I've been meaning to ask the group about this for a while and the recent
threads that referred to blowing up traps and traps catching fire have
reminded me to do so.
I have an Alpha 87A linear amplifier, which has a pretty comprehensive set
of computer-controlled error detection and protection circuits that
instantly shut down the amp in the presence of any of about 40 abnormal or
unsafe conditions during transmit, including high SWR, mistuning,
over-driving, arcing, incorrect voltage or current values, high temperature,
tuning motor failures, etc., etc., etc.
While operating at high-power during CW contests, every now and then the amp
shuts down with a fault status that indicates either severe mistuning (not
likely) or an RF arc. It happens most often on 10M or 15M. I think it might
have happened once or twice on 20M, but I'm not sure. All three of those
bands are on the TH-7. I've never seen it happen on the 40M 4-square, 80M
inverted vee, or dummy load. It's a hard problem to drive deliberately. I
don't recall it ever happening while chasing DX; it only seems to happen
after running 1500W for a while with CT generating high-speed (30 WPM+) CW.
Pulling the power down a couple of hundred watts below 1500 always makes it
stop happening.
For a while I thought a keying transient might be firing one of the two
Polyphaser 2KW lightning protectors I have in that line, but some
experimentation without the units and with substituting higher power (3KW)
Polyphaser units seemed to rule that out.
Thinking there could be a problem with the amp, I called Alpha/Power about
it a while back. The tech there asked what kind of antenna I was using and
said, "Oh, the traps on the TH-7 are notorious for arcing, especially on
10M." Needless to say, I was surprised to hear this, since the antenna is
supposed to be rated for the full legal limit. Also, I have 250 feet of
LMR400UF and 80 feet of RG/213 between the amp and the antenna. I'm probably
not getting more than 700-800 watts to the antenna on 10M and 15M.
Nonetheless, the symptoms do seem to agree with his assessment. I can't say
for sure, but I think the problem has happened more often in moist weather,
which is another bit of evidence against the traps. By the way, the antenna
is only one year old, so I'm a bit doubtful about insects living in the
traps.
I do recall seeing the same problem occur once in a while on 20M on my old
GAP Titan vertical. That antenna has design problems with arcing through the
standoff insulators, especially in wet weather, so I'm reasonably sure that
the problem with the TH-7 is caused by something arcing in the system.
What's the general consensus out there about TH-7 traps? Has anyone had
problems with them arcing in dry or wet weather? Could a mere 800W be
causing the high-band traps to arc? Can keying transients (perhaps
exacerbated by the speed and computer generation) cause a big RF spike that
might in turn be causing the higher band traps to arc?
73, Dick, WC1M
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