K2 U10a on control board oscillates on receive

[Marshall Jose]

My K2's ALC circuit on the control board oscillates at around 50 Hz
during receive. During receive, the "V PWR" voltage is set to zero, so
maybe there's some instability associated with driving U10a into its
non-linear region.

At any rate, it has no effect on receive operation since the BFO buffer
the "V ALC" signal controls is effectively blocked further down the xmit
signal path. But I've noticed that it can cause trouble because when
it's in that state it won't re-enter a linear mode until the power
control is increased during xmit above a certain level.

And I've noticed some rather severe limitations on the power I can
transmit in the higher bands, and have wondered about the words in the
troubleshooting section: "If you have seen a slow (10-20Hz) oscillation
superimposed on the transmitter's output signal, it could be due to ALC
modulation. Increase the value of R98 (RF board) to the largest size
that permits full output on 10 m."

Has anyone seen this behavior? Can anyone suggest a course of action? I
have a full suite of RF test equipment, so I can chase a signal through
the whole unit if necessary.

Thanks,

--
Marshall Jose, WA3VPZ

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[Marshall Jose]

Followup: After receiving a wonderful and long list of suggestions from
Gary, I understood that the U10a oscillation on receive is not a
pathological matter and that I shouldn't worry about it.

And, embarrassingly, in pursuit of the low-TX-power problem I discovered
an unsoldered joint on a relay coil pin, which made contact long enough
for the initial alignment to fool me well, but then opened up later and
caused detuning of the preselector/driver filters. Mea culpa.

K2 #5715 now lives, and in perfect operating condition. Cheers!

Marshall, WA3VPZ

Marshall Jose wrote:

> My K2's ALC circuit on the control board oscillates at around 50 Hz
> during receive. During receive, the "V PWR" voltage is set to zero, so
> maybe there's some instability associated with driving U10a into its
> non-linear region.
>
> At any rate, it has no effect on receive operation since the BFO buffer
> the "V ALC" signal controls is effectively blocked further down the xmit
> signal path. But I've noticed that it can cause trouble because when
> it's in that state it won't re-enter a linear mode until the power
> control is increased during xmit above a certain level.
>
> And I've noticed some rather severe limitations on the power I can
> transmit in the higher bands, and have wondered about the words in the
> troubleshooting section: "If you have seen a slow (10-20Hz) oscillation
> superimposed on the transmitter's output signal, it could be due to ALC
> modulation. Increase the value of R98 (RF board) to the largest size
> that permits full output on 10 m."
>
> <http://www.elecraft.com>
>

--
Marshall Jose, WA3VPZ

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