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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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19th Apr 2007, 1:15 pm | #1 |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,864
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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. |
20th Apr 2007, 10:42 am | #2 |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
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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 |
15th May 2007, 4:48 pm | #3 | |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
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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:
Andrew |
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