[R-390] Rice Krispies Exorcised from my 390A

[wabate]

Thought I was done with the restoration project.  Even had the Cosmos 
tracking well when a lightning storm was heard in the speaker.  Narrowed 
it down to the IF module.  Thought I had it when I discovered a previous 
owner had substituted a 6AU6A for a 6BA6.  Thought I killed two birds 
with one stone when it came back.  Sometimes it took a few hours to 
occur.  That's how I found it, I was doing an 8 hour burn in.

Thought I had it a few times replacing some discs.  Then I narrowed it 
down to the 3 resistors around V501.  Seemed strange but I have come 
across resistors doing that.  So I bypassed each resistor with another 
but got no noticeable change.  Then I noticed the plate choke.  On a 
whim I bypassed the choke and the noise disappeared.

Dang!  I replaced the choke (L505) with something from the junk box and 
the receiver has been quiet for 7 hours.

I offer this up for anyone on a similar mission.  Even though the choke 
is sealed there must have been some arcing between winding layers.

Bill, K3PGB