[Elecraft] K3/KPA500 question

[Bob Wilson, N6TV]

Does your automatic antenna switch use the Unified Microsystems BCD-10 or
BCD-14 band decoders, or is your design similar to the 4O3A Antenna Genius?

These products all use opto-Isolators on the four Band Data lines in the
AUX cable.  Wiring this type of decoder in parallel with the
K3/KAT500/KPA500 AUX cable via a 15-pin Y-cable or similar *will usually
cause automatic band changes on the KPA500 to stop working*, until you
disconnect the antenna switch's band decoder, or send some RF so that the
KPA500 frequency counter takes over.  Not cool.

Opto-isolators draw too much current from the K3 ACC pins, leaving too
little voltage for the KPA500's band decoder to track properly.  This is no
problem for the KPA1500, only the KPA500, with or without the KAT500.

The solution I discovered was to connect four 4.7K pull-up resistors to 12V
to each band data pin in the K3 ACC cable, assuming the band decoder in the
antenna switch can handle 12V instead of 5V (most can).  The KAT500 and
KPA500 will have no problem with 12V on the band data lines.

The Y-BOX v3.0 (not pictured on my website because I have not had time to
update it) includes these four pull-up resistors.  They can conveniently be
wired internally to the band data pins, and band switching should work
again when all the devices are connected in parallel to the K3 ACC via the
Y-BOX.  This assumes your KPA500 "RADIO" menu is set to "K3" or to "BCD" so
it tracks properly when the automatic antenna switch is disconnected from
the ACC/AUX cables.

FYI, you can still tap the BAND buttons on the KPA500 to change bands on
the K3, because that uses the AUXBUS line (ACC Pin 2), not the BAND DATA
lines.  Many folks may have never discovered this cool feature of the
KPA500 and KPA1500.

73,
Bob, N6TV
https://bit.ly/Y-BOX


On Fri, Apr 23, 2021 at 6:18 PM Joe K2UF <joe at k2uf.com> wrote:

> I have my K3-KAT500-KPA500 working great.  The amp is off to the left side
> and I can't see the display easily.  while working on building an auto
> antenna switch  I noticed  that the band display on the amp does not synch
> with the K3.  It seems to be random.  If I hit  one dit on the key  the amp
> will synch the band with the K3.  I am not sure if this has been working
> like this right along and just did not  notice.
>
> Any ideas guys.
>
> 73 Joe K2UF
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