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

[Llgpt@aol.com]

Tom,

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.

Les Locklear


In a message dated 1/21/03 6:06:17 PM, courir26@yahoo.com writes:

<< Gentlemen?

Someone asked about this.
In the original article about the R-725/URR, I wrote that the extra 
transformer
was for IF filaments.  But then I got some info from Servo which stated
otherwise (mea culpa!).

"A hum-bucking circuit has been added to the R-390A/URR to improve 
performance.
 This modification places a positive dc voltage on the filament of the 
variable
frequency oscillator (vfo) (V701) in the receiver, reducing the
heater-to-cathode leakage of 60 Hz into the signal."

Further on the hum thing.

"The following modifications have been made to reduce hum.

(a) Open small enclosure at end of vfo unit and lift pin 3 of V701 from 
ground.
(b) Connect a 0.01-uF capacitor between V701-3 and ground.
(c) Connect V701-3 to J709-C.  <==(this connects new transformer. Ed.)
(d) Connect P109-C to P121-E (filament return)
(e) Disconnect and dead-end wire (with tape) from P112-8."

End of Servo info.

Hope this helps!

73 N5OFF


ps The Servo info also indicated provision has been made for use of a 
dynamotor
for DC operation (anyone seen this?).

 >>