[ARC5] Reverse engineeing the BC 456

Sun Mar 19 14:44:02 EDT 2017

    FWIW, I measured the resistance of an RS-38A which I know to work 
correctly, it is about 1700 ohms. I measured the capsule from a WW-2 
vintage EE-8 field phone, which also works correctly, it is about 500 
ohms. I also measured my only T-17D and got several thousand ohms. This 
mic is damaged and was when I got it so its not typical.
    The resistance of carbon mics depends on the condition of the 
carbon. Most will change resistance widely if shaken. I think most are 
somewhere around 500 to 2K ohms.
    The output depends on the current through the element. It should be 
as low is possible consistent with adequate output. I don't know how 
much voltage the BC-456 puts across the mic but, FWIW, an ordinary 
telephone handset puts about 4.5 volts across it. Higher voltage 
produces higher output but also increases the chances of the carbon 
particles burning and packing. If this is extensive the mic is ruined 
and the only fix is to replace the carbon.
    Carbon mics are also vulnerable to moisture. The degree of 
vulnerability depends on the design. Bell System T-1 and F-1 elements 
are very well protected, many communications types are not. Sometimes 
baking the element in a very slow oven, say not over 150F for several 
hours will dry out the carbon. I think an electric oven or a box with a 
light bulb is better than a gas oven because the latter tend to produce 
some moisture from the burning gas.
    From past experience and reading on the web I think the T-17 was not 
the most reliable of mics. However, they were made by several 
manufacturers, each with a somewhat different design, so this likely 
varies with make.

On 3/19/2017 10:12 AM, J Mcvey via ARC5 wrote:
> Got a link to that subject?
>
> Meanwhile, I've been doing some baseline tests on what amplitude is
> produced by a carbon mike.
> Here's what  i found with a "whistle test"  ( whistling a tone in the mike)
> power supply 18VDC and a load resistor of 300 ohms . This simulates the
> 60ma current limit supplied by the BC456.
>
> After several tries, the best amplitude from the T-17 was about 1.12
> volts p-p.
> Doing the same test with an old telephone carbon element  yielded over 3
> volts p-p.
>
> I suspect that the T-17 is supposed to be more like the telephone
> element and that's why the modulation is so weak.
>
> The electrets I've tested only give me about 70-100mv for the same test.
> Could have sworn I seen a lot higher outputs than that.
>
> looks like this is going to need an amp with an emitter follower to work.
>
> I'm working on the premise that 4v-pp will definitely over-drive the
> modulator. What say you folks?
>
>
>
>
> On Sunday, March 19, 2017 8:34 AM, John Hutchins <jphutch60bj at gmail.com>
> wrote:
>
>
> Chris -
>  there was many recent emails, last year, on milsurplus at mailman.qth.net
> <mailto:milsurplus at mailman.qth.net> on this subject.
> Schematic and parts list.
> Hutch
>
> On 3/18/2017 2:45 PM, Chris Bowne wrote:
> Many years back someone had a simple circuit published in ER of an
> electret element mic and I believe a source follower low impedance
>  output stage with appropriate gain ahead of it to provide more than
> ample drive for the various WW2 sets that were designed to be used with
> a T-17 carbon mic, maybe someone can supply the particular issue to look
> for it in.  If you can't fit all the components in the T-17 case, why
> not just have them in an in-line tube of appropriate size on the mic cable?
>
> Sent from my iPhone
>
> On Mar 18, 2017, at 13:46, J Mcvey via ARC5 <arc5 at mailman.qth.net
> <mailto:arc5 at mailman.qth.net>> wrote:
>
>> My idea to to modulate the bc456 with a 'level shifted" electret mike
>> hit a snag.
>>
>> The mike input transformer has ratio of 1:10 primary to secondary
>> which is terminated with about 3000 ohms which means that the primary
>> is looking to match a 30 ohm device?
>>
>> That explains why the modulation is so poor. My T-17 likes to be
>> around 2K.
>> It also precludes using the electret output directly because the
>> internal fet needs a drain resistor of 2- 5k to provide the gain,
>> requiring a medium Z to match.
>> Bummer...
>> Nothing is ever simple...
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL