[Amps] SB-220

[kwasny]

> 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

Thanks to all that replied. I am being very cautious before applying power
to the amp again. I have looked all over the meter area, the band switch
area and everything else in that area can cannot find any evidence of any
arcing. I checked the zener diode and it seems OK. I checked the meter board
and all of the diodes and resisters test OK also. I am still trying to find
out what might have arced in that area, any ideas? I would like to get it
working again before making any other changes to improve the parasitic
oscillation problem. After I get it working I will make the modifications. I
have been wanting to get the paracitic kit from Harbach but he is not
accepting any orders at this time, perhaps in a few weeks. I have checked
the tubes for any shorts from one element to another and both tubes show no
shorts. I realize that when cold that they may not show up. Any futher
information or help would be appreciated.

Thanks, Darryl K5WAS