[Amps] Ten-Tec Centurion

[R.Measures]

On Nov 10, 2004, at 3:16 PM, Georgens, Tom wrote:

>
> I have a Centurion at my contest station in Barbados that works 
> flawlessly
> on 10-80 but will arc on 160.  It is tuned into a resonant antenna 
> with a
> reasonable SWR.
>
> On 160, the amp will occasionally arc immediately on key down.  If it 
> does
> not arc immediately, it will not arc if the key is held down.  Also, 
> when it
> does not arc, it tunes smoothly and at the predicted settings.  All of 
> the
> arcing is in the Tune cap and is not always in the same place.
>
> My theory is that there may be power overshoot from the TS850 on the 
> leading
> edge of the carrier that is causing the arcing.  It is hard to see 
> this on
> the wattmeter and my 3 other amps all work fine.  Does this seem like a
> reasonable theory and is there any way to compensate for this?

Tom -- Since the 3-other amplifiers do not exhibit this problem with 
the TS-850, overshoot is not a very logical  explanation.  The first 
thing I would check is the resistance of the vhf suppressor resistors.  
If the resistance is >50% higher than the marked value, the possibility 
exists that vhf energy damaged the vhf suppressor resistors -- 
especially if the outside of the resistors shows no sign of 
overheating.  However, if the amplifier has been used for 10m RTTY, 
there may be signs of over-heating and higher resistance.   A common 
problem with 2, 3-500Zs is that their 0.3pF of anode-cathode (feedback) 
C amounts to –j4500-ohms at 110MHz.  The workaround is to reduce the 
vhf gain (and the amplitude of the vhf ringing as well) of the 
amplifier by using low vhf Q parasitic suppressors -- the trade-off of 
which is c 2% less power output at 29MHz.
>
> On my most recent trip, I performed a flawed experiment.  For 
> convenience, I
> used a different TS850 to drive the amp and the problem occurred only 
> once
> out of dozens of tries.  I then moved it to the TS850 where I had 
> previously
> noted the problem.  In this case, I was never able to reproduce the 
> problem.
> I tried mistuning to some degree but no hint of the problem.  However, 
> when
> taking the whole thing apart, I noticed that I had never connected the 
> amp
> relay cable.  The amp was using RF sensing to key the relay.

RF sensed switching invariably results in the amplifier hot-switching 
because RF is already present when the relays in the amplifier begin to 
switch.  Hot-switching creates current transients and those in turn 
ring the anode circuit's vhf resonant circuit formed by the Tune-C, the 
Anode-C and the anode lead's L.  In my opinion, such ringing is the 
seed signal that gets fed back by anode-cathode C and initiates a 
regenerative condition.
> ...
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org