[R-390] Z503 Replacement

[Dan Rae]

On 4/2/2011 1:06 PM, Drew P. wrote:
>   Remember, Z503, while adjustable, doesn't even have a hole in the can for adjustment; this shows the relatively "loose" nature of the part.
Well, in my both R-390 and my EAC R-390A this coil is adjustable, but I 
do understand that some are not.

In my R-390 I found recently that the coil in the Z503 had gone open 
circuit.  It proved to be totally impossible to take the ferrite cup 
core apart to re-wind so I replaced it with another inductor from my 
junk box, also with cup core and adjustment from one end and was able to 
mount that into the original can with the same hardware.  The original L 
as Todd points out was around 1.5 mH.   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.

The original C in my R-390 version Z503 was split into approx half 
silver mica and half ceramic,  total 82 pF from the schematic.  I'm 
guessing this was for temperature compensation.
>
>
> 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.
>
>
This was exactly what Todd was suggesting, just recently and also 
earlier; if I had not found a suitable inductor I would have tried to do 
it his way.

Dan

ac6ao