[R-390] Toasted R216 (2nd mixer plate resistor)

[John KA1XC]

Tim,
actually it's easy to toast R216, and also R212 in the first mixer and R205 
in the RF Amp by accident.

In all of these Variable IF or RF stages the coil electrically closest to 
its associated tube has the B+ going through it, which means the trimmer cap 
and its adjusting screw in that coil assembly are hot with B+ as well. So if 
someone adjusted the cap with a metallic screwdriver it would be easy to 
cause a short circuit to ground and fry these decoupling resistors. Same 
situation with the R-390 too, that's why they say in the TM to use an 
insulated tuning tool.

73,
John

>>>>>>>>>>
In my yellow-striper's R-390A RF deck there's an obviously toasted,baked, 
and
swelled R216.  It's changed value to something in the hundreds of ohms
from its original 2200 ohms.

If this was part of a pentode mixer I would think that bad voltages on a 
control grid was
causing too much plate current.  But it's part of the V203 2nd mixer.  Could
really wacky AGC voltages (which sets the DC bias on the 2nd mixer) have 
caused
so much current flow that the plate resistor smoked?  I'd think that this 
would take
tens of volts

An alternative explanation might be a short in V203 (a 6C4), plate to 
filament, and
that doesn't sound likely.  Or (and this is the scariest) a short to ground 
inside Z216?

R215 on the cathode seems fine.  Normally whatever DC current goes through 
R216
would go through R215 too.

Any thoughts, experience?

Tim.