[ARC5] Selectivity and the BC-455

[Mike Feher]

Ian -

Well, I cannot find fault in your measurement method or your math, yet the
number just does not seem right. I just took the cover off of an unmolested
7277 and indeed the pi does have a lot of turns, I still would not expect it
to be in the 2 mH range. Time for me to make a measurement myself and then
put on the thinking cap, HI. Thanks & 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: Sunday, October 26, 2014 11:17 AM
To: ARC5
Cc: Mike Feher
Subject: Re: [ARC5] Selectivity and the BC-455

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
>
>
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