[ARC5] 274N and ATA: Not Exactly Identical

[David Stinson]

While it's often been said (even by me) that ATA and 274N sets are 
electrically identical, that's not precisely true.  There are 
differences in the respective Modulator microphone circuits.  The ATA 
modulator was designed for use with the RS-38 microphone.  The 274N 
BC-456 modulator was intended for use with the T-17, which (when new) 
could deliver considerably more mic current/output than an RS-38.

Here is a function diagram of the BC-456 modulator and a circuit 
description:
https://photos.app.goo.gl/Sxt5TSEdu921HcLt7
https://photos.app.goo.gl/8qJ1ojJu3CLrN7zf7
Please note there is a typo in the ATA description; R55 is 15K, not 1.5K.

Here is the function diagram and description of the BC-456:
https://photos.app.goo.gl/MdY37NJyx4Q63vzB9
https://photos.app.goo.gl/HjbfF2Svs4MevjC28
R55 and R56 both load the microphone transformer more heavily and divide
the drive voltage to adapt the T-17's higher output specification.
In the BC-456, the microphone current limiting resistor, R-54, is
80% higher in value than the one in the ATA modulator, further
damping the T-17 drive.

Here are comparison photos with red lines to the differing parts.
ATA:
https://photos.app.goo.gl/HjbfF2Svs4MevjC28
BC-456:
https://photos.app.goo.gl/HjbfF2Svs4MevjC28

I have a couple of T-17s which, now lacking the output to drive a 
BC-456, will still drive the ATA modulator nicely.

GL OM ES 73 DE Dave AB5S


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