[R-390] R-725/URR Hum Bucking Transformer
Tom M.
[email protected]
Tue, 21 Jan 2003 16:02:55 -0800 (PST)
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?).
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