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What kind of capacitors do I use?
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Ernie  
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 More options Apr 20 2012, 10:10 am
Newsgroups: rec.radio.amateur.boatanchors
From: "Ernie" <sag...@brrr.net>
Date: Fri, 20 Apr 2012 09:10:37 -0500
Local: Fri, Apr 20 2012 10:10 am
Subject: What kind of capacitors do I use?
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My old receiver's IF wouldn't align and I finally traced it down to the IF
can. A grid dip meter shows the primaries and secondaries of each coil are
anywhere from 20 to 40 kc away from the design frequency (460kc). The
trimmers trim OK, but just not close to 460 kc.
I can bring each coil back to the IF frequency by soldering another small
value cap (100 pf or less) parallel to the trimmer. The capacitor sellers
have all sorts of variations and I am wondering if anyone can give me a
suggestion as to what kind would be the best to use for this application?
Thank you.

 
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Scott Dorsey  
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 More options Apr 20 2012, 10:18 am
Newsgroups: rec.radio.amateur.boatanchors
From: klu...@panix.com (Scott Dorsey)
Date: 20 Apr 2012 10:18:21 -0400
Local: Fri, Apr 20 2012 10:18 am
Subject: Re: What kind of capacitors do I use?
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Ernie <sag...@brrr.net> wrote:
>My old receiver's IF wouldn't align and I finally traced it down to the IF
>can. A grid dip meter shows the primaries and secondaries of each coil are
>anywhere from 20 to 40 kc away from the design frequency (460kc). The
>trimmers trim OK, but just not close to 460 kc.
>I can bring each coil back to the IF frequency by soldering another small
>value cap (100 pf or less) parallel to the trimmer. The capacitor sellers
>have all sorts of variations and I am wondering if anyone can give me a
>suggestion as to what kind would be the best to use for this application?

Silver mica.  In a pinch, an NPO ceramic would probably be fine.

BUT... this happens when the capacitor inside the IF can fails.  If it
failed completely, you're fine.  But if it failed partially then adding the
cap will only be a temporary measure...eventually it will fail completely
and then the value will change again.

So I would suggest going into the can and clipping out the old capacitor
if you're willing to risk it, then putting a new silver mica in.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


 
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Ernie  
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 More options Apr 20 2012, 10:29 am
Newsgroups: rec.radio.amateur.boatanchors
From: "Ernie" <sag...@brrr.net>
Date: Fri, 20 Apr 2012 09:29:35 -0500
Local: Fri, Apr 20 2012 10:29 am
Subject: Re: What kind of capacitors do I use?
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Thanks Scott. Silver mica it is. That's a good suggestion about eliminating
the old capacitor entirely and I will do so.


 
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Jim Mueller  
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 More options Apr 21 2012, 12:27 am
Newsgroups: rec.radio.amateur.boatanchors
From: Jim Mueller <wrongn...@nospam.com>
Date: 21 Apr 2012 04:27:30 GMT
Local: Sat, Apr 21 2012 12:27 am
Subject: Re: What kind of capacitors do I use?
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On Fri, 20 Apr 2012 09:10:37 -0500, Ernie wrote:
> My old receiver's IF wouldn't align and I finally traced it down to the
> IF can. A grid dip meter shows the primaries and secondaries of each
> coil are anywhere from 20 to 40 kc away from the design frequency
> (460kc). The trimmers trim OK, but just not close to 460 kc. I can bring
> each coil back to the IF frequency by soldering another small value cap
> (100 pf or less) parallel to the trimmer. The capacitor sellers have all
> sorts of variations and I am wondering if anyone can give me a
> suggestion as to what kind would be the best to use for this
> application? Thank you.

Grid dip meters are notoriously inaccurate.  If the IF is 460 KHz, listen
for the meter's 2nd harmonic on 920 KHz with a broadcast radio before you
do something you can't undo.

It's especially suspicious that ALL the IF cans are off by the same
amount.

--
Jim Mueller wrongn...@nospam.com

To get my real email address, replace wrongname with dadoheadman.
Then replace nospam with fastmail.  Lastly, replace com with us.


 
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