[ARC5] Driving the BC-456

Sat Apr 1 17:10:43 EDT 2017

Fixed the signal generator today , so I anxious to try testing to see what amplitude input results in 100% modulation with the BC456.
The maximum modulation attainable looked to be only about 50-60% before the carrier could no longer tolerate an increase in 
AF input and started to clip. That was at about 2.5Vpp using a 1khz sine wave. The input is clean, but the carrier clips.

Is this because it's screen modulated or is there another issue?

    On Sunday, March 26, 2017 12:14 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:

 It makes sense that the text was referring to rms voltage across the T-17 . I don't think there would be much modulation with 1.2 to 1.7 vrms at the grid. By turns ratio , the voltage at the secondary (not the grid) should be 12 to 17 Vrms .
I didn't get too specific about measuring the  exact AC Z, but it's very low.  Maybe I should do that too. Somehow,I manged to blow up my audio generator when I attempted drive the modulator. It was isolated with a cap and all...perhaps it was just a coincidence?It may have been backfeed of RF that did it, as that seems to an issue, but easily remedied with a .01 cap across the  mike terminals. (hindsight is 20/20).
Got to fix it before I can do anything more specific...

    On Sunday, March 26, 2017 11:20 AM, Dave Jackson <cjack93907 at razzolink.com> wrote:

 #yiv2901666835 -- filtered {font-family:Helvetica;panose-1:2 11 6 4 2 2 2 2 2 4;}#yiv2901666835 filtered {panose-1:2 4 5 3 5 4 6 3 2 4;}#yiv2901666835 filtered {font-family:Calibri;panose-1:2 15 5 2 2 2 4 3 2 4;}#yiv2901666835 filtered {font-family:Tahoma;panose-1:2 11 6 4 3 5 4 4 2 4;}#yiv2901666835 filtered {font-family:Consolas;panose-1:2 11 6 9 2 2 4 3 2 4;}#yiv2901666835 p.yiv2901666835MsoNormal, #yiv2901666835 li.yiv2901666835MsoNormal, #yiv2901666835 div.yiv2901666835MsoNormal {margin:0in;margin-bottom:.0001pt;font-size:12.0pt;}#yiv2901666835 a:link, #yiv2901666835 span.yiv2901666835MsoHyperlink {color:blue;text-decoration:underline;}#yiv2901666835 a:visited, #yiv2901666835 span.yiv2901666835MsoHyperlinkFollowed {color:purple;text-decoration:underline;}#yiv2901666835 p.yiv2901666835MsoPlainText, #yiv2901666835 li.yiv2901666835MsoPlainText, #yiv2901666835 div.yiv2901666835MsoPlainText {margin:0in;margin-bottom:.0001pt;font-size:10.5pt;font-family:Consolas;}#yiv2901666835 span.yiv2901666835EmailStyle17 {color:#1F497D;}#yiv2901666835 span.yiv2901666835PlainTextChar {font-family:Consolas;}#yiv2901666835 .yiv2901666835MsoChpDefault {font-size:10.0pt;}#yiv2901666835 filtered {margin:1.0in 1.0in 1.0in 1.0in;}#yiv2901666835 div.yiv2901666835WordSection1 {}#yiv2901666835 Jim:  I am a bit confused in where the 1.7 volts is measured.  The text says “These resistors are so chosen that the voltage applied to the modulator while transmitting on VOICE is sufficient to produce 85% average modulation with from 1.2 to1.7 volts rms input” This isn’t clear if the voltage is measured at the mic jack or on the grid of the 1625.  My guess is that it is in fact at the mic jack.  It wouldn’t make much sense to require the tech to measure the voltage on the grid but stranger things have happened.  The tech writer was probably using the engineering notes to write the instruction manual and it may not have been clear.  In my experience, it is hard to get over 80% or so using screen grid modulation only, as the SCR-274N does, without a lot of distortion.  You guesstimate of the input impedance is in line with my experience with carbon microphones as well as several mic technical descriptions.  See TM 11-487M, TB SIG 330, Shure 104C mic spec sheet, etc…  Most of these are dual button, noise cancelling microphones so the impedance is higher than the single button like the T-17 or RS-38.  Since this is in a “voltage” transfer mode rather than a “power” transfer mode, it makes sense that the input impedance may be higher than the mic itself, so you might  even be a bit low on the impedance.  I have a NIB BC-4556 modulator and will measure the turns ratio of the input transformer when it get a chance.  Also, It would be difficult for a multi-K ohm mic to draw 60 ma in this circuit.  Just my take and not “gospel”.  Dave, WA4BOJ      From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of J Mcvey via ARC5
Sent: Sunday, March 26, 2017 9:48 AM
To: ARC5 at mailman.qth.net
Subject: [ARC5] Driving the BC-456  I was noodling with the electret mike circuit today.It turns out that the initial reverse engineering results were pretty close.The input Z is very low, in the 30-50 ohm range. 4Vpp DOES NOT overdrive the modulator!This falls in the T-17 mike spec which was a max of 1.7 Vrms ( 4Vpp = 1.41Vrms)It looks to be about 80% modulation@ 4Vpp. I thought that would be enough , but it looks like it would be good to have more adjustable range to5Vpp or 6Vpp.  The internal tone modulation looks like 100%+  So far, the circuit consists of two active devices. A LM3900 and MPSA06 npn transistor.A LT1006 or equivalent would probably work as well-maybe better, but I have the LM3900's on hand.    

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