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

Fri May 8 15:21:33 EDT 2009

Hi All,

I responded to Bill's remarks inline.  Not especially personal but 
maybe a little clearer at this point.

At 12:02 PM 5/8/2009 -0500, Bill Hawkins wrote:
>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.

Actually I've been watching audio at the Diode Load output of the 
detector (V506B).  I've disconnected the Limiter and replaced 
Limiter's load resistors (R526 and R527) with a 56k to ground.  I've 
got a small RF bypass cap across the 56k resistor to bypass the 
remaining 455 kHz RF that shows up there.

>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.

Sounds good to me.  After isolating the filament transformer I could 
also isolate the B+ supply if necessary.  Controlling sneak paths 
could start to become confusing!

>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.

Don't want to go DC except as a last resort.  The kludge factor is 
high considering transformers, rectifiers, capacitors and most likely 
a Variac to deal with Roy's warning.  I'm already having problems 
trying minimizing the amount of cobbing needed to run these tests.

>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.

Initially I did observe the oscillation I think you're 
describing.  Running the IF gain control to max clockwise rotation 
would send the strip into oscillation.  Backing the gain down a 
little calmed it right down again.  It's behavior was very 
"honest".  I ended up arbitrarily setting the IF gain to mid-rotation 
for the time being.  If you expect oscillation to occur at lower gain 
settings, perhaps there is a problem with the module and it does not 
have enough gain.

Anyway, since then, something's changed.  I now see continual but 
random bursts of oscillation at the IF test point.  Somewhat similar 
in appearance the color burst signal in good old analog 
TV.  Recovered audio appears as a negative going spikey sort of pulse 
for the duration of the burst.  Appearance of this spikey pulse is 
reminiscent of "motorboating" behavior when AGC or B+ between stages 
is not decoupled well enough.  Can't find evidence of this 
motorboating behavior on B+, AGC or Cathode Bias Lines.  I'll also 
add that the bursting is not affected by the setting of IF gain 
control.  The behavior is indeed NOT honest.

It could be the fact that I disconnected the AGC line from the rest 
of the receiver it P112 and "tied back" the AGC to the IF module by 
connecting P112 pins 4 and 6.  It's hard to picture this as a cause 
but I don't know at this point.  Perhaps the "flailing" AGC in the 
rest of the receiver is causing problems.  Maybe I should ground it 
and stop it's flailing and also try reconnecting it.

>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.

Have never considered myself the know-it-all type.  The college thing 
doesn't replace experience.  I know that dealing with equipment like 
these R-390 receivers does take a lot of finesse.  Also realizing 
that it's so difficult to troubleshoot by remote control, I'm passing 
my observations along to the group as carefully as I know how so the 
group knowledge base can be as involved as possible.  Hopefully some 
worthwhile ideas for everyone will come out of all this.

Best Regards,
Jon Schlegel

>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
>
>
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