[Elecraft] KPA100 voltage measurements?

[Don Wilhelm]

Tom,

I have not yet seen a reply on the reflector to your questions, so I will
try to give you some information.

The voltage at U5 pin 1 will depend on the reflected voltage sensed by D17
and the setting of R27 - if you are measuring the voltage under transmit
conditions, do you have a good dummy load connected?  Pickup on an antenna
wire connected to the output MIGHT do strange things during receive periods
too.  You should have the same voltage on U5 pin 1 and pin 3 because U5A is
a unity gain op amp.  If the voltage on pin 1 is much different than the
voltage on pin 3 the problem will be found after the op amp U5A, but if they
are the same, the problem is in the SWR bridge or its adjustments.

I am at a loss to explain 28 volts on U6 pin 6 - that is another op amp and
it is powered from a 5 volt supply, so the pin 6 voltage will never be
brought higher than 5 volts by anything in U6 itself.  A regular RS232
computer cable plugged into the KPA100 and connected to a computer might
cause that kind of voltage - DO NOT USE A REGULAR COMPUTER SERIAL CABLE -
BAD THINGS WILL HAPPEN.  Check for something tied into the VRFDET line
somewhere in the base K2 or on the KPA100 board's AUX IO umbilical cord
and/or the DB-9 connector. Follow the PC board traces from U6 pin 6 to the
connection with the cable and the DB-9 connector for a short to an adjacent
trace or solder pad.

That is all I can tell you with the info I have right now.

73,
Don W3FPR

----- Original Message ----- 
>
> Am on page 23; KPA100 test points and have two problems:
>
> U5 Pin 1 should be 0 to 0.1; I get 2.4
>
> U6 Pin6 should be 0 to 0.5;  I get 28v!
>
> Any ideas or where to look? All other measurements were either right on or
> within the +/- 10%.
>