power connections for GPO loudspeaking telephone - UK Vintage Radio Repair and Restoration Discussion Forum

Dave Moll · 4th May 2010, 12:34 pm

Just a quick question this time:

The LST4 series of GPO loudspeaking telephones requires an external 50V DC power supply. Whereas I assume that this is connected to the additional two wires (orange and black) in the line cord, can someone please confirm which way round it needs to go (and indeed whether this assumption is correct). Although intuitively black says negative, leaving orange as positive, I wouldn't wish to risk blowing the circuitry by getting it wrong - nor do I fancy another session of circuit tracing just to work it out.

Dave Moll · 4th May 2010, 2:40 pm

Having now opened up my telephone power supply unit and being rather surprised to find that it doesn't have a 50V tapping, I would like to add a supplementary question: the options being 10V, 20V, 40V, 70V - would 40V be adequate?

Afterthought: having noticed that it is currently set to give 30V between the 10V and 40V outputs, presumably the solution to the above problem is to connect between 20V and 70V to give 50V.

AndiiT · 4th May 2010, 3:52 pm

Hi,
40 Volts should be adequate to power the telephone, but connecting between the 20 and 70 volt tappings would have most likely been done "back in the day", as it were.

Looking at the diagram shown Here it appears that Blue is the Negative connection and Orange the Positive one which fits in with standard wiring practice of the time (i.e. Blue - Battery, Orange - "Earth") the Black lead being a spare.

Regards
Andrew

Dave Moll · 4th May 2010, 5:59 pm

Spot on! Thank you for that - and for pointing me to the right place on the samhallas web site for the N diagrams.

I have now tried it and it works!

edit: as the problem is solved, I'll close the thread.

4th May 2010, 12:34 pm	#1
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,130	power connections for GPO loudspeaking telephone Just a quick question this time: The LST4 series of GPO loudspeaking telephones requires an external 50V DC power supply. Whereas I assume that this is connected to the additional two wires (orange and black) in the line cord, can someone please confirm which way round it needs to go (and indeed whether this assumption is correct). Although intuitively black says negative, leaving orange as positive, I wouldn't wish to risk blowing the circuitry by getting it wrong - nor do I fancy another session of circuit tracing just to work it out. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

4th May 2010, 2:40 pm	#2
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,130	Re: power connections for GPO loudspeaking telephone Having now opened up my telephone power supply unit and being rather surprised to find that it doesn't have a 50V tapping, I would like to add a supplementary question: the options being 10V, 20V, 40V, 70V - would 40V be adequate? Afterthought: having noticed that it is currently set to give 30V between the 10V and 40V outputs, presumably the solution to the above problem is to connect between 20V and 70V to give 50V. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley) Last edited by Dave Moll; 4th May 2010 at 2:49 pm. Reason: noticed alternative solution

4th May 2010, 3:52 pm	#3
AndiiT Octode Join Date: Aug 2005 Location: Saltburn-East, Cleveland, UK. Posts: 1,786	Re: power connections for GPO loudspeaking telephone Hi, 40 Volts should be adequate to power the telephone, but connecting between the 20 and 70 volt tappings would have most likely been done "back in the day", as it were. Looking at the diagram shown Here it appears that Blue is the Negative connection and Orange the Positive one which fits in with standard wiring practice of the time (i.e. Blue - Battery, Orange - "Earth") the Black lead being a spare. Regards Andrew

4th May 2010, 5:59 pm	#4
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,130	Re: power connections for GPO loudspeaking telephone Spot on! Thank you for that - and for pointing me to the right place on the samhallas web site for the N diagrams. I have now tried it and it works! edit: as the problem is solved, I'll close the thread. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)