[ARC5] BC-456 and the microphone circuit. (Was reverse engineering the BC-456)

[J Mcvey]

Those observations are pretty close to my conclusions. 
The 1.2 vrms to 1.7v rms is a very handy piece of information.
1.2 Vrms is approx 3.4 Vpp which is the low end of the spec.
My t-17  was only giving me 1.12 Vpp @60ma limit which is only 0.424 Vrms.No wonder the modulation is low!
It should be more like the 3 Vpp that the telephone carbon element produced. 
 

    On Sunday, March 19, 2017 9:41 PM, Dave Jackson <cjack93907 at razzolink.com> wrote:
 

 Les and list:

My take on the matter.

>From the SCR-274N operation and maintenance manual, Army Air Forces
Technical Order 08-10-50, par. 15 page 42:

" (3) T-51 is a transformer, the primary of which is in the microphone
circuit.  The control grid of tube V-51 (Tube VT-136) is connected to the
junction of R-55 and R-56 which, together, act as a voltage divider and as a
load across the secondary of the microphone transformer.  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.  Circuit elements throughout the voice modulation
circuits have been designed on the basis of the maximum output from an
average Microphone T-17.  The direct current thorough Microphone T-17 is
approximately 62 milliamperes (assuming that R-66 is short circuited by S-53
in Radio Control Box BC-451A).  Switch S-53 is opened only when using
microphones not now supplied to the United States Arm Air Corps."
---------------------------------------
If the mic has 62 ma. flowing and using 27.5 volts for the DC supply:

(360 + 25) * 0.062= 23.87 volts drop

[360 ohms (R-54) + 25 (T-51 primary dc resistance from table 14, pg 71)]

27.5-23.87= 3.63 volts across the T-17 mic

3.63 / 0.062 = 58 ohms for the T-17 mic

Hope this helps.

Dave, WA4OBJ



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