[Elecraft] K2 BFO Test: BFO low freq is really low

[G3VVT at aol.com]

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