[R-390] R-725/URR Hum Bucking Transformer

[Drew Papanek]

On Tue, 21 Jan 2003 Les Locklear wrote:

>You forgot to add the original R-725/URR receivers had a MU-Metal shield
>around the PTO. No mention was ever made of this in their (Servos) blurb, 
>but
>obviously was placed arounf the PTO to eliminate any spurious emissions 
>from
>the PTO that would/could have interfered with the DF'ing.

Mu metal is used for magnetic shielding and is often found encasing 
electrostatic deflection CRT's in oscilloscopes.  There it prevents trace 
deflection caused by stray magnetic fields from 'scope's power transformer.  
Magnetic shielding for PTO in R-725 would prevent FM-ing of PTO signal due 
to transformer induced 60 Hz  "permeability modulation".  The aluminum PTO 
cans would not provide magnetic shielding but would provide effective RF 
shielding thereby minimizing the aforementioned spurious RFemissions.

Drew

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