[ARC5] Selectivity and the BC-455

[Ian Wilson]

Hi Mike,

I tried an alternate way to measure the pie-wound coil in the original
IFT - 1000pF capacitor in parallel, tiny capacitor to my signal generator
(this is my standard way of testing IFTs, but of course without adding
an external capacitor). The resonant frequency is about 109kHz, which
corresponds to an inductance of 2.13mH. Pretty close to the AADE
measurement. This also agrees with eyeballing the two modified IFTs
that I have here: someone seems to have removed at least 3/4 of the
turns from the coil.

Also noted the DC resistance: 43 ohms.

A single pie 2mH inductor will be self-resonant well below 2830kc.
A quick measurement suggests that there would be a voltage insertion
loss of about 7x between the plate of the mixer tube and the grid of
the 1st IF amplifier (my calculations say that this makes the overall
gain of the mixer stage about unity - but I could be wrong there).
I looked in the Radiotron Designer's Handbook, 4th Ed, but I couldn't
find the typical loss of a double-tuned IFT for comparison.

73, ian K3IMW


On Sat, Oct 25, 2014 at 11:06 AM, Mike Feher <n4fs at eozinc.com> wrote:

> Ian - you may want to try and measure that winding again. 2 mH is a lot and
> would not be used at RF frequencies. Just compare it to a typical 2.5 mH RF
> choke which has multiple pi sections and is used as a choke to prevent
> conductive RF leakage. 73 - Mike
>
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960
>
>
> -----Original Message-----
> From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Ian Wilson
> Sent: Saturday, October 25, 2014 1:27 PM
> To: Kenneth G Gordon
> Cc: ARC5
> Subject: Re: [ARC5] Selectivity and the BC-455
>
> Update on my measurements: I went through my ARC-5 spares box and
> discovered
> three 7277 (1st) IFTs. On closer inspection, two of these have been
> modified. The third unit had unbroken red paint over the screws so I think
> it's safe to assume this one is kosher.
>
> The pi winding in the intact 7277 measures about 2mH. This is drastically
> different from the other two: 60uH and 180uH. My guess is that someone was
> trying to turn the second winding into a tuned one.
>
> So, I no longer think that the pie-wound winding was meant to resonate
> approximately with the input capacitance of the second stage.
>
> 73, ian K3IMW
>
>