[ARC5] Reverse engineeing the BC 456

David Stinson arc5 at ix.netcom.com
Mon Mar 20 08:01:00 EDT 2017 

Please note that the some of these "simple" circuits
turned out to have flaw.  With no base path
to ground, the mic element could go Hi-Z
and the little preamp could start "motorboating."
I kind of expected others would refine and
post about it but no one ever did as far as I know.
I haven't gone back and addressed the problem
yet.  Will eventually.
Dave S.

----- Original Message ----- 
From: "Mark K3MSB" <mark.k3msb at gmail.com>
Subject: Re: [ARC5] Reverse engineeing the BC 456


> Took me about an hour.
>
> Here's the contents.
>
> (25 Mar 2016) [ARC5] Carbon mic replacements
> (27 Mar 2016) [ARC5] Carbon Mic Replacements
> (29 May 2016) [Milsurplus] Carbon mic booster ?
> (30 May 2016) [Milsurplus] Carbon mics, more.....
> (30 May 2016) [Milsurplus] Aw Heck (Was: SIMPLE Carbon 
> Microphone Sub)
> (30 May 2016) [Boatanchors] SIMPLE Carbon Microphone Substitute
> (22 Jun 2016) [ARC5] More on Bashless T-17 Microphone 
> Resurrection
> (16 Sep 2016) Re: [ARC5] Microphone Element
>
> I'll send you and others that have expressed interest in 
> private email.
>
> 73 Mark K3MSB
>
>
> On Sun, Mar 19, 2017 at 2:45 PM, J Mcvey <ac2eu at yahoo.com> 
> wrote:
>
>> Mark,
>>
>> that sounds like a lot to wade through.
>> I was just curious to see the conclusion/circuits made, if 
>> any. Don't
>> spend a lot of time doing a compilation, especially since it 
>> wasn't what
>> you expected.
>> My next venture will be to tone modulate the BC456 to verify 
>> what input it
>> needs to get to 100% modulation.
>> I'm waiting for some pl-68's to arrive so I can do that.
>>
>> When I'm done with this project, it will have ample modulation 
>> to be heard.
>> The goal is to participate in some of the AM "military" nets 
>> on 75  meters
>> as well as CW.
>>
>>
>> On Sunday, March 19, 2017 1:50 PM, Mark K3MSB 
>> <mark.k3msb at gmail.com>
>> wrote:
>>
>>
>> J
>>
>> I kept all the emails for that those threads as I need to get 
>> my BC-456
>> working here.   I tried it a few times for test, and it wasn't 
>> impressive,
>> but then again I didn't really work on it that much for AM.
>>
>> There are probably 50 to 75 emails in those threads;  I'll try 
>> to put them
>> all in a pdf for you (and anyone else that's interested).
>>
>> Mark K3MSB
>>
>> On Sun, Mar 19, 2017 at 1:12 PM, J Mcvey via ARC5 
>> <arc5 at mailman.qth.net>
>> 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
>> 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> 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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