"Floyd Sense" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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A couple of the list members have steered me to the discovery that a
previous owner had replaced the final output transistors with MRF421s. It
seems that the 421s have less gain on the higher bands than the original
transistors and that may explain why I still don't have good output on 10
when the ALC is set up as it should be on 20 meters. The good news is that
the 2SC2879 transistors that have much better gain at 30MHz are available in
matched pairs from RF Parts and I have a pair on the way.
Today, I put the TR7 in the operating position with the intention of trying
it out on all bands and driving the amplifier with it to see if there were
any interface problems. I first tested it into a dummy load on all bands at
100W out, and all was well. Then I tried it with a real antenna, first on
40 meters, and found that when I sent CW the output level was constant - not
following the keying - and the SWR noted on an external meter was high.
Very strange. I first thought that I had RF getting into the TR7 through
one of the interconnects, but that turned out not to be true. The problem
was that the pre-driver was apparently oscillating ( I had the pre-driver
pot turned all the way up to get 10 meter output). And - it wasn't
oscillating on the frequency the TR7 was tuned to, hence the high SWR. I
removed the cover and turned the pre-driver gain pot back down to just below
1/2, reassembled things, and the problem was gone. Good output on the lower
bands, but much worse on 10 and 12. So, will wait until the new finals are
installed and see what happens. The 2SC2879s are the same finals that are
used in the Elecraft K2 and FlexRadio SDR-1000 and I know the K2 can get
100W out with them and reasonable drive.
So, be forewarned that if you screw around with the pre-driver gain pot and
crank it up high, you may run into odd problems on 40 meters. I had read
that running the gain up too high could cause "instability" on 40, but in my
case it didn't show up until all the covers were on and a very slightly
reactive antenna load was used. That may make some sense to the engineers
in the crowd, but it caught me by surprise.
73, Floyd - K8AC
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