[Elecraft] [KPA500] stuck on TX when AUXBUS attached

[Jack Brindle]

This is a matter of physics. A driver pulls its output to a power  
rail, either power or ground. In the case of the drivers used in the  
K3 for the AUX signals, pullups are used, to the positive rail to  
provide the pullup function. This means that when the driver is off,  
its output will be pulled to whatever voltage the power rail is  
sitting at. When the K3 is powered off, this voltage goes to zero,  
thus causing devices connected to the driver to see a zero value at  
their inputs. Thus the KPA sees a 60 meter input, and the key line  
appears to be active. Note that the Auxbus signal itself is only an  
output from the KPA, and has a diode block on the input. Thus it is  
_not_ part of this situation.

There is another, potentially bigger problem that occurs when you  
parallel two devices in this way. When one device is powered, and the  
other off, the powered device can actually inject power into the  
"unpowered" device through the IO lines. This will actually power up  
the device, at least partially. The device is in a dangerous state at  
this point, with transistor junctions improperly biased and processors  
running in very strange situations. It is actually possible to blow  
ICs in this situation. Beyond this, it is sucking power from the other  
device. This scenario is called "incidental power," and is a major  
problem in the telco co-processing arena where we specifically design  
to protect against it from happening.

Now, with all this going on, do you _really_ want to parallel the K3  
and KX3 control signals to the KPA, or even leave one connected while  
the other is driving the amplifier? By far the best thing to do is to  
pull the plug from the K3 when you use the other transceiver, then  
plug it back in when you use the K3 with the KPA. Why risk damage to  
any of this precious gear we have invested in?

Jack Brindle, W6FB


On Sep 14, 2012, at 6:25 AM, Gary Gregory wrote:

> *Art
>
> The Auxbus cable without the adapter carries the PTT function also  
> so maybe
> that is also an issue that FW may not be able to change?
>
> With the addition of the KAT500 into the K-Line this may become even  
> more
> difficult as the Auxbus cable will run from the K3 to the KAT500 and  
> then
> to the KPA500 to enable really tight integration of the K-Line and  
> this may
> also make a FW option in the KPA500 difficult.
>
> Just thinking.......Jack may jump back in and comment?
>
> 73
> *
> On 14 September 2012 23:16, Art Nienhouse <ka9zap at gmail.com> wrote:
>
>> It would be nice if the function of the AUXBUS cable could be  
>> disabled
>> from a menu choice on the KPA500 instead of disconnecting of the  
>> cable
>> between the K3 and the KPA500.
>>
>> Would this be something that could be done in the KPA500 software?
>>
>> Regards
>> Art
>> ka9zap
>>> Tonight I tried to use my KX3 with my KPA500, but when I turn off  
>>> the
>>> K3 that it is attached to via AUXBUS, then put the KPA500 into  
>>> OPERATE
>>> mode, the asterisk comes on and it is in TX, which means that the
>>> receiver of the KX3 goes deaf.  Is this the way it is supposed to
>>> work, or have I messed up a setting?  Will I have to remove the  
>>> AUXBUS
>>> cable from the K3--even though the K3 is turned off--whenever I want
>>> to use the KPA500 with another rig?
>>>
>>>
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>
>
>
> -- 
> *Gary*
> *Start the day off slow, then taper off.........*
> K3 #679
> KPA500FT #18
> KAT500FT 007
> P3 #1629
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