[ARC5] Unstable VFO

[D C *Mac* Macdonald]

Basically it was the same problem that has shown up in
polystyrene dialectric trimmer capacitors in Icom PLL
circuits.  What I remember was that the plasticizer would
migrate out of the plastic.  This is the same process that
causes leaching in coax cables and that which causes the
vinyl in automobiles to "outgas" and coat the windows
(most noticably) as if cigarette smokers inhabited the vehicle.

73  ---  Mac, K2GKK/5




----Original Message Follows----
From: "Brad Hernlem" <alihernlem at hotmail.com>
To: k2gkk at hotmail.com
CC: arc5 at mailman.qth.net
Subject: Re: [ARC5] Unstable VFO
Date: Tue, 03 May 2005 17:21:55 +0000

Mac,

I am curious what exactly was the nature of the unstability. My 
understanding is that these parts have excellent properties, in general, 
i.e. excellent accuracy and leakage, and good temperature stability (with 
reliable tempco) but sensitive to overheating (so must be installed 
carefully with adequate heatsinking). Depending upon how they are 
manufactured, they might not have very low inductance. Do you recall what 
exactly this speaker complained about?

Thanks.

Brad
KG6IOE

>From: "D C *Mac* Macdonald" <k2gkk at hotmail.com>
>To: arc5 at mailman.qth.net
>Subject: Re: [ARC5] Unstable VFO
>Date: Tue, 03 May 2005 03:39:31 +0000
>
>Many years ago, one of the ham radio clubs in Oklahoma City
>had a guest speaker that I think was employed in ARRL lab.
>He presented quite a talk on VFO stability and part of that
>talk was the info that POLYSTYRENE capacitors tend to be VERY
>unstable beasts and should not be used in frequency determining
>circuitry.
>
>Mac, K2GKK/5
>
>
>
>----Original Message Follows----
>From: David Stinson <arc5 at ix.netcom.com>
>To: arc5 at mailman.qth.net
>Subject: Re: [ARC5] Unstable VFO
>Date: Mon, 02 May 2005 08:34:06 -0500
>
>
>
>Ian Wilson wrote:
>
>>Posted a while back about a haunted ARC-5 VFO on 40m. Strange 'warbling'
>>instability that I just couldn't defeat despite changing many parts.
>>
>>Someone - Ken Gordon, I think - suggested a bad joint around the tube
>>socket. The answer turned out to be a bad joint somewhere in the grid
>>circuit.
>>
>>The VFO is quite stable now, running from a 182V stabilized (Zeners,
>>sorry) supply. The original dogbone ceramic capacitor has been replaced
>>with a polystyrene equivalent that appears to be doing a fine job.
>>
>>This has taken way too much time, need to get the rig back on the air
>>now.
>
>Good news.  I think I'll take that suggestion on a BC-459
>with a similar problem.
>
>Since we're talking about this, it reminded me of the suggestion
>to use ferrite beads to kill VHF parasitics, if that were the problem.
>I have a similar situation that is certainly rooted in VHF parasitics.
>What is the proper procedure for selecting the "right" ferrite
>for a problem?
>D.S.
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