[Elecraft] Seeking Help Solving KAF2 Problem

[Don Wilhelm]

Dennis,

Are you certain your KAF2 is tuned to your sidetone pitch?  If it is not it
will severely attenuate your signal.

There is no need for the AC voltages at the headers pin 1 to match the
voltages at pin 2.  The filter and the input to the audio amplifier are
differential inputs and the measurement must be between the two pins to
indicate how it is working.  It is more than just a matter of subtracting
the two AC voltages to obtain the drive levels because the phase of the
signals must be considered too. If you have an ungrounded AC meter, measure
the voltage between the pins.  If you are observing with a 'scope, remember
that the scope is grounded and you must use two probes - invert one channel
and add the two to see the differential voltage.

I would suggest that you check the components on the KAF2 board for correct
values and good soldering.  Especially check R9, R10, and C16 because these
operate a phase shifter to restore the balanced output (the filter sections
themselves are single ended).

The temporarily lost sidetone is not related - the sidetone is injected into
the LM380 audio amplifier  by a separate path from the receiver audio.

73,
Don W3FPR

> -----Original Message-----
>
> I haven't received any comments about my audio filter problem yet, so
> tonight I tried a few more things to no avail, and took some VAC
> measurements with a fairly accurate voltmeter. The signal came from the
> 10Mhz. oscillator shown in the manual.
> With the filter switched out, J1 & J2 pin 1 measured .030 and pin
> 2 measured
> .018 (shouldn't they be equal?). At the LM380 (had to solder a
> couple wires
> to the pins & stick them out from between the boards) pin 2 measured .004
> and pin 3 measured .008. At the speaker (volume control centered) it
> measured .292 and the volume was very good.
> With the filter switched in, J1-1 measured .036 and J2-1 measured
> .056; J1-2
> measured .022 and J2-2 measured .053. At the LM380 the voltage at
> pins 2 & 3
> then measured .058 VAC on both, and the voltage at the speaker
> was way down
> to .008.
> I also took a couple DC measurements. With the filter out, the
> four pins had
> 5.1V. on them, and with it switched in, J1's pins were still at 5.1V. and
> J2's pins measured 6.03V. Do these additional measurements I took give
> anybody any ideas?
> Thanks again,
> Denny Payton     N9JXY
> ..................................................
> >A couple weeks ago, I got out my K2 and operated the ARRL
> International DX
> > Contest. During the contest, my audio output was so low I had to
> > constantly
> > use the preamp just to hear on my headphones, so afterwards I
> opened the
> > rig
> > up and tracked the problem down to the audio filter. Shorting
> pins 1 & 2
> > of
> > J1 & J2 gave me full audio with the board removed, and flipping the
> > board-mounted switch to "out" gave me full audio with the board
> installed.
> >
> > That told me the problem 'had' to be in the filter board, but after
> > checking
> > the DC voltages from the chart and thoroughly examining each component's
> > location and solder joints, I found nothing. I decided to use a RS
> > speaker-amp to follow the audio signal through the filter to
> see where it
> > was getting attenuated, but was very surprised to not see it getting
> > attenuated anywhere. The filter appears to work properly from
> beginning to
> > end. The audio at pins 1 & 2 of J1 & J2 all sound fine to me,
> whether the
> > filter is switched in or out, but the K2's audio is 'greatly' attenuated
> > when the filter is switched in. Although I'd made sure the filters were
> > aligned with each other when I first started troubleshooting, I thought
> > that
> > was all that was left, so I started checking them again. Then, at one
> > point,
> > I hit the "spot" button and discovered I suddenly had no
> sidetone. I spent
> > quite a bit of time trying to figure out what happened, but
> couldn't get
> > any
> > power from pin 25 of the MCU whether I keyed the rig, hit "spot", or
> > turned
> > the rig off & on. Then as a curiosity after giving up, I
> removed the audio
> > filter board. That gave me sidetone again! Next, I reinstalled
> the board
> > and
> > I still had sidetone, so something must have "locked up"? Now, I've got
> > the
> > filter removed again though, and have decided that I'm at the end of my
> > ability.
> >
> > Something else I think I should mention is that I measure 5.15 volts on
> > pins
> > 1 & 2 of  J1 on the control board. I hope that's expected. Also, I don't
> > know if I've had this problem since the beginning or if it just
> popped up.
> > I
> > haven't operated the K2 since installing the KAF2 quite some
> time ago, and
> > I
> > just don't remember how I left it.
>
>
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