[R-390] Z503 replacement

[ToddRoberts2001 at aol.com]

In a message dated 4/2/2011 1:58:17 P.M.  Eastern Daylight Time, 
comcents at bellsouth.net writes:
As we know today -  there are lots of new and 
exotic/precision/nifty parts that didn't exist  back then.  
If we were to establish the parameters of Z503 (inductance,  
capacitance, Q, bandwidth, etc.) then using modern 
components fabricate  one that duplicates those parameters - 
I sincerely doubt there would be any  measurable difference 
between the original and the replacement "in  circuit".  
Might cost a lot more - take more time to make; each have to  
be "hand tweaked" (rather than the economy of mass 
production) - but  we're not needing production quantities, 
either.  


Hi All,
A replacement Z503 was discussed on this list about 3 weeks ago.
I have a spare NOS Z503 that I recently measured.
 
The inductance of the coil L-514 measured 1.38-1.39mH (millihenries) using  
an LCR meter.
The resonating capacitor C-563 measured 84pF.
 
The Q of the coil and capacitor inside Z503 measured a meager 25Q at 410KHz 
 using an HP4342A Q-meter.
 
It would be recommended to use a ferrite shielded inductor for a  
replacement to minimize stray effects of the shielding can.
 
Mouser has some suitable small shielded inductors of 1.2mH and 1.5mH at  
current prices of $1.20 each.
 
I have a similar 1.0mH shielded inductor from Mouser and it measured with a 
 Q of 170 at 455KHz.
There was no change in inductance when I inserted the inductor inside the  
Z503 shield can.
Unshielded inductors show a large change in inductance when inserted inside 
 the metal Z503 can.
 
Using a fixed inductor would require a small trimmer cap to adjust to  
resonance. Mouser has
suitable miniature trimmer caps that will easily fit inside the Z503  can. 
The assembly could be 
peaked with the can off, then replace the shield can as there will be no  
change of inductance with the can on.
 
To match the low Q of the original inductor a resistor could be placed in  
parallel with the Mouser inductor and 
cap tuned circuit to lower the Q down to the 25-50Q range which should work 
 very close to the original Z503 circuit.
 
73 - Todd WD4NGG