[R-390] Hum

John KA1XC [email protected]
Wed, 11 Feb 2004 21:53:44 -0500


Hi Jim,
the hum might not be in the audio deck. Here's some tips.....

Does it go away when you click ON the noise limiter? If so bypass cap C536
in the IF deck could be open.

You could also have filament to cathode leakage in one of the detector or
noise limiter tubes.

To further isolate the trouble, disconnect the Diode Load jumper (TB103
terminals 14&15) which will break the connection from the detector and see
what happens.

John
----- Original Message ----- 
From: "Jim Brannigan" <[email protected]>
To: <[email protected]>
Sent: Wednesday, February 11, 2004 5:18 PM
Subject: [R-390] Hum


> I have a '67 EAC R-390A with a low level hum that is making me crazy.
>
> The hum is controlled by the AF gain control and therefore before it.  It
is
> influenced by the 800cps bandpass filter and disappears when V601 (1st AF
> amp) is removed.
>
> The plug in Capacitors have been replaced with new electrolytics.  C609 in
> the cathode circuit has been replaced and the Rippel audio mod installed.
> The tube has been swapped for another and the hum still persists.
>
> Short of replacing every component in the 1st AF amp, I'm stumped!!
>
> Any suggestions?
>
> Thanks in advance,
>
> Jim
>
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