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Old 25th Apr 2023, 4:19 pm   #1
vic0239
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Default GPO Bell-set No 20 Magneto Help

Hello All.

Iíve got myself a GPO bell-set No 20 in the perhaps forlorn hope of getting it to function with two telephones (main & extension) as it would have in its heyday. It appears to be complete and still has the original snipped cables attached to the terminal strip. It has been ďupgradedĒ to the mark 4 version I believe.

My main worry apart from the complexity of connecting all the components is the magneto generator. It does turn, but is not smooth and has no momentum, not like my vague recollection of using this apparatus in the fifties. Can anyone advise if the magneto can be rejuvenated and whatís involved it so?

Many thanks .. Andy.
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Old 25th Apr 2023, 4:51 pm   #2
Pellseinydd
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Default Re: GPO Bell-set No 20 Magneto Help

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Originally Posted by vic0239 View Post
Hello All.

Iíve got myself a GPO bell-set No 20 in the perhaps forlorn hope of getting it to function with two telephones (main & extension) as it would have in its heyday. It appears to be complete and still has the original snipped cables attached to the terminal strip. It has been ďupgradedĒ to the mark 4 version I believe.

My main worry apart from the complexity of connecting all the components is the magneto generator. It does turn, but is not smooth and has no momentum, not like my vague recollection of using this apparatus in the fifties. Can anyone advise if the magneto can be rejuvenated and whatís involved it so?

Many thanks .. Andy.
Hi Andy,

First thing I would do is remove the bits of old cabling that are connected to the terminals. If they are shorting out, you'll find the generator could be shorted out giving a 'rough' turn. I've just tried a Mk 4 version with a Generator No 26. Shorted out, the generator26 is quite rough but without the short it is still not very smooth either - has some resistance to turning. Only way it to see how it is when you set it up.

Ian J.
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Old 25th Apr 2023, 5:56 pm   #3
vic0239
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Default Re: GPO Bell-set No 20 Magneto Help

Hi Ian.

Thanks for the suggestion, unfortunately removing the old cabling made no difference. However it made me wonder if having the bell-set on its back might have an influence so I sat it up and that seemed to help. Encouraged, I cranked it up a bit and it eased off some more.

Iím seeing ~50v across the Extn terminals on my meter while winding the handle and (drum roll) a connected telephone rings in unison! So it looks to be working.

Thanks again. Andy.
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Old 25th Apr 2023, 9:45 pm   #4
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Default Re: GPO Bell-set No 20 Magneto Help

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Originally Posted by vic0239 View Post
Hi Ian.

Thanks for the suggestion, unfortunately removing the old cabling made no difference. However it made me wonder if having the bell-set on its back might have an influence so I sat it up and that seemed to help. Encouraged, I cranked it up a bit and it eased off some more.

Iím seeing ~50v across the Extn terminals on my meter while winding the handle and (drum roll) a connected telephone rings in unison! So it looks to be working.

Thanks again. Andy.
Are you intending to wire it up for use as an intercom with just two local battery powered phones ? Or set it up with an 'exchange line' as well which can be switched through? This is my Bellset 20 with a Tele 121 as the main phone. The 'outside line' comes from an Analogue Terminal Adapter and a VoIP number on our replica of the old GPO public network of the 1960's.
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Old 26th Apr 2023, 6:50 am   #5
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Default Re: GPO Bell-set No 20 Magneto Help

Not sure about what's the primary question, but regarding the generator, after a drop of thin oil (sewing machine oil) it should turn easily if no load, when you short it it should be much harder to turn.
You may also take a look at this thread: https://www.vintage-radio.net/forum/...d.php?t=137666
And this page: https://www.britishtelephones.com/bellst20.htm
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Old 26th Apr 2023, 7:31 am   #6
vic0239
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Default Re: GPO Bell-set No 20 Magneto Help

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Originally Posted by Pellseinydd View Post
Are you intending to wire it up for use as an intercom with just two local battery powered phones ? Or set it up with an 'exchange line' as well which can be switched through? This is my Bellset 20 with a Tele 121 as the main phone. The 'outside line' comes from an Analogue Terminal Adapter and a VoIP number on our replica of the old GPO public network of the 1960's.
I intend to set it up initially with a 1/232 as the main phone and an exchange line run from my BT Revelation PBX (2xVoIP & 1xPSTN lines). Thereafter I'd like to add an extension, probably a 300 series phone. I understand I'll need an external power supply for that, but unsure as to the voltage. Do you have an extension connected?
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Old 26th Apr 2023, 7:39 am   #7
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Default Re: GPO Bell-set No 20 Magneto Help

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Originally Posted by dagskarlsen View Post
Not sure about what's the primary question, but regarding the generator, after a drop of thin oil (sewing machine oil) it should turn easily if no load, when you short it it should be much harder to turn.
You may also take a look at this thread: https://www.vintage-radio.net/forum/...d.php?t=137666
And this page: https://www.britishtelephones.com/bellst20.htm
Thanks for the links, I'd already seen the latter which describes the task of wiring it up as "onerous".

