Carbon microphone distortion - UK Vintage Radio Repair and Restoration Discussion Forum

richrussell · 14th Jun 2012, 9:39 pm

I've got my two Sterling U125 phones working now. They have the common sort of setup of a carbon microphone and magnetic earphone in series.

Whilst the sound is perfectly loud enough, it's quite distorted and occasionally goes quiet. If one of the microphones is tilted so it's horizontal, the sound disappears completely. I intend to check for broken wires and poor connections, but something is no doubt not quite right with one or both of the carbon granule microphones. Unsurprising really, given they probably haven't worked properly for the best part of 80 years, and have been knocking around in my garage, before that my dad's garage, and before that my grandad's garage (after he removed them from their house in the 1940s).

I'm currently powering the phones (really just an intercom) with four 1.2V rechargeable AA cells (it's wired as two batteries, one at each end). Would I benefit from increasing the voltage here?

What are the best ways to go about reviving the microphones? I've read that tapping them can help - but should I dismantle them and clean up anything inside? Is it worth looking at the earpieces too?

G6Tanuki · 14th Jun 2012, 10:08 pm

I've had problems with carbon-mics where the granules have over the years got damp and 'clumped' together.

Try putting the mic-inserts in an airtight jar/bag with some dry Silica Gel, and storing them at 20-centigrade-or-above for a few months!

Alternatively, if you can find someone with an industrial vacuum-desiccator they can do the same job in a few hours.

--G6Tanuki.

richrussell · 15th Jun 2012, 8:25 am

As they've been stored in garages for several decades it's quite possible they've got damp. I'll see what I can do about drying them out.

14th Jun 2012, 9:39 pm	#1
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Carbon microphone distortion I've got my two Sterling U125 phones working now. They have the common sort of setup of a carbon microphone and magnetic earphone in series. Whilst the sound is perfectly loud enough, it's quite distorted and occasionally goes quiet. If one of the microphones is tilted so it's horizontal, the sound disappears completely. I intend to check for broken wires and poor connections, but something is no doubt not quite right with one or both of the carbon granule microphones. Unsurprising really, given they probably haven't worked properly for the best part of 80 years, and have been knocking around in my garage, before that my dad's garage, and before that my grandad's garage (after he removed them from their house in the 1940s). I'm currently powering the phones (really just an intercom) with four 1.2V rechargeable AA cells (it's wired as two batteries, one at each end). Would I benefit from increasing the voltage here? What are the best ways to go about reviving the microphones? I've read that tapping them can help - but should I dismantle them and clean up anything inside? Is it worth looking at the earpieces too?

14th Jun 2012, 10:08 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Carbon microphone distortion I've had problems with carbon-mics where the granules have over the years got damp and 'clumped' together. Try putting the mic-inserts in an airtight jar/bag with some dry Silica Gel, and storing them at 20-centigrade-or-above for a few months! Alternatively, if you can find someone with an industrial vacuum-desiccator they can do the same job in a few hours. --G6Tanuki.

15th Jun 2012, 8:25 am	#3
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: Carbon microphone distortion As they've been stored in garages for several decades it's quite possible they've got damp. I'll see what I can do about drying them out.