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Old 19th Apr 2007, 1:15 pm   #1
Nickthedentist
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Default Acoustic shock suppressors

Hello everyone,

I have a couple of pristine ex-GPO 706 telephones in daily use here.

They work perfectly, after nothing more than cleaning the gravity switch and dial contacts.

My only complaint is the very loud click that's audiable in the receiver whenever the gravity switch is operated (e.g. when ending one call before dialing another number, without replacing the handset).

The 746-series instruments have a little semiconductor device wired between T1 and T2 to prevent this, and I would like to add this to my phones. It could be soldered to the print side of the PCB to keep things looking authentic from the component side.

But what should I use? Would a couple of diodes do? In series or parallel? And will 1N400X do, or should I use Zeners etc? I've got no idea

Thanks for your help,
Nick.
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Old 20th Apr 2007, 10:42 am   #2
AndiiT
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Default Re: Acoustic shock supressors for 706/746?

Hi,
Looking at the information on this http://www.britishtelephones.com/tranchan.htm page, a "rectifier element 205" is in fact nothing more than two diodes connected in parallel with opposite polarity.

Looking at the circuit diagram of a 706 telephone the section of the Induction coil that feeds the telephone receiver (the bit that the rectifier is connected across) is only 6 ohms, A series resistor of 15 ohms being fitted between one side of the induction coil and the transmitter.

I can't see why a couple of small diodes connected across T1 - T2 in the telephone shouldn't work as well as the "real thing" as it were, and seem to recall some later 746s having two diodes soldered on to the PCB to do the job.

I am not sure how "rectifier element 205" does it's job, but I think that Telephones are more current than voltage driven which might give some clues as to exact circuit operation.

For anyone who wants to have a go at figuring it out I have enclosed a link to the 746 N - Diagram http://www.samhallas.co.uk/repositor.../0000/N846.pdf

Regards
Andrew
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Old 15th May 2007, 4:48 pm   #3
AndiiT
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Default Re: Acoustic shock supressors for 706/746?

Hi just found this in another forum, hopefully it explains the operation of Aucoustic shock suppressors better than I can

Quote:
Rectifiers are one way devices, they pass current in one direction only. However I suspect you are referring to the units used to lessen acoustic shocks via the earpiece. All rectifiers have a property which is normally an embarrassment, they drop a volt or two before/ when passing current, due to the junction barrier voltage. In the telephone this is useful, two diodes back to back ( one working in a forward direction the other reversed) allow the voltage to rise to about 0.7 of a volt in either direction. That is a nice value for the earpiece volume. Along comes your gigantic click (the cause of the acoustic shock), the voltage might be as high as 100 volts. Our little rectifier can now turn on ( the barrier voltage has been exceeded) and maintains the voltage on the earpiece at 0.7 whilst it bypasses the current majority hence no acoustic shock.
Most of the time it does nothing.Try dialling or tapping the receiver rest without it, the click is pretty loud. Be Warned.
Hope this helps. If not I can find a site that explains the principles of the Diode to give it the generic name.
Regards
Ron
Regards
Andrew
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