Regarding my original question I was just looking for tips as to how I could improve the operation of the generator as I didn't want to ruin it. I have some thin oil so I'll try that. Many thanks.
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Old 26th Apr 2023, 8:48 am   #8
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Default Re: GPO Bell-set No 20 Magneto Help

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Originally Posted by vic0239 View Post
I'd like to add an extension, probably a 300 series phone. I understand I'll need an external power supply for that, but unsure as to the voltage. Do you have an extension connected?
Communication between the main and the extension will require external power. As far as I recall, I used a couple of 6V lantern batteries to give me 12V for my setup, though the voltage isn't critical - a lower voltage will simply provide a little less volume.

You may find diagram N520 helpful if you haven't already seen it.
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Old 26th Apr 2023, 9:45 am   #9
Pellseinydd
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Default Re: GPO Bell-set No 20 Magneto Help

Quote:
Originally Posted by dagskarlsen View Post
Not sure about what's the primary question, but regarding the generator, after a drop of thin oil (sewing machine oil) it should turn easily if no load, when you short it it should be much harder to turn.
You may also take a look at this thread: https://www.vintage-radio.net/forum/...d.php?t=137666
And this page: https://www.britishtelephones.com/bellst20.htm
A 'Generator 26' as used in a Mk4 version of the Bellset No 20 does provide some 'resistance to turning without any load. As mentioned in my previous - and it is a brand new unused BS20 that I tried.
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Old 26th Apr 2023, 9:55 am   #10
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Default Re: GPO Bell-set No 20 Magneto Help

Quote:
Originally Posted by vic0239 View Post

My main worry apart from the complexity of connecting all the components is the magneto generator. It does turn, but is not smooth and has no momentum, not like my vague recollection of using this apparatus in the fifties. Can anyone advise if the magneto can be rejuvenated and whatís involved it so?
The magneto generator is quite straightforward to work on. I've rewound one from a Beethoven shot-firing exploder; they're the same device, but the exploder has a slightly different crank-handle connection. I once rewound an Alnico telephone magneto with thicker wire to get a low voltage but I digress...

To strip completely, remove the top strap across the magnet and loosen the side screws holding the magnet securing washers in place. The magnet will lift off the pole-pieces with a bit of effort - they're very strong magnets. You may need to remove the side-screws entirely and slide the magnet off. You needn't actually do this but it makes inspection easier.

The two-pole armature is connected to the terminals on the pull-to-changeover switch on the back by a couple of brushes on sliprings. Loosen off each large screw on insulated mounts - on the handle-end it's the lowest big screw - and the brushes sit inside. Do it on a tray, though, as they may spring out.

The handle-end may now be withdrawn by removing the four screws that connect it to the lower pole-pieces, and the armature withdrawn. It's then possible to get at the gears and clean off any caked-up grease before cleaning and re-lubricating them. You can withdraw the armature without removing the magnet.

You should get 50 - 75V at a reasonable cranking speed at the brush terminal screws. These magnetos are designed to be turned clockwise.
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Old 26th Apr 2023, 10:36 am   #11
vic0239
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Default Re: GPO Bell-set No 20 Magneto Help

Thanks to everyone for your contributions, plenty for me to consider. However, knowing the generator can be dismantled and my inquisitive nature, that might be an itch I just have to scratch!
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Old 26th Apr 2023, 12:36 pm   #12
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Default Re: GPO Bell-set No 20 Magneto Help

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Originally Posted by russell_w_b View Post
To strip completely, remove the top strap across the magnet and loosen the side screws holding the magnet securing washers in place. The magnet will lift off the pole-pieces with a bit of effort - they're very strong magnets. You may need to remove the side-screws entirely and slide the magnet off. You needn't actually do this but it makes inspection easier.
Do you need to / is it a good idea to put a keeper across the poles of the magnet when you remove it?
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Old 26th Apr 2023, 12:54 pm   #13
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Default Re: GPO Bell-set No 20 Magneto Help

[QUOTE=TonyDuell;1554215]
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Do you need to / is it a good idea to put a keeper across the poles of the magnet when you remove it?
I think it's best to do that, Tony. I parked mine on my steel filing cabinet next to my bench, away from anything meter-ish or magnetically sensitive.

That said, I don't think it would lose its magnetism that quickly, but it's always good practice. I've never seen an Alnico magneto with a poor magnet in it, but I've seen some of the older horseshoe type with hardly any magnetism left at all.
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Old 26th Apr 2023, 3:53 pm   #14
vic0239
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Default Re: GPO Bell-set No 20 Magneto Help

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...that might be an itch I just have to scratch!
Well I did scratch and got the armature and winding mechanism out without removing the magnet. Didn't look too bad, but devoid of lubrication, so cleaned the bearings and winding shaft and re-greased. Working much smoother now with around 70 volts measured at the terminals.
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Old 27th Apr 2023, 6:46 am   #15
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Default Re: GPO Bell-set No 20 Magneto Help

Regarding batteries, to high voltage may be harmful for the transmitters if the current rises to much, using small 9V batteries (with pretty limitedcurrent capacity that risk is considerably reduced, I would have started with one, if that is to little, jut add one more in series and you get 18V but still pretty limited current.

Regarding the generator, my only ruined generator was used on a power plant, and I have to guess that it was an erro making it burn up. By shortin/disconnecting and turning it around by hand, you are not able to do anything wrong.
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