[Elecraft] voltage display on K2 -- KB3FBR

[Don Wilhelm]

Joe,

Taking things one at a time from the end to the beginning!

Before we delve into the proper operation, let me say one thing about 
intermittent operation - that is usually a sign of a poor solder 
connection.  See my post earlier today about what to look for when 
inspecting solder connections.  If your soldering does not look like the 
diagrams in the Soldering Tutorial sheets sent with your K2 (and 
available on the Elecraft website) with a nice small fillet of solder 
tapering out to a very thin edge on both the solder pad and the 
component lead, you should wick away the excess solder and re-solder 
things with a sufficiently hot iron and minimal amounts of solder 
(excess solder will mask a poor solder connection).

There is no concise written description of the function of each 
microprocessor pin.  Some information can be derived by examining the 
signal names along with a study of the K2 operation in the manual.  
There used to be a 'Theory of Operation' section to the manual, but it 
was dropped - I imagine due to a lack of interest from most of the 
builders.  Note that U6 and U8 are connected by serial data and clocking 
lines, so U6 pin 21 is "DAC  Chip Select" and pin 22 is "EEPROM Chip 
Select", pin 23 is Serial Data to EEPROM, pin 24 is "Serial Data Out", 
etc.  Pin 25 is Transmit Data (for the RS232 signal TXD) and pin 26 is 
Receive Data (from the RS232 signalling).  U8 behaves both as a DAC and 
as an expansion of U6 output pins.  Of course, all that has nothing to 
do with your problem.

Now for your problem - to troubleshoot, I suggest that you start at the 
beginning.  U6 pin 27 should be a logic high (near 5 volts) during 
transmit periods and a logic low (near 0 volts) during receive.  The RX 
signal at pin 28 should be just the opposite, high during receive and 
low during transmit.  Note that there are no bars on the signal names 
denoting that they are at a high state when the signal is active 
(compare with the /DASH signal at pin 29 which is active when low).

The TX and RX signals drive the gates of Q3 and Q4.  Q3 should conduct 
during transmit and Q4 should conduct during receive (assuming the FETs 
were not zapped during assembly).  These FETs in turn control the 
conduction of Q1 and Q2 to switch the 8T and 8R voltage rails on and 
off.  8T should be close to 8 volts during transmit and near zero at 
other times while 8R should be near 8 volts during receive and near zero 
during transmit.

There is one other device in the transmit and receive voltage rails, and 
that is RF Board Q23 which should conduct during transmit periods and 
its purpose is to pull 8R down to zero in case there is some residual 
voltage on it when the transmit 8T signal comes active.

So now you have the conditions for the proper operation of 8R and 8T as 
well as the driving signals (RX and TX) from the microprocessor.  If you 
follow that information from source to result, you should be able to 
figure out where it is failing - or better yet, verifying that all is 
working correctly.

Once you have verified that the 8T and 8R voltage rails are operating 
properly, we can address the switching that occurs at D6 and D7 - one 
step at a time, the result depends on the input.  "Shotgunning" by 
trying to 'fix' everything and anything sometimes works, but an orderly 
and straightforward approach will usually lead to a solution much more 
quickly and with certain results and conclusions.

73,
Don W3FPR

MARY HAYS CARNEY wrote:
> Don:
>
> Your email prompted me to make these measurements and clarify my statement about the voltage at D6 and D7:  
> The anode voltage at D7 is a zero with key up; however it does not go to 8 v on key down.  The anode at D6 is 7.9 volts with key up (receive) but only drops to about 0.6 v on key down.  I think the problem goes back to U6. I.e. with key up, the voltage on pin 27(TX) of U6 is 0 volts.  On key down, it stays at 0 v.  Thus the base of Q1 (ctrl brd) is not turned on.  At the same time the voltage on pin 28(RX) of U6 remains at 5 volts.    
>
> To complicate things, this behavior is intermittant; on some bands I see the described behavior and on some not, and it even occurs/doesn't occur on a particular band at seemingly random times.  Unless you have the great AHA! with these symptoms you might hold off on a reply until I replace the U6 on the ctrl brd with extra antistatic care and recheck measurements.  Also, I will recheck the low pass filter and T4 before firing it up again.
>
> One more question.  Is there a list somewhere that describes the function of each pin of U6 (ctrl brd)?  For example, I understand the functions of pins 27 and 28, but  don't know which one translates V RFDET to a value for the DAC, U8 as well as functions of pins 21-26.
>
>