Carbon microphones - UK Vintage Radio Repair and Restoration Discussion Forum

Alf · 11th Jun 2008, 11:58 pm

The carbon microphone on the GPO 200 series telephone, I know it has a number but I can't re-call it. Audio from it is very crackley and intermittent, tapping it sometimes gives some improvment. Take the unit out, shake it, and you can hear what I presume to be the carbon granules moving about. It has a problem as it's been swapped over to another phone and the problem is the same. Can these be repaired

?

dagskarlsen · 12th Jun 2008, 7:48 am

Generally carbon microphone capsules are not easy repaired. Often the problem comes by moist from speaking makes the granulate making it unstable. When the microphone has ben stored in a dry place for a peride, the moist is gone, then rolling the capsule on an uneven surface, as cardboard, or just a mat the granulate wil spread more evenly in the capsule, and even lighter burned surfaces may be scaped inside the capsule. The capsule will often be usable after this threatment.
In addition to moist, to high voltage results in to high current and brocen microphone capsuls. This is more often on local battery telphones, where somone rises the voltage (add more batteries) to get a louder signal.

dsk

AndiiT · 12th Jun 2008, 5:09 pm

Hi,
There is some information here http://www.britishtelephones.com/tranchan.htm on how to fit a modern "electronic" 21A insert in to a 200 or 300 type handset.

Regards
Andrew

11th Jun 2008, 11:58 pm	#1
Alf Hexode Join Date: May 2005 Location: Wigan, Greater Manchester, UK. Posts: 479	Carbon microphones The carbon microphone on the GPO 200 series telephone, I know it has a number but I can't re-call it. Audio from it is very crackley and intermittent, tapping it sometimes gives some improvment. Take the unit out, shake it, and you can hear what I presume to be the carbon granules moving about. It has a problem as it's been swapped over to another phone and the problem is the same. Can these be repaired? Last edited by Alf; 12th Jun 2008 at 12:09 am. Reason: error

12th Jun 2008, 7:48 am	#2
dagskarlsen Heptode Join Date: Jan 2008 Location: Hakadal, Norway Posts: 643	Re: Carbon microphones Generally carbon microphone capsules are not easy repaired. Often the problem comes by moist from speaking makes the granulate making it unstable. When the microphone has ben stored in a dry place for a peride, the moist is gone, then rolling the capsule on an uneven surface, as cardboard, or just a mat the granulate wil spread more evenly in the capsule, and even lighter burned surfaces may be scaped inside the capsule. The capsule will often be usable after this threatment. In addition to moist, to high voltage results in to high current and brocen microphone capsuls. This is more often on local battery telphones, where somone rises the voltage (add more batteries) to get a louder signal. dsk

12th Jun 2008, 5:09 pm	#3
AndiiT Octode Join Date: Aug 2005 Location: Saltburn-East, Cleveland, UK. Posts: 1,786	Re: Carbon microphones Hi, There is some information here http://www.britishtelephones.com/tranchan.htm on how to fit a modern "electronic" 21A insert in to a 200 or 300 type handset. Regards Andrew