[R-390] 60 Hz Hum Output from R-390A IF Module

[Bill Hawkins]

Another note:

First, separate the problem into IF or Audio modules. With the IF
module exposed, short pins 1 and 2 (they're connected) of the
limiter V507 to ground AT THE AUDIO MODULE. If you still have hum,
the problem is in the audio deck.

Seems like the main thing is that the IF module generates the lowest
level of audio, from the detector. The audio deck, then, has high
audio gain. Hum is generated in the resistance of the two ground
IF extension wires, which also carry heater current.

You can avoid that hum by using a separate 6.3 volt transformer
that is connected directly to the IF module, after disconnecting the
6.3 volt extension wire at J512 pin 20. No heater current will flow
in the ground extension wires at pins 7 and 17.

The lowest level audio leaves on J512 pin 5, goes out to DIODE
LOAD terminal 14, through a jumper to terminal 15 and back in
through pin 3. Make sure the shields on those harness wires are
grounded.

If you do want to try DC, use a separate supply, or put the diodes
and cap at the radio. That way the cap charging current spike will
not induce worse hum on the ground extension wire. Heed Roy's warning.

If the IF module oscillates, adjust its gain control as described
in the manual. It's easy to get too much gain if you just set the
pot in the center. This ought to be covered in the Y2K compendium
of group knowledge.

Most of what I know about these fine radios came from this group.
I learned the principle of solving a problem by halves back when I
got over being a know-it-all college graduate, about 1962.

Bill Hawkins


-----Original Message-----
From: Roy Morgan
Sent: Friday, May 08, 2009 3:19 AM
To: Jon Schlegel

On May 7, 2009, at 11:44 PM, Jon Schlegel wrote:

> Hi All,
>
> I'm still working on the problem. ... If I'm still seeing 60 Hz, then 
> I'll call in Dave's big guns with a rectifier brick and capacitus 
> gigundus on the filaments.

NOTE:  If you put a simple rectifier and humongus capacitor on a 6.3
volt AC filament supply, you'll get around 10 volts DC.  That may blow
out your IF strip tube filaments in short order.

Roy


Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073
Home: 607-898-3607
Cell: 301-927-7794