----- Original Message -----
From: "R.Measures" <r@somis.org>
To: "kwasny" <kwasny@netzero.net>
Cc: <amps@contesting.com>
Sent: Monday, November 01, 2004 8:38 PM
Subject: Re: [Amps] SB-220
Richard, How do I go about getting these parasitic mods an instructions from
you? I hate to use the smoke test to find where it arced but I will power it
up without the first and if that goes well will try one tube then both. At
this time I have a .5mH choke to replace the one that fried and have removed
that smoked 200pF cap. Should this be OK to try it? I will not drive it at
this time.
>Problem is that the parasitic may occur next time you try it out. I'd do
the modifications first before any power was applied. My opinion is that
the Harbach >parasitic mods are kinda expensive; you can get the same stuff
cheaper direct from Rich Measures. But the Harbach step/start circuit can't
be beat and the >replacement fan from them is tons better than the original.
Bought my 220 used 14 years ago, had spitting and arcing in the tune cap,
did all the mods, it runs like a >champ. Watch out for that HV! Gd luck,
Phil KB9CRY
Darryl - K5WAS
> On Nov 1, 2004, at 4:50 PM, kwasny wrote:
>
> > I smoked one of 1mH chokes on my sb-220.
>
> Darryl -- This is caused by a parasitic oscillation ar c. 110MHz.
> Replacing the choke with resistors will not prevent vhf oscillations.
> There's an article about parasites in the 220 on my web site that
> explains this phenomenon and a method of reducing vhf gain in the 220.
> .
>
> > Also saw some arcing around the meter area. I am going to replace the
> > chokes with resisters. Have to replace one of the 220pF caps that got
> > smoked up. Does anyone know what may have arced near the meter area?
> > Fact is, what made the rf choke smoke?
>
> The high grid-I during a vhf parasite creates a lateral EMF on the hot
> filament helix. This occasionally bends the filament far enough
> sideways to short against the grid -- which is grounded through the
> choke. Since the filament xfmr is connected to the +110vdc power
> supply on Rx, it shorts the supply to ground, causing a large current
> flow-- which smokes the choke.
>
> > What other damage might I have done. What is a good source for the
> > 220pF caps and any other parts that I may need?
> >
> > Thanks to all, Darryl
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> >
>
> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>
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>
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