[Elecraft] K2 - KPA100 RS232 interface repair

[Floyd Sense]

Hi Don.  I'm sure you're aware that the RS232 spec includes the 
description of voltage levels, and if you're going to refer to an 
interface as RS232 (as Elecraft does in many places) then you're 
implying that the normal RS232 signal levels will be used.  A user 
shouldn't be required to read a one hundred page manual before plugging 
in a connector when every other such connector works in the conventional 
way.  Having been a designer using RS232 interfaces 45 years ago, I 
would have been executed had I designed something like this.

Anyway, maybe you can help me determine what areas to focus on in the 
repair.  Here's what I know:

1. The RF chokes RFC6-9 are all intact.
2. The ALC is functioning properly - the power control properly controls 
the output
3. U1 in the KPS-100 appears to be functioning as the relays are being 
activated at the right time as I switch through the bands.
4. Data sent from Hyperterminal through the USB/serial converter to the 
K2 shows up on Pin 6 of the KPA-100 U4 (the Max1706) and can be seen at 
pin 26 of the K2 MCU (U6).  But, the K2 doesn't act on the commands that 
are  sent.
5. The voltage on pin 5 of U4 (MAX1406) is -15.4 VDC, seems that 
indicates U4 is toast?

I haven't observed any other problems.  I found the document written by 
Gary Surrency and Tom Hammond regarding this situation.  Which leads me 
to some questions:

1.  Reference is made to possible damage to the KSB2.  Do you know what 
the symptom would be in that case?
2.  Apparently, any damage to the MCU IC might be isolated to just the 
input from the interface - pin 26.  I say that because everything else 
appears to function normally.  Are you aware of anything else I might 
check for regarding the MCU function?

73, Floyd


On 8/22/2015 3:16 PM, Don Wilhelm wrote:
> Anyone who has purchased a used K2 needs to take notice, lest it 
> happen to you too.
>
> That connector is marked AUX I/O for exactly that reason, it is not a 
> computer RS-232 connector even though it contains TXD, RXD and signal 
> ground.  As a result, one must always use the special cable that is 
> built as part of the KPA100 to connect to a computer serial port.
>
> You are not likely to have damaged anything in the KPA100 when 
> connecting a standard serial cable to the K2, although KPA100 U6 may 
> have a damaged output at pin 6, and the inductors located just behind 
> the DE-9 connector should be checked for continuity.
> The TXD and RXD signals in the serial cable will connect to U4 just 
> the same as if the special cable were used.
>
> You could have also damaged the computer serial port because pin 1 was 
> grounded by the K2.
>
> You will find most of the damage will be in the base K2.
> Internal signals AUXBUS and VRFDET could have caused the MCU IC to be 
> damaged as well as the KSB2.
> The fact that the 8R voltage rail appears on that connector, that may 
> have caused further damage.
>
> 73,
> Don W3FPR
>