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Old 29th Aug 2009, 5:18 pm   #1
thaistyle
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Default GPO 248 telephone on Bellset 44

I have just completed a rebuild of a GPO 248 telephone on a Bellset 44.
It all went smoothly, and the conversion was made easy by the information available on britishtelephones.com.
However, I am still having a problem with solving the excessively loud "click" in the earpiece whenever the switch-hooks are toggled, and also to a lesser degree (but still annoying) when operating the dialer.
Even with the use of 205 rectifiers and back-to-back diodes I have never really solved this problem on the 332L's I have previously restored.
The switch-hook contacts have been cleaned in each case, without making a lot of difference.
Incidentally, on the 248 handset, I have left in the original earpiece and transmitter components whereas previously I upgraded with 3T/4T and 21A's.
I would welcome any help/suggestions or step-thro's to solve this niggle.
Thanks.
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Old 29th Aug 2009, 10:10 pm   #2
russell_w_b
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Default Re: GPO 248 telephone on Bellset 44

Are the 'Dial-Off-Normal' contacts clean and making contact satisfactorily, the ones that short out the receiver when dialling?
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Old 30th Aug 2009, 10:15 am   #3
thaistyle
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Default Re: GPO 248 telephone on Bellset 44

Thanks for the reply Russell.
Assuming you mean the dial-off contacts are actually on the dial (which is a No.12), can you be more specific as to which are the dial-off contacts...or should I just clean all of the contacts I can see on the dial ?
ie. with crocus-paper/solvent etc
Will cleaning these contacts reduce the loud clicks when depressing the switch-hooks, which in fact are more pronounced than the dial clicks.
Is there a test I can do with a meter to check correct contact functioning ?
Could you possibly confirm where I should be fitting the 205 rectifier ?
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Old 30th Aug 2009, 10:23 am   #4
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Default Re: GPO 248 telephone on Bellset 44

I would simply clean all the dial's contacts, and check their operation with a multimeter on the lowest ohms range and with the dial disconnected from the phone. The ones in question are the two pairs which close and stay closed while the dial's actually in use, only to be opened when the dial is back at its rest position.

No, doing this won't affect clicks when operating the gravity (hook)switch. The only way of dealing with these is by use of the rectifier element, should be connected straight across the receiver's wires in the phone (i.e. the green and red handset wires), polarity not important.

Good luck,

Nick.
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Old 31st Aug 2009, 7:11 pm   #5
thaistyle
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Default Re: GPO 248 telephone on Bellset 44

Thanks for the reply Nick.
I haven't yet cleaned the contacts in the dial as you suggested, but I will be pursuing that suggestion shortly.
Meanwhile I have fitted a 205 rectifier across the connectors, actually in the ear-piece itself - I was able to do this by replacing the diaphragm/magnet assembly with a new 2/3T receiver, which gave me enough space to do this.
Much better - the click is now definitely attenuated and easily bearable.
I have another question, which I hope is not too silly - in the bellset 44 (base unit) there are no bells but there is a buzzer - would it be possible to wire in the buzzer to buzz on incoming calls ?
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Old 31st Aug 2009, 7:33 pm   #6
Dave Moll
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Default Re: GPO 248 telephone on Bellset 44

Quote:
Originally Posted by thaistyle View Post
in the bellset 44 (base unit) there are no bells but there is a buzzer - would it be possible to wire in the buzzer to buzz on incoming calls ?
A good starting point might be to measure the resistance across the buzzer coil(s).

Also, does it have a contact which opens when the buzzer arm moves? This is known as a trembler and means that the buzzer is designed to work on DC and therefore not compatible with the AC ringing current.
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Old 31st Aug 2009, 8:09 pm   #7
Nickthedentist
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Default Re: GPO 248 telephone on Bellset 44

I fear that the answer's no.

See here: http://www.britishtelephones.com/bellst44.htm

It would appear that the Bellset is designed to be used with an external bell, and the buzzer is a DC device, used for internal signalling.

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Old 31st Aug 2009, 9:41 pm   #8
thaistyle
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Default Re: GPO 248 telephone on Bellset 44

Ok thanks for the replies.
Will download the schematics and see if I can find a way to connect up a 9v battery and then trigger the buzzer somehow.
But first I will do as Dave suggests and verify the buzzer resistance(s).
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Old 31st Aug 2009, 9:59 pm   #9
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Default Re: GPO 248 telephone on Bellset 44

Hi,
A 9 volt battery should operate the buzzer in your telephone and it may be possible to build something similar to the lamp signaling unit shown at the following link http://www.samhallas.co.uk/repositor...1000/N1880.pdf in order to operate the buzzer in sympathy with the incoming ringing cadence, substituting the lamp in the original circuit for the buzzer in your bellset.
You will still need an external 9 Volt supply for the buzzer and some rewiring of the Bellset might be required (which may be enough to not make this a viable proposition).

If you are not sure the lamp signaling unit is shown in the bold broken line at the right of the diagram

I have succeeded to do this with a more modern 740 type phone but this already had the signaling unit internally fitted.

Regards
Andrew
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Old 5th Sep 2009, 10:53 am   #10
thaistyle
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Default Re: GPO 248 telephone on Bellset 44

Thanks Andrew for your suggestion.
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