[R-390] Open Z-503 AGC IF Transformer

[ToddRoberts2001 at aol.com]

In a message dated 1/10/2006 8:53:34 AM Eastern Standard Time, 
JMILLER1706 at cfl.rr.com writes:
Believe it or not I have rewound RF/IF transformers like this that have 
been fried.  If you can unravel the turns, just measure the total 
length after getting the wire off, go buy some small diameter magnet 
wire of the same gauge and length, or close, and wind it back on.  It 
won't look pretty but it will probably work.  Glue or wax to hold it in 
place.  I did that with an RF coil in a 51S1 once.  Worked great.  
Alternately, it is possible that a coil from generic 455 khz IF can 
would work.   JM

An open winding in  coil L514 in the Tuned Circuit can Z503 seems to be a 
common failure mode in the R-390A IF deck. I have run across several 
open-circuited ones over the years. It may be corrosion eventually breaks the tiny wire 
leads from the coil or it may be something to do with the insulation breaking 
down as this coil has about 180VDC running through it and some current. If 
anyone is interested I measured the inductance of L514 in Z503 as 1.385 mH. This 
would require a capacitance of 88pF to resonate at 455KHz. I see that Mouser has 
some Hi-Q miniature radial inductors with a Q of 100 in a 1.2mH and 1.5mH 
size. These would easily fit inside the Z503 can. The 1.2mH inductor would 
require 102pF to resonate at 455KHz and the 1.5mH inductor would require 82pF. The 
peak may be broad enough that you could get by with a fixed cap across the coil 
but one could also mount a miniature trimmer cap and a slightly smaller fixed 
cap inside the can to get the best peaking on the coil. I haven't measured 
the Q of the original coil yet but I do have a Heathkit Q-meter and I will check 
it out. I would think a coil with a Q of 100 would be suitable here. In the 
future the Z-503's are going to be hard to find. It would be nice if an 80 cent 
inductor from Mouser and a small mica cap would get it working again. 73 Todd 
WD4NGG