[Elecraft] [K2] No Transmit Power

[Don Wilhelm]

  Brian,

On your RF Probe, that 200 mV is noise pickup.  Do you have a relatively 
short ground wire (4 to 6 inches) with the alligator clip connected to 
the board near the tip?  That alligator clip should connect to one of 
the ground loops you installed on the K2 boards.  A short grounding wire 
like that should reduce the noise pickup level - if it does not, check 
the soldering.  The DMM reading should go to a very low millivolt level 
when the ground wire and the tip are connected together.

If you have a 'scope, it will give you good readings if you use a 10X 
probe and both the probe and 'scope have adequate frequency ratings for 
the frequency in use.  In general, a 'scope and probe can be used for 
reliable amplitude measurement if their frequency rating is 5 times the 
frequency of the signal.  It is still usable above that frequency for 
relative RF voltage measurements.  Remember that the 'scope will display 
the peak to peak amplitude (the RF Probe will indicate RMS values), so 
multiply the 'expected' voltages in the manual by 2.8 to obtain the 
corresponding peak to peak amplitude.

Regarding the DC voltage at CB U8 pin 2, read the entire paragraph 
carefully. it is high because your K2 power output is lower than the 
requested value (reason "a" of the two reasons listed). Since you have 
checked the RF Detector, go to the end of the paragraph and do what it 
says "If you can't find a problem with the RF Detector, continue with 
the next signal tracing section (transmit mixer, etc)."

Check the Transmit stages in the order listed - if the stage is working, 
the RF Voltage will be HIGHER than the expected value listed (that stage 
is trying to drive harder to increase the power output).
When you come to the stage where the RF Voltage is significantly lower 
than the expected value, that is the output of the stage with the 
problem - stop and analyze that stage - Do the DC Voltage measurements 
associated with that stage, then move on to the schematic armed with 
that voltage information and see which component is likely to cause that 
problem.  Hint:  it is usually NOT the transistor.

That process is consistent with the 3 steps for any generalized 
troubleshooting process.
1) Find the failing stage.
2) Make measurements of that stage
3) Analyze the circuit to determine what is causing abnormalities in the 
measurements.

73,
Don W3FPR

On 8/27/2010 11:41 PM, Brian Denley wrote:
> I have built the RF probe but I am unable to get any signal less than about
> 200 mV even when not touching anything.  I have tried three DMMMs with no
> better luck.   I do know that I am transmitting something because I can pick
> up the transceiver signal easily on any of my HF receivers.  I have been
> able to establish that pin 2 of U8 on the Control Board reads 5.0 volts key
> down and that means (according to page 14) that I should check the
> components around the RF detector...and I will as soon as I can find a way
> sense RF.  Maybe I will switch to the scope tomorrow and try that.  Any
> other ideas will be welcome.
>
> BTW, I have checked the RF detector again and again.  It's a pretty simple
> circuit and the diode is oriented correctly.
>