[Amps] SB-220

[Alek Petkovic]

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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>>Amps at contesting.com
>>http://lists.contesting.com/mailman/listinfo/amps
>>
>
>Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
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