Friends,
The transmitter I have borrowed from Steve, works 24/7 and has an excellent
output wave form through the scopematch on its way to the antenna. This amp
uses a single layer solenoid transformer for its output.
For all the current tests, the dummyload is being used instead. Scott lent
me his prototype amp which is designed for a pair of 34NB20 FETs, and I have
been trying to do what I thought would be a "simple" task... which is to
build the final output circuit, consisting of the output transformer, feed
choke and capacitor bypass.
The problem is that whenever I switch to a rig with an output transformer
with a TV Horizontal output core, the waveform goes bad. (Yes, into the
dummy load!)
I need to paint a word picture of the wave form because I find that whenever
I try to send even small picture attachments of 30K into the reflector, the
message is blocked.
Start with a sine wave, flatten both the positive and negative waveform
peaks. On each of these flats on the positive and negative half cycles, the
leading and trailing edges each have a smaller peak with a valley in the
flat between them. The peak on the leading edge is a little higher than the
trailing edge with a slight dip between these peaks. The closest thing I
can use to describe one half of the wave form is a two-hump camel back. The
opposite half of the waveform is identical to the first with the leading
peak slightly higher than the one which follows.
The Low Pass Filter, Scopematch and all its connections are the unchanged
between the use of the two different amps.
The only variable which I know of is the output transformer, final choke and
RF bypass capacitor. Since I suspected the output transformer I have taken
it apart and rewound it with several windings, on two different cores, all
suggested by different friends who are technically sound. There are NO
insulator disks between the core halves, and at 56 Watts of input the core
cannot be saturating. The latest winding matches the Jim Moritz, M0BMU
alternative for the G0MRF amp on page 77 of LF Today. with taps on the
output winding to set power output to lower values
As I try different windings I find myself making mistakes and filling the
burned FET can, so I am doing these experiments with the less expensive
IRF640s, in Scotts prototype until I get things right.
The current protective circuit does work, is set at its lowest setting and I
am choosing the low taps and keep the TX current at about 4 amps at 14 Volts
supply.
Please think about what I could be doing wrong with this simple circuit.
There may be something I have done wrong with the or to the amp, but I
cannot imagine what it is. It is drawing current, and providing output.
Both FETs have input pulses with the key down.
I am at a loss having spent the last few days trying to get this working.
When the output circuit is fixed, it will be used on a similar amp to the
prototype, which I have acquired most of the parts for, and expect to have
built in the next couple of weeks. It will have a pair of 34nb20s, and is
intended to bring the power level up to 1 W ierp, the license limit. A few
parts are still needed for an SWR protective circuit with diode OR similar
to the protection in the G0MRF design.
I realize that this problem has been discussed before, but no solution or
even cause has been found. Please brainstorm. No idea will be considered
foolish and even if wrong, may give someone a sideways inspiration to a
structured method of locating and solving the real problem.
Thanks,
J.
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