[Collins] Back trouble shooting the KWS-1 Need Help

[Gerald]

On Thu, 2005-06-23 at 15:53 -0500, Dave Meitzen wrote:
> Symptom: When I adjust the bias to set the PA idle current at 100Ma it
> slowly declines to zero.
> 
> Bias circuit is good. Swapped the finals so next checked the screen circuit.
> This is what I see.
> 
> The screen voltage slowly declines (screen current as well) to ~200vdc. I
> scoped the screen voltage and do not see any ripple. This tells me the
> screen supply has good caps. The voltage does not decline.
> 
> If there was a bad bypass I would think it would blow the fuse?? The screen
> fuse is 0.25 amp.
> 
> Before I disassemble the amp I thought I would ask.
> 
> Anyone out there seen this before?
> 
> Thanks
> Dave

The tube screens aren't connected to the supply well. The electron flow
or its lack when idling will tend to make the screen go negative.
There's lots of impedance or an open (which could be the screen fuse)
between the supply and the PA screens. Could also be an overload relay
with dirty contacts.

If there was a short, the screen voltage would be zero, wouldn't go
negative.

Floating screens can go far enough from rated voltage to blow the screen
bypass capacitors.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer