[R-390] Z 503... "non-A" vs. "A"

[2002tii]

Bob wrote:

>Todd's solution is only good for the R-390A/URR.  This is one of the
>Cost Reduction Changes that grew into the production of the R-390A/URR.
>
>The fact of the matter is, the R-390A/URR IF transformers are ALL
>identical with a higher Q than what they first went to the Signal Corps
>with.

There you go again Bob, spouting off about things you clearly do not 
understand in yet another stunning display of ignorance.  The amount 
of flatly wrong information you inflict on this list, despite the 
best efforts of people who do understand and try to explain things 
correctly, is both pathetic and potentially harmful to folks who 
might not know better.  Please stop.  It is painfully obvious that 
you don't have even the slightest idea how an R390 or 390A works.

Z503 (in both the R390 and R390A) is NOT AN IF TRANSFORMER.  As 
anyone who can read a schematic can see, and several of us have 
explained in detail, it is neither a transformer (there is no 
secondary -- it is just an inductor), nor is it part of the IF signal 
chain, nor is it a high-Q circuit.  In both radios it is simply a 
low-Q tank loading the AGC amplifier and feeding the AGC detector.

Any LC circuit tuned to 455 kHz, with the appropriate loaded Q (about 
25), voltage rating, DC resistance, and DC current capacity, will 
work in either radio.  It needs to be reasonably well shielded 
because the following AGC rectifier creates harmonics, which you 
don't want radiated back into the RF and IF stages by an unshielded inductor.

I have not tried using Z503 from an R390A in an R390, but I see no 
reason why it should not work just as well as the original.  As will 
any homebrew solution that satisfies the requirements in the previous 
paragraph.

As Todd said:

>I think one could get a shielded inductor, small value mica cap and a small
>trimmer capacitor from Mouser and build a "new" Z-503 for under $5 bucks.
>Instead of using an adjustable inductor use a small trimmer cap to "peak"
>the coil. Electrically the result should be the same.

Exactly.  You may need to parallel the LC with a resistor to get the 
loaded tank Q down to 25 or so.  As someone else pointed out a while 
back, the fact that the tank is loaded by the low impedance of the 
AGC detector will go at least some way toward lowering the Q of Z503.

Best regards,

Don


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