K2 Frequency Counter Sensitivity

[Don Ehrlich-2]

I am having trouble reading out the frequency of the signal at TP3,
the first step of checkout after part 2 of wiring up the RF board.
The Frequency Counter probe is fine and all components of the
frequency counter from the input through the flip-flop Q9/Q10 check
out perfectly with capacitance and ohmmeters.  Supply voltage to the
flip-flop at U6-10 is 4.75 volts.

The frequency readout from the K2 varies from nothing to almost normal
(12+ MHz) depending on whether or not I have scope or frequency
counter probes attached ( The additional capacitance from the probes
increases the signal level slightly and the counter counts, but
unsteadily.

My external frequency counter reads the signal at TP3 just fine and
frequencies are within specification.

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.

Don  K7FJ


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[Mike S-8]

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.

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[Don Wilhelm-3]

Don,

I did not see an answer to your question on the reflector, so I pulled the
left side panel from my K2 and took some readings.

The peak to peak voltage at my TP3 is about 2 1/2 volts with the probe
attached and the K2 in CAL FCTR mode, so yours is within reason.  This level
signal is amplified and shaped by Control Board Q9 and Q10 to a clipped and
shaped 4 volts p-p at the collector of Q10 (you can read the voltages at RP4
pins 1 thru 4).  Q9 and Q10 are amplifiers rather than flip/flops - with
this front-end amplifier on the counter, it is quite sensitive and should
read input levels as low as 0.1 volts or so.

Check out the soldering on the Q9/Q10, RP4/RP5 area and you should find th
problem.
Also, check to see that the frequency probe is connected to the Control
Board properly (the shield should be nearest the left side panel).

73,
Don W3FPR

----- Original Message -----

>I am having trouble reading out the frequency of the signal at TP3,
> the first step of checkout after part 2 of wiring up the RF board.
> The Frequency Counter probe is fine and all components of the
> frequency counter from the input through the flip-flop Q9/Q10 check
> out perfectly with capacitance and ohmmeters.  Supply voltage to the
> flip-flop at U6-10 is 4.75 volts.
>
> The frequency readout from the K2 varies from nothing to almost normal
> (12+ MHz) depending on whether or not I have scope or frequency
> counter probes attached ( The additional capacitance from the probes
> increases the signal level slightly and the counter counts, but
> unsteadily.
>
> My external frequency counter reads the signal at TP3 just fine and
> frequencies are within specification.
>
> 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.
>

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[Don Brown-4]

Hi

Also check that you have not reversed the pins in the frequency counter probe connector or installed the connector on the control board backwards. There was also a post recently about low gain on some of the 5179 transistors causing low sensitivity on the frequency counter.

Don Brown


  ----- Original Message -----
  From: W3FPR - Don Wilhelm<mailto:[hidