[R-390] Z503 Replacement

[2002tii]

Dan wrote:

>My replacement at 0.5 mH needed
>more C to resonate, around 310 pF, but with a measured Q of 170 gives a
>nice peak at 455 kc/s and good AGC action.  I am not sure why the Q was
>deliberately lowered seemingly in later radios, but my relatively high Q
>coil works fine, and I see no need to lower it with parallel R.

As others have pointed out, the sole function of Z503 is to load the 
AGC IF amplifier.**

You want the bandwidth of the IF at the AGC detector to be determined 
by the main IF chain, NOT by the AGC IF amplifier, so you want Z503 
to be at least as broadband as the widest IF filter (16 kHz), and 
preferably just a little wider, so that signals at the edge of the IF 
passband are not ignored by the AGC.

Since Q = center frequency / bandpass, an LC circuit with a bandwidth 
of 16 kHz at 455 kHz should have a Q of 455/16 = 28.4.  If we want a 
bandwidth just a little wider than 16 kHz, the Q should be slightly 
lower -- say, Q = 25, in excellent agreement with Todd's measurement 
of an actual Z503.  (Note that this is the loaded Q of the LC 
circuit, NOT the unloaded Q of the inductor.  I will leave it as an 
exercise for the reader to calculate what the unloaded Q of the 
inductor must be to achieve a loaded Q of 25 in this circuit, and 
what value of parallel resistance would be required to bring the 
loaded Q to 25 if one used an inductor with an unloaded Q of 170.)

Best regards,

Don


** For those who still cannot seem to figure out how the R390A IF 
system works, and therefore worry that Z503 will affect the IF tuning 
(or maybe affect the non-existent 455 kHz "oscillator" that they 
think is the "heart" of the 455 kHz IF) -- Z503 has no effect on the 
IF, except to the infinitessimal degree that the nonlinearity of the 
AGC rectifier (V509A) may propagate backwards through the AGC IF 
amplifier (V508 -- a pentode, therefore having excellent reverse 
isolation), then through the V508 grid coupling capacitor (only 47 
pF), then backwards through the IF cathode follower (V501B -- which, 
being a cathode follower, also has excellent reverse isolation) and, 
finally, to the grid of the 4th IF amplifier, V504.  All told, the 
reverse isolation of this path must be well over 100 dB -- thus, Z503 
has no effect on the IF, tuning or otherwise.


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