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Old 29th Jun 2021, 2:15 pm   #1
john777
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Default Clunking sound on Type 12 dial when dialling out.

Hello
I have a problem with a couple of dials when dialling out.
It is two different dials, a No 10 and a No 12. They dial out and connect fine, but when the dial returns I am getting a clunk when it comes to rest which isn't pleasant on the ear. It is very frustrating as have spent some time getting the phones into working order.
From looking at some previous posts it would appear to be a problem with the points. I have cleaned the points but the problem continues. I have also tried them on different phones.
Does anyone know what could be causing this and whether there is a remedy
Thanks for reading
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Old 30th Jun 2021, 3:59 pm   #2
Oldcodger
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Default Re: Clunking sound on Type 12 dial when dialling out.

John, going back a loooong time to my days at GPO /PO training schools, I seem to remember that there was a pair of contacts on the dial designed to S/C the receiver on dialling out and these should break when the dial has returned to normal preventing "clacks" in the ear.
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Old 30th Jun 2021, 8:49 pm   #3
dagskarlsen
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Default Re: Clunking sound on Type 12 dial when dialling out.

2 diodes in opposite direction shunting the receiver may also help.
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Old 3rd Jul 2021, 2:05 pm   #4
john777
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Default Re: Clunking sound on Type 12 dial when dialling out.

thanks for reply
So could anyone give me an indication of which set of points this is, and what is the normal resting position(closed or open0 to prevent this clunk when the dial returns.
Also with regard to the pair of diodes for shunting the receiver, what type of diodes and where should they be located
Many thanks for reading
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Old 3rd Jul 2021, 6:50 pm   #5
dagskarlsen
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Default Re: Clunking sound on Type 12 dial when dialling out.

The diodes could be almost any rectifier diodes, and could be put on the receiver terminals in the handset, or on the terminals for the handset cord in the phone. They will short out all peaks of approx 0.7V.
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Old 4th Jul 2021, 3:58 pm   #6
Oldcodger
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Default Re: Clunking sound on Type 12 dial when dialling out.

JON- have a look at https://www.bing.com/search?pc=COSP&...q=N%20DIAGRAMS
(N diagrams for PO telephones always gave the diagram now a 100 more than the phone. I've looked at a diagram for a 706 phone, where the diagram was 806). Is it a physical clunk or more one in the ear. Physical clunk suggest a mechanical problem. Bang in ear suggests electronical problem.
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Old 4th Jul 2021, 8:24 pm   #7
john777
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Default Re: Clunking sound on Type 12 dial when dialling out.

Hello
Thanks for the answers
For the diodes I do have some here which I can try
Should it be one diode for each terminal (three) or are they put across the terminals, in which case what order should they be put in?
I think it is definitely an electronic problem. It seems to be one set of points. So not the point that controls the pulses. One of the ones on the other side (the inner one) if you flick it when the phone is plugged in you can almost reproduce the sound, which isn't pleasant on the ear.
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Old 5th Jul 2021, 8:28 am   #8
Dave Moll
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Default Re: Clunking sound on Type 12 dial when dialling out.

Quote:
Originally Posted by john777 View Post
Should it be one diode for each terminal (three) or are they put across the terminals, in which case what order should they be put in?
The two diodes are connected in parallel across the receiver terminals. They are, in fact, not being used as diodes as such, but relying on a characteristic of diodes that they require a minimum voltage to conduct. As long as this minimum is greater than the voltages normally encountered when receiving speech signals, their presence has no effect. If a higher-voltage pulse (such as line-voltage switching) is encountered, they will conduct and short out the pulse.

Quote:
Originally Posted by john777 View Post
It seems to be one set of points. So not the point that controls the pulses. One of the ones on the other side (the inner one) if you flick it when the phone is plugged in you can almost reproduce the sound, which isn't pleasant on the ear.
I assume this is one of the "dial-off-normal" contact pairs (probably the pair shorting out the receiver). These are normally-open, but close while the dial is being operated and open again when it returns to rest (its "normal" position). It seems that they are not opening cleanly, resulting in a burst of arcing.

Try running a strip of paper between the contacts a few times, while holding them closed, to remove any deposits. Don't use anything more abrasive than that or it may remove the plating from the terminals.
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Last edited by Dave Moll; 5th Jul 2021 at 8:33 am. Reason: addition
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Old 8th Jul 2021, 6:35 pm   #9
john777
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Default Re: Clunking sound on Type 12 dial when dialling out.

thanks for reply
Which two receiver terminals is it. There are three wires going from the handset to the main phone.
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Old 8th Jul 2021, 8:50 pm   #10
Dave Moll
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Default Re: Clunking sound on Type 12 dial when dialling out.

Assuming the handset cord follows the standard colour convention, the receiver is green and the common return is red, so the "rectifier" goes between green and red. The white wire is the transmitter.
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