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
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