[ARC5] Reverse engineeing the BC 456

[J Mcvey]

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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