[R-390] R-390/URR Z-503

[bavarianradio at comcast.net]

Hi Z-503 group, Just a note, my coil has 2 capacitors in paralell, one is a silver mica and the other is a ceramic dog bone. I didn't note the values, I disconnected them when I "gave it the juice"  Ross W1EKG
----- ToddRoberts2001 at aol.com wrote:

In a message dated 3/10/2011 10:01:06 A.M. Eastern Standard Time,
tisha.hayes at gmail.com writes:
It looks simple enough;
Tuned Circuit, Intermediate Frequency: 455 kHz  center frequency;
Including
L514 and C563, (13499) drawing no.  278-0235-00.
I have a spare good IF deck, this weekend I will open up  Z-503 and
unsolder
a leg of the cap, take measurements of the capacitor and  the inductor
to
get
some values for what L514 and C563 are.
Ms. Tisha  Hayes/ AA4HA



Hi Tisha,
I Have the parts from a Z-503 assembly that was taken apart years ago
with
a broken coil.
I also have a spare Z-503 with a good coil.

I measured the inductance of the good coil coil L-514 as 1.38mH (
millihenries )
The resonating capacitor C-563 measured 84pF.

A resonant frequency calculator shows the resonant frequency as  467KHz.
The calculator shows the capacitor should be close to 88pF to resonate
the
coil near 455KHz.

The capacitor was taken from a different Z-503 assembly so it could be
those parts are mixed
and matched at the factory to resonate at 455KHz. There could also be
some
stray capacity
in the circuit that needs to be accounted for.

If someone was going to replace the coil I would recommend using a
ferrite
shielded coil
to minimize the effects of the shielding can.

Mouser Electronics has small ferrite shielded inductors in values of 1.2
and 1.5mH.
If a 1.5mH inductor was used a capacitor of around 80-81pF should  work.
The 1.5mH inductor is part no. 432-02-152J and the current cost is $1.20
each.

Mouser has the small leaded Mica capacitors in 82pF values. One could
also
mix and match two smaller value caps in series to get close to the best
value.

The original circuit seems to be fairly broad tuning so the Q does not
need
 to be extremely high.
A broadly-tuned resonant circuit around 455KHz should work with the
1.5mH
inductor in the circuit.

73 - Todd WD4NGG


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