[Elecraft] K2 signal tracing question

[Don Wilhelm]

Paul,

That is not enough signal to saturate the AGC for certain - but then if 
you have the Extremely Strong Signal Handling diodes in place you will 
likely find a similar condition because the diodes clip on those very 
large signals.
All is not lost, you can still do the signal tracing with a lower level 
signal as long as you 'use your head' instead of the RF voltages 
indicated in the manual.  You can approximate the gain/loss of each 
stage and check for that amount of gain/loss with the scope reading on 
the output vs the input of any stage.

Did you really mean 4.9V signal?  That is 1.75 volts RMS and is well in 
excess of what is required - if you meant 4.9 mV peak to peak, then 
fine, but if you are really measuring 4.9 volts, then something is wrong 
(sounds like a strong oscillation).

Check the BFO input to the Product Detector at U11 pin 6 - it should be 
in excess of 200 mV peak to peak.  Likewise check the RX VFO signal to 
the mixer - it must be +7dBm  (5 mW) which into a 50 ohm load translates 
into 0.5 volts RMS or 1.4 volts peak to peak.  A weak BFO signal can 
explain both bad receive and low transmit output.  On transmit, the 
Transmit Mixer also needs more than 200 mV peak to peak from the VFO to 
U10 pin 6.

Be certain the Post-Mixer Amp is amplifying,  as well as the IF amplifier.
If the input to the IF amplifier is low, it may be because of a KSB2 
board problem - remove the KSB2 and put jumpers from pins 1 to 3 of both 
J9 and J10.

Has this K2 worked before and suddenly failed?  Or is this a new build 
of an older kit?  In both cases, look for a solder problem, like an 
unsoldered connection or a poorly soldered joint.  It is not unusual for 
a bad solder connection to work fine until enough oxidation builds up to 
cause a failure.

73,
Don W3FPR

Paul Fletcher wrote:
> I'm fault finding a K2 that is deaf and has low TX output and have a basic
> question regarding the sig gen I'm using. My generator will only produce
> 145mV pk to pk and I want to know if this should be enough to drive the AGC
> into saturation. I'm only measuring 4.9V on pin 1 of U2 which seems way low
> compared to the spec. Is this enough input as the spec is 145mV RMS from a
> RF probe (which I don't have - I'm using a scope). Initial checks haven't
> shown up anything significant but I will start from scratch again tomorrow
> as this week has been lost repairing my scope.
>
> Serial no. is 2756 and it has the rev A boards BTW.
>
> 73 Paul M1PAF
>   
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