We all feel GB Ron's pain with the blown mosfets in his amp. I am
supposing that the problem is a parasitic oscillation while keying.
Maybe GB Fred K1FMS, can give that a look and make some
recommendations. I will be looking into it. The circuit has a nice chip
cap bypass on the bias line, but it may need a more robust treatment. I
will do some work there and let people know what I found. In my previous
experience, I have not seen any instability in the 500 watt amps.
I had my own problems with equipment troubles and have been running them
all down to be fixed. I had horrendous VSWR that would go away after
pumping a KW into the antenna for a few minutes. I also lost my WD5AGO
cavity preamp in a thunderstorm a few Tuesday nights ago. The preamp is
hosed with a 30 dB noise figure. . All the relays are fine, and I found
a few droplets of water in the flex section between the preamp box and
the 222 4 X yagi array. A look on the TDR showed one connector with a
problem. I took things apart and dried it out. As far as I could see,
the cable dielectric was still OK. We had been on the tower fixing
stuff before, and must have disturbed this connection on the preamp box
to allow some water ingress. Anyway, I put the 1/2" Superflex cable
back together and it tested perfectly. I replaced it on the tower on
Monday after the June Contest. VSWR in the connector was now 1.0zip, or
about 35 dB RL. I still am working on getting the preamp fixed, but at
least I am back on the air.
I will be on 222 this evening starting at a little before 23:00 UT. The
Moon will be in the western sky and nowhere near the horizon, but I will
also monitor the HB9Q EME chat page for 222 MHz. Incidentally, I have
a portable EME setup at the house that is now running on 432 and later
on will be on 1296. It is courtesy of Frank and Bob, NC1I and W1QA. It
uses four rear mount 12 element yagis. The secret is 3 ft long 1/2"
heliax phasing lines and a low loss power divider with a low noise
preamp right there at the PD. It is very effective when you consider the
small size of the array..
Anyway, dust off the 222 gear. Put down your bong pipe, and get on 222
MHz this evening. Remember to check in on the ON4KST Chat page (144/432
Region 2) and let folks know you are out there. Meteors will be more
plentiful. I hope to try a sked or two. I am even bringing 5 gallons of
diesel fuel if it is a long night!!!
73
Dave K1WHS
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