Buzz on the line! - UK Vintage Radio Repair and Restoration Discussion Forum

trevwgb · 21st Aug 2008, 11:56 pm

Hi
Having found one of those 1960 skeleton phone repros with nothing inside I thought it would be entertaining to make it work. I have achieved this with a number 27 induction coil and wired the whole lot up as per ( I think ) the 232 N diagram without a dial. It works fairly well, but there is much more noise ( continuous buzz ) in the receiver compared to my other old phones. It is the same in all phone sockets and with and without a broadband filter. Could the experts give me some clues as to what things might cause such noise?
Many thanks
Regards
Trev

Chipmunk · 13th Sep 2008, 6:15 pm

Hi Trev, 2 things I know of can cause a buzz like that, firstly, are you using the innards of a 21A mic rather than the original? I have had several go bad, producing a loud hum such as you mention (I haven't checked the waveform to find out whether it's mains hum being picked up in the wiring tho).

Also, if you're using a 3 wire connection (IE using the bell wire as the GPO would have), having the A/B wires reversed can cause hum in some situations (Although more often the result is just very poor speaking as the gpo would say)

russell_w_b · 13th Sep 2008, 8:57 pm

Does it sound like 100Hz mains hum? If not, then what does it sound like, frequency-wise?

russell_w_b · 15th Sep 2008, 5:07 pm

Quote:

Originally Posted by russell_w_b

Does it sound like 100Hz mains hum? If not, then what does it sound like, frequency-wise?

It occurred to me as well that if you have disembowelled a 21A red mic insert and fixed the electret capsule is at a distance from the IC, then you might need screened cable to link them.

21st Aug 2008, 11:56 pm	#1
trevwgb Tetrode Join Date: Feb 2007 Location: Haywards Heath, West Sussex, UK. Posts: 81	Buzz on the line! Hi Having found one of those 1960 skeleton phone repros with nothing inside I thought it would be entertaining to make it work. I have achieved this with a number 27 induction coil and wired the whole lot up as per ( I think ) the 232 N diagram without a dial. It works fairly well, but there is much more noise ( continuous buzz ) in the receiver compared to my other old phones. It is the same in all phone sockets and with and without a broadband filter. Could the experts give me some clues as to what things might cause such noise? Many thanks Regards Trev

13th Sep 2008, 6:15 pm	#2
Chipmunk Triode Join Date: Mar 2007 Location: Southampton, Hampshire, UK. Posts: 30	Re: Buzz on the line! Hi Trev, 2 things I know of can cause a buzz like that, firstly, are you using the innards of a 21A mic rather than the original? I have had several go bad, producing a loud hum such as you mention (I haven't checked the waveform to find out whether it's mains hum being picked up in the wiring tho). Also, if you're using a 3 wire connection (IE using the bell wire as the GPO would have), having the A/B wires reversed can cause hum in some situations (Although more often the result is just very poor speaking as the gpo would say)

13th Sep 2008, 8:57 pm	#3
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Buzz on the line! Does it sound like 100Hz mains hum? If not, then what does it sound like, frequency-wise? __________________ Regds, Russell W. B. G4YLI.