[Elecraft] KX1 ATU is resetting from TUN to CAL by itself

[Michael Madden]

Hi Ron and the group,

Thanks for the feedback.  I checked C4, C5, RFC1, and
C11 and they all all have good connections.  I can
make this happen to a 300W 50 ohm dummy load, so could
it still be stray RF from the antenna?

Two other builders reported this problem in April
2005, and below I have copied Wayne's reply.  Any
resolution found?  Is the offer to analyze and repair
still good? :)

Thanks!

Mike, N9OHW






The KX1 should definitely not lose power output or the
KXAT1 change its L-network settings under any normal
circumstances. We've tested the tuner and the rig
extensively, and I've never seen this happen.

The first things that come to mind are intermittent
component failures or power supply brownouts that
reset the MCU. However, using the tuner with a very
poor (or non-existent) ground, and/or an antenna that
is close to a half wavelength or a multiple thereof
could also cause trouble. RFI in such cases is a
challenge for almost any ATU. The usual remedies are
(1) better ground counterpoise, (2) adjustment to
antenna length, (3) balun. A 4:1 balun can be quite
effective at reducing high RF voltages that might be
present on the chassis or antenna, in many cases
transforming a difficult match into one that's in
range of the tuner.

If anyone has a KX1 that exhibits a consistent failure
pattern, please contact me directly. I would need to
know exactly how the failure occurs so I can determine
whether it's likely to be due to RFI, a component
failure, or some other cause. I'll also need to know
what L, C, and network selections were in effect at
the time. (The KX1, like the K1 and K2, provide ATU
menu entries that allow you to determine how the tuner
is configured. See the manual for details.) I will
analyze and repair any KX1 that is demonstrated to
have such a problem. But let's discuss it by e-mail
first ;)

I should mention one other possibility. If you have an
antenna that you know presents high RFI to the rig,
but you'd like to use it anyway, or if you just want
to "ruggedize" the KXAT1, you could add a few bypass
capacitors. I would place a very small .01 uF
capacitor from each of the MCU's relay drive pins to
ground. Since relays K1-K3 are so close to the KXAT1's
MCU, you could place capacitors directly across the
relay coil drive pins. The other four relays (K4-K7)
are farther from the MCU, so it would be best to put
the bypass capacitors right at the MCU itself, i.e.
from pins 10, 11, and 12 to pin 5 (ground).

The Elecraft part number for these bypass capacitors
is E530019. Anyone who wants to add these to their
KXAT1 can request seven of them at no charge by
e-mailing parts at elecraft.com.

Note that our new T1 ATU has such bypass capacitors on
all relay drive lines (surface-mount capacitors,
pre-installed on the board). We felt that the T1
should have this protection because it is a very
wide-range tuner, capable of operation on 6 meters
where RFI problems can be significantly worse than on
the HF bands. Also, the T1 is rated at 20 watts, and
has been successfully used at twice this level on 40
meters and up (with "backpacker" amps, usually). The
KXAT1, in contrast, is never going to be operated at
high power levels and is only intended to cover 40,
30, and 20 meters. But in extreme cases, it too could
benefit from the extra bypassing.

73,
Wayne
N6KR

--- Ron D'Eau Claire <rondec at easystreet.com> wrote:

> That sounds a lot like RF getting back into the
> controller in the ATU.
> Handling RF around a tuner is tricky. A whole lot
> depends upon the impedance
> presented by the antenna. The higher the impedance
> (which depends upon the
> electrical length of the wire) the more RF voltage
> and the more easily RF
> voltage can be capacitively coupled into the wrong
> circuits. It doesn't take
> a whole lot of RF to cause the controller to
> misbehave. 
> 
> Suggest you take a close look at the RF filtering
> provided in the design
> first, to make sure none of it has been bypassed
> accidentally by a solder
> bridge or unsoldered connection. That would include
> the 6V line filter C4,
> RFC1 and C5. If either C4 or C5 isn't grounded, it
> would greatly reduce the
> effectiveness of the filter without interfering with
> the operation of the
> tuner otherwise. A solder bridge across RFC1 would
> have the same effect.
> 
> The other critical item is C11 which bypasses one
> side of the relay coils to
> ground for RF. 
> 
> Unwanted RF feedback usually goes away when the
> power is reduced simply
> because lower power levels reduce the RF voltages
> present.
> 
> Ron AC7AC
> 
> -----Original Message-----
> Just a follow-up to my previous post...  It does not
> always reset from TUN to CAL.  I have seen it reset
> from TUN to other ATU settings like G03 or some
> other
> random fixed ATU state.  
> 
> When I use only the internal AA's, I don't see this
> problem happening.  Only on external 12V from 10
> NiMH's.  
> 
> But I don't want to have to give up those nice 3dB I
> get from the 12V and 4W vs. 2W!
> 
> Thanks,
> Mike N9OHW 
> 
> 


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