Do what Rich says. You won't be sorry. His kits have now tamed many a wild
amplifier in the land down under.
73, Alek
VK6APK
At 09:38 AM 2/11/2004, R.Measures wrote:
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
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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