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

[J Mcvey]

In my case all is as it should be ( including the closed switch in the BC-451) , but the modulation is still rather weak.
You won't have to worry about the mike jack if you don't have the BC456 modulator!Are you building your own modulator?Before I aquired the real thing, I made a dyno supply to run cw with the same design as seen in the BC456. It worked well.I was going to add a solid state "amp pack" for the modulation, but the BC456 came along and that project was abandoned.All the switches and jacks were built right into the home-brew chassis . It was about the same dimensions as the BC456 too.
 

    On Sunday, March 19, 2017 6:28 PM, Leslie Smith <vk2bcu at operamail.com> wrote:
 

 Fellow list-readers.

Looking at the circuit diagram Fig 27-1 and the parts list I see:

The carbon mic is in series with two resistors, R-54 and R-66.  Para 9-f
states the R-66 is normally "out" of the circuit, and gives instructions
to check this is so.

R-66 has a value of 360 ohms.  
The transformer has the following characteristics:  
Primary resistance: 280 ohms.  Turns ratio 1:10 (step-up)
Not stated:  impedance step up: 1:100, but this is easily calculated.

The circuit is fed with the heater supply:  24V,  

This suggests the mike current is in the order of 20 to 50mA
I understand that the output to the 1625 (grid) is in the order of 3 to
10 volts.
This suggests (by inspection and commonsense) that the gain of the PA
stage is in the order of x40 to x100.
(Perhaps less if the modulation transformer has a significant turns
ratio).

The thing is this:  The mic jack is mounted on the FT-451-A control box.
What other arrangements do people here use when the BC-456 modulator or
FT-451-A aren't available?

Les Smith.
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