[Amps] SB-220

[R.Measures]

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 at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org