Posted by
G3VVT on
Nov 15, 2004; 12:52pm
URL: http://elecraft.85.s1.nabble.com/K2-BFO-Test-BFO-low-freq-is-really-low-tp371631p371632.html
A possible cause of misreading the BFO frequency at TP2 may be due to the K2
frequency counter being insensitive.
See Elecraft Builders Alert #21 of October 19, 2004.
Recent kits issued with the blue colored MPS5179 used in the K2 frequency
counter input circuit have been found to have low sensitivity and should be
replaced with the black version of the transistor. It is also worthwhile
checking the K2 frequency counter input on the K2 Control Board has the components
place correctly and soldered joints are good.
The significance of this is that TP2 signal levels are much lower than TP1
or TP3 and where the counter may work on the other TP's, it could fail or
become random on TP2. The given voltage in K2 Troubleshooting when measured with
the K2 RF probe is between 0.025 and 0.070 V rms which is a much lower level
than TP1 where 0.60 to 0.75 V rms is expected and TP3 where somewhere
approaching 1V rms can be the norm.
Checking the setting of C22 and the 4Mhz oscillator with an external
frequency counter can have pitfalls.
Firstly the counter has to have sufficient accuracy and resolution to fulfil
the task.
Secondly the external frequency counter, whilst it may have very good
accuracy, could load the K2 4MHz oscillator and cause misleading measurements. I
found my Racal Dana 9915M bench counter with xtal ovened clock did this and
needed to connected via a 10X scope probe to minimise the loading affect. You
will note that the K2 frequency counter probe has a 10pF capacitor in series
which should reduce the loading effect of the internal counter to a degree.
Using the recommended C22 adjustment techniques given by Elecraft and other
experts may still be the best solution with a better final accuracy.
Regards,
Bob, G3VVT
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