[ARC5] Reverse engineeing the BC 456

Sun Mar 19 14:45:54 EDT 2017

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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