[ARC5] Driving the BC-456

[Dave Jackson]

Oops! Missed a word.

 

 

Should be “..modulator tube  while transmitting.”

 

 

 

From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Dave Jackson
Sent: Sunday, March 26, 2017 11:20 AM
To: 'J Mcvey'
Cc: ARC5 at mailman.qth.net
Subject: Re: [ARC5] Driving the BC-456

 

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 to

1.7 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.

 

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20170326/a4266d06/attachment.html>