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

[Larry H]

Hi Nelson,  Are you using SS recs in your rx?  If so, the power-on voltage could go higher than the 260 V with tube recs, and if you're turning it on by switching to AGC immediately, you should check C411 at 300 V, otherwise at 275V (even though the max is about 255 V).  However, if your line voltage is more than 115 VAC, you should also account for that.

Regards, Larry


________________________________
 From: "ngietz at mymts.net" <ngietz at mymts.net>
To: R390 <r-390 at mailman.qth.net> 
Sent: Monday, January 26, 2015 8:42 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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