[ARC5] Reverse engineeing the BC 456

[Mark K3MSB]

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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