[Heath] Heath Digest, Vol 94, Issue 9

[Al Skierkiewicz]

Chris, both of the i.f. cans in the AR3 are the same part number.  To
calculate the capacitance, I simply measured the dimensions of the screened
silver areas, subtracting for the circular area and then pulled out the
reference books on calculating capacitance based on the thickness of the
mica sheet.  I calculated the larger area to be 0.086238 in.sq. and the
smaller to be 0.0689 in.sq.
>From the Handbook C (pf)=.224((dielectric constant x area)/spacing between
plates)  I chose 5.4 for the dielectric constant of mica and using a
calipers, I measured the thickness of the sheet at 0.001 in.  So C=.224((5.4
x .08638)/.001)=104.3pf for the larger cap and substituting 0.0689 I
calculated 83pf for the smaller cap.
Al



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Subject: Heath Digest, Vol 94, Issue 9

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Today's Topics:

   1. Re: Heath Digest, Vol 94, Issue 7 (ChrisIwata)


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Message: 1
Date: Sat, 20 Oct 2018 17:36:46 -0700
From: ChrisIwata <chrisiwata at aol.com>
To: heath at puck.nether.net
Subject: Re: [Heath] Heath Digest, Vol 94, Issue 7
Message-ID: <5BCBCA1E.60807 at aol.com>
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Good result!

My AR-2 had the same problem with the IF cans.

Calculating the capacitance from the surface area and the width of the mica
wafer must have been hard:  How did you do the measurements?

For those who don't have an inductance meter to measure the coils with the
internal cap disabled,  the value for the caps is around 110pF.  It will be
slightly different for primary vs secondary winding, and 1st vs 2nd IF can.

You might be able to pinpoint which cap(s) is bad by wiggling each IF can
solder lug successively.  Do this with the radio ON and an insulated wand.
The mica wafer is physically attached to the solder lug.  Anyway, that's
what worked for me.  Thus, I didn't have to replace all 4 caps.

Chris, KL7DM

On 10/20/2018 1:31 PM, Al Skierkiewicz wrote:
> Guys,
> I have just finished restoring an AR-3.  It was a labor of love as I 
> my first ham receiver AR-3.  I had purchased it used for $15 from 
> George at the Radio TV Lab in Chicago.  I saw this one at the hamfest 
> in Oak Creek, Wisc for $25 and just wanted to get it going.  My cap 
> was not frozen but very stiff.  I just shot some Lubriplate white 
> grease into the ball bearings that make up the reduction.  I waited 
> for a day and just started moving the shaft until the grease got
distributed.  Worked wonders.
> I had more problems with the fact that the kit wasn't fully correctly 
> wired and that the silver mica caps in the i.f. transformers were all
noisy.
> (Thunder syndrome)  If you tap on the cans and you hear tons of 
> crackling and thunder, then you have the same problem.  I removed the 
> mica wafers and replaced the caps with dipped silver micas.  I 
> calculated the values from the area of the mica that was plated with 
> silver.  The fact that there were wiring errors meant that the radio 
> had never been aligned.  A RF signal generator is kind of necessary 
> for alignment of this box.  Although I got it working, I also bought a 
> Q multiplier that wouldn't work at all.  Turns out, that the i.f. was 
> 50kHz low.  Once I got everything correctly tuned, the Q mult worked great
as well.
> Al
> WB9UVJ
>
>


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