[Elecraft] [K2] single tone test - output seems amplitude-modulated

[dl0tuh]

Hi,

my question originates from recent one in this list, but I'd like to put it
in into 
a new thread, to avoid confusions.

Intending to do a two-tone test of my K2, I decided to try a 1000 Hz single
tone 
first. On the ocsilloscope on antenna jack I expected a signal much like
when holding 
key down in CW.

Instead, I get something like an AM signal. It's modulated with the
frequency of the 
single tone test signal.

http://www.mydarc.de/dl2ipu/kpa100/barefoot/AntJack_1_watt_1_tone_1000Hz.jpg
AntJack_1_watt_1_tone_1000Hz.jpg 

To see if it is really the test tone frequency or may be +-50 or 100 Hz, I
checked
RF board D9 cathode (out detector) and registered 1kHz sine overlayed on DC
level.
 
http://www.mydarc.de/dl2ipu/kpa100/barefoot/RF_D9_catode_1_watt_1_tone_1000Hz.jpg
RF_D9_catode_1_watt_1_tone_1000Hz.jpg 

I varied the test tone and cross-checked oscilloscope's time base, it really
seems to 
be the test tone frequency.

Emitter of Q8 on control board (ALC) has a constant level.

In CW - nice constant amplitude and usual K2's outstanding shaping.

I hope somebody can help me explaining the first oscillogram - where is this
AM 
coming from?

I did balance the carrier on KSB2 successfully. With mic jack disconnected,
RF 
amplitude is less than 100mV (checked both with the same scope probe and
K2's RF 
probe). Filters were aligned with noise generator and Spectran, lower
roll-off freq of FL1
is about 400 Hz.

Test conditions: PC as tone generator, K2 power set to 1 watt, CAL probe
removed, 40m 
band, both LSB and USB tried, ALC shows one LED, dummy load on antenna jack,
K2 
powered from a 12V 7Ah battery. KPA100 removed; KSB2, KNB2 and K160RX are
in.

73! Oleg

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