any ideas where I might look next to fault find the system and also any clues as to why the low voltage side is fused?

Also, what will be effect of putting 500ma 250v fuses in place of the 12v ones?

Thank you.

 

[Spark123]

Have the fuses popped though? You should be able to check them with a multimeter set to ohms range. I have seen PCB fuse holders go loose before causing a bad contact, just need nipped up. It doesn't really matter if you put a 240v fuse in a 12v system as long as it is the same current rating and type. It does matter the other way around i.e. if you were to put a 12v car fuse in a 240v circuit. Can you draw us a schematic diagram or post some pics of what you have?

 

[breezer]

proaby the flag mechanism is sticking, or the button to press is worn corroded

 

[crt]

Thanks for that. At least I am reassured on the fuses for the time being. When I'm next at the house at the end of the week I will do a diagram and take some pictures.

 

[RF Lighting]

Is it a system with one of these type indicator panels?

 

[crt]

Yes it is very similiar to that though the bell and transformer is to one side. The transformer is in a 4"x2" metal box marked "GEC" and next to it the bell has an external, sprung clapper.

As well as the external door bells there is a bell push in a bedroom linked to the indicator box (most of the flags/apertures are not connected to anything so far as I can see). The sole purpose of this is to allow children to ring the thing incessantly (when it works).

As to the suggestion (for which thanks) about one of the bell pushes being corroded/worn, or the flags sticking, would one being worn/sticking stop the rest working?

Pics will be out for the lads on Friday.

Thanks all.

 

[TicklyT]

If it's a DC bell, the contact breaker on the bell clapper may be worn/pitted, so only making intermittent contact. They are usually shunted with a capacitor to stop them arcing and burning, much the same as a 'proper' contact breaker in a car distributor. The capacitor can fail, or the contacts can get bunged up with dust and cobwebs etc.

If it's DC, there may be an old metal oxide rectifier in the circuit somewhere, looking like a pile of washers threaded onto a piece of studding, with 2 or maybe 4 wires connected to tags along the side of it. These can fail,leaving a DC bell etc. trying to work on AC.

A modern diode (2 Wire) or bridge rectifier package (4 Wire) may provide a convenient replacement.



If it's an AC bell, that blows those ideas out of the water

 

[JohnD]

The older ones are usually happiest on DC, they were mostly installed before houses had mains electricity and were run off large batteries.

The transformer may have been added as recently as 1925.

All contacts will be worn, tarnished and pitted.

Much of the wiring is in thin, hard drawn copper and tends to break. It has various odd kinds of insulation on it.

If you are re-making connections,you may do better to solder a new, flexible bit of stranded copper wire onto the thin old stuff. You can clean the old with a bit of wire wool first but it will tarnish very quickly.

Put a concealed switch on the one the children use.

 

[crt]

That's helpful. So what you're suggesting, I think, is replace all the hard copper old stuff with modern, stranded wire and clean up all the contacts. Correct?

What about putting a new transformer in the box?

 

[JohnD]

keep the old wire if you can. It's an original architectural antique, like your corniced ceilings and panelled doors. Just remake the ends if you have any bad contacts. Try to clean up contactswith a toothbrush and metal polish, not sandpaper which will wear them away.

Only replace faulty bits, and keep any modern additions out of sight.

I like TicklyT's idea of a capacitor to reduce spark erosion of contacts.

 

[breezer]

you can also put a diode across the contacts to stop arcing (back emf) but you have to put it the "wrong way round"

I would opt for the fact that if it is like the picture its just old and worn out, some tlc might make it better

several times i have seen the old bell wires to ring the bell in the servants area, not electric cable but a steel cable with pulleys so the cable can move to pull the bell (like Lady Penelope does in Thunderbirds) never seen a whole one though

 

[RF Lighting]

One of these?

There is a wooden fuse box in the bottom right of the pic too

 

[breezer]

sort of, no mais required. just a loft a nd wall cavities full of wires and pulleys (ok not wall cavaties a such, but false wall cavaties)

 

[RF Lighting]

Sorry, the bell works just on the pully and wires system, no electric. The fusebox is just another feature retained when the house was refurbised about 12 years ago

 

[crt]

Finally got round to looking at it again, and after pulling it all about and vacuuming out decades of dust discovered the fault was with the adjusting screw/switch on the bell - a couple of turns after bridging the gap with a wire from the transformer to find the fault and all sorted out.