[R-390] [Fwd: Reconditioned Amelco R-390A Part Failure]

[Tom Frobase]

Last fall I had the rare opportunity to pick up 2 R-390A's at the 
Belton, Texas hamfest.  I rebuilt the nicest of the units first, a circa 
1962 Almalco,  with the goal of putting it in my shack.  I did the 
normal, completely  dissembling and cleaning the radio.  I completed the 
radio figuring it needed to run in the shack for a bit to discover and 
anomalies not found in its reconstruction.

After being in the shack for a couple of weeks the radio failed 
outright.  I pulled it apart this morning to find the 2200 resistor R508 
to be smoked.  This is the dropping resistor that feeds the 12 MH choke 
L505 to the plate of the 1st IF driving the mechanical filters.  I 
always change C558 which connects to the filters, you all have warned me 
many times!

My first thought was to check the .05 ceramic to ground, wasn't it, then 
I pulled the tube in case it was shorted, it was not.  I thought of the 
irony of having a defective cap which I replaced to protect the filters 
but indeed it was good as well.

The problem ended up being the 12MH inductor L505, shorted dead to 
ground.  Go figure in thirty years of repairing these things I have 
never seen that before, has anyone else?

tom, N3LLL