[Amps] 3-500Z amp producing output with no input, why?

[Vic K2VCO]

It's become an oscillator! The high swr is an indication that it is oscillating on a 
frequency removed from the resonant frequency of the antenna.

How to fix it could be complicated. You need to determine what frequency it is oscillating 
on and why. Is it a grounded-grid amplifier? Then make sure that the grid is effectively 
rounded for RF.

On 6/16/2010 7:54 AM, Bill Dzurilla wrote:
> Hello,
>
> My homebrew 6 meter 3-500Z amp has made the trip to Europe.  The only visible damage
> was that one of the lines to the filament pin of the 3-500Z had broken off.  I soldered
> it back on.
>
> The amp powers up normally but something goes wrong when the PTT is pressed.  Even with
> the rig set to SSB and the rig's power control fully counter-clockwise, the amp
> immediately produces hundreds of watts of output, even with no modulation.  In fact,
> the amp does this even when the RF input line from the rig is disconnected.  And if the
> RF input line is connected, the rig's internal power meter shows about 4 watts of
> output (the meter shows zero if the amp is not connected).  Also, the SWR is way off.
> Barefoot, I read a 1.2:1 SWR to the antenna.  With the amp on and generating the weird
> output, the SWR is over 5:1.
>
> Any thoughts on what might be wrong and how to fix?  I tested it and it was working
> fine just before I packed it up to send to Europe.
>
> 73, Bill NZ5N/OM9ACA


-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/