John Sandin wrote:

Regarding the 3-watt transmitter I'm building (and having trouble
with), here's an update.

I removed the lowpass filter and loosened the wiring, and for a while
it seemed to help. The transistor didn't get as hot. But the output
stayed the same (less than 1 watt). But then I noticed, as I've
noticed before, that the voltage measured at the dummy load doesn't
always drop when the key is released. And the transistor stays hot
even after the key activity has stopped.

I did as one poster suggested. I removed the antenna from my
receiver, tuned around while transmitting, and discovered the
following:
(Crystal is 7110 kHz)
at 3554 kHz, signal is S8
at 7110 kHz, signal is S6
at 14218 kHz, signal is S6
at 21326 kHz, signal is S7
at 28433 kHz, signal is S5

Even I can see that this isn't good.

This IS good - it is what you should expect with the LP filter
removed. (In fact, you may very well "see" these frequencies
even with the LP filter installed.) You're "seeing" the harmonics
with the receiver. It proves the thing is producing oscillation
in the Q1 stage, which is a desired effect. But it doesn't tell
you anything about self oscillation or the "health" of the Q2
circuit. If you disconnect the base of Q2 and the receiver doesn't
hear those frequencies any more, or hears them at a lower level,
that proves that the Q2 stage was amplifying.

What is of interest is the fact that you saw voltage across the
dummy load, after the key was released. That is bad. Q2 should
be cut off by the 33 and 56 ohm resistors with no signal on L2,
and even if it was shorted, there should be no voltage across
the dummy load. That suggests that Q1 must still be producing
a signal, or self oscillation in the Q2 circuit. Check the
capacitor values in the Q1 circuit - make sure it is a 10 uf
electrolytic, and the 220 is 220 pF. The resistance to ground
from the base of Q2 should be about 90 ohms. The components
connected to the base of Q2 should have short leads, and you
want to keep wires and components connecting to the Q2 collector
away from the Q2 base and all of the components to the left
(on the schematic) of Q2.

And the emitter of Q2 connects
to the same trace as everything else that's grounded in the circuit,
and it's not a very big trace.

I'm probably going to rebuild this rig, without the circuit board.
Many thanks to all who gave me advice on this. Any further comments
are also much appreciated.

I put the schematic on my website. The design calls for 28 VDC for a
5-watt output, but I am using 12, which is also ok according to the
article.

http://www.kcnet.com/~oyster/transmi...ansmitter.html

-John Sandin KC0QWE

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