[R-390] Z503 Replacement

[Drew P.]

On Z503 replacement, Perry Sandeen wrote:

"I was somewhat aware of the complicated construction.

Is it possible that this was an extremely over-engineered part?

My question really was: why can?t a satisfactory electrical equivalent be made out of components we can buy?  After all it is a 455 KHz part.

I?d rather make one or two up for spares now, even if I never need them, than to have to scrounge up a replacement for a dead one in the future.

Has anyone on the list pursued an alternative replacement method or made one?"


As Roger had previously written, the AGC section is the least critical part of the IF system. By that point in the system, the IF amplifier bandwidth has already been established by the IF interstage transformers and the encountered signal bandwidth has been defined by funneling through the mechanical filters ahead of most of the IF amplifier. Also, Z503 is coupled to V590A, the AGC recifier, a relatively low impedance point, via an appreciable coupling capacitor. Z503 is therefore unavoidably heavily damped and would have a broad bandwidth, wider than that of the rest of the system. Remember, Z503, while adjustable, doesn't even have a hole in the can for adjustment; this shows the relatively "loose" nature of the part.

I believe that Z503's coil was "over engineered" with the pot core, pie windings, and epoxy simply for manufacturing convenience - to enable it to be built on the same production line as the already-proven IF transformers (which are also somewhat similarly constructed) and without incurring need for extra source component inventory.

Insofar as shielding provided by the pot core is concerned, Z503 is already in a can - how much more shielding do you need? I see little reason why the "guts" of Z503 could not be replaced with those out of a tube-type 455 KC IF can salvaged from an AA5, those from a detector transformer just to be proper.

It should also be practical to use here an inexpensive commercially available inductor, with a similarly inexpensive trimmer cap, perhaps paralleled with a fixed cap, for adjustment. IIRC, someone in this forum recently mentioned having done just that.

Drew