[Elecraft] K2 Frequency Counter Sensitivity

[Mike S]

At 04:59 PM 9/29/2004, Don Ehrlich wrote...
>The signal at TP3 is about 2 volts peak-to-peak.  Is this a normal
>value?  It looks marginal to me for triggering the Q9/Q10 flip-flop.
>If so then the PLL Ref Oscillator has a problem.

Q9 & Q10 form an amp/buffer to the U9-15 input. U9-10 provides power to this circuit when the frequency counter is active. 

U9-15 must have levels which cross normal TTL Schmidtt trigger input thresholds, (<~1.0v for low, >~4.0v for high by spec, but probably more lenient than that in reality). The p-p voltage at U9-15 doesn't matter, but the absolute levels do. If it doesn't go above/below those thresholds, it might not count (literally). It sounds like the levels on your K2 may be marginal. Since you've got a scope, look there. Q9 and Q10 are static sensitive, and one or both may be damaged/defective.