[Elecraft] K2 build completed. RS-232 no go yet

[W. Wright, W5XD]

Hello list,
This weekend I completed the assembly of my K2 and KPA100. Everything seems
to be working OK (haven't used it much yet, so can't be certain there aren't
undiscovered problems), but I can't make the RS-232 link (on the KPA100)
work at all in either direction.

I do make mistakes while building, but I (normally) have the skills to find
them (one solder bridge on the KPA100 and one omitted component in the K2
itself) but this one escapes me.

I have eliminated the cable as a possible problem (by using it on another
rig).

And I am 95% sure that U4 on the KPA100 is OK on its receive, RXD, side
(because my DVM can "see" the characters on pin 11). The DVM can see the
same signals over on the control board at pin 26 on U6.

Its also no-go in the other, TXD, direction (which is a completely
independent signal path). Using the PORT ON menu entry on the K2 and tapping
DISPLAY gives the expected one or two second "test" display, which is
documented to be accompanied by a message coming out of pin 25 on U6 Control
Board. Neither my DVM nor my software at the other end of the RS-232 cable
sees anything.

Next comes a puzzling behavior that I wonder if anyone else sees:

After using the PORT ON to generate the "test" message, I then tape MENU
twice to get back to the normal frequency display. Next I TAP the MODE
button on the K2 front panel (as if to switch to LSB, for example), and
that's the last the K2 will do. Its completely frozen--no response to any
front panel switch or the VFO knob.

My firmware is revsion 2.03d.

Can anyone else reproduce the "dead K2 after PORT test message"?

I would like to know whether I have some troubleshooting to do on the
Control Board, or whether the firmware does this on someone else's machine.

Thanks,
Wayne, W5XD