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

[mikea]

On Wed, Jun 16, 2010 at 07:54:48AM -0700, 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.

Sounds like something has gone seriously wrong with the neutralization,
OM, and it's going into oscillation when keyed. The high SWR tells us
that it's not oscillating in the 6m band, either. 

It may be time to go through the amp carefully with an eye to whatever
the design uses for neutralizing. 

Remember that an oscillator is just an amp with positive feedback at the
frequenc(y|ies) of oscillation.

Good luck! Please let us know what you find. 

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin