[R-390] R390A 2nd osc. hot R406, R407

[Roger Ruszkowski]

Nelson,
 
Charles nailed this one for you.
 
So once upon a time V401 went short and the resistors got burnt.
The tube got replaced.
The receiver worked OK
and things were left.
 
You found some parts that had long ago been stressed.
 
As Charles pointed out, these receivers can take a lot of abuse and still work.
 
But as long as you are there, I would replace the seriously cooked parts.

Roger AI4NI
 
 
-----Original Message-----
From: ngietz <ngietz at mymts.net>
To: R390 <r-390 at mailman.qth.net>
Sent: Tue, Jan 27, 2015 1:15 pm
Subject: [R-390] R390A 2nd osc. hot R406, R407


    Having gone through this RF deck, I’m hoping you more experienced guys can 
explain this.  The two resistors in the B+ supply line to T401 and V401 were 
seriously cooked, to the point of darkening the phenolic on the turret board 
where they were mounted.  They each measured about 1K instead of the spec’d 
3.9K.  I subbed them, but want to avoid a recurrence.  
    The grid resistor R404 was high at 137K instead of 120K, and it has been 
replaced as well.  I used a digital meter and then a cap checker to check for 
leakage to ground from the B+ at 250V.  They both show infinite resistance with 
the band switch rotated through its full range.
    The windings of T401 are in spec, and in a visual check look very healthy.
    Any thoughts on what might have cooked those two resistors?  (I seem to 
recall seeing something about an overdriven V401 doing that, but how would it be 
overdriven?)
    Nelson
    
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