Posted by
Don Wilhelm-4 on
Nov 04, 2009; 11:08pm
URL: http://elecraft.85.s1.nabble.com/K2-signal-tracing-question-tp3948589p3948974.html
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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