[ARC5] 274N and ATA: Not Exactly Identical

[David Stinson]

CRUD
Please trash this post.  I'll correct it and repost.


On 4/2/2020 9:26 AM, David Stinson wrote:
> 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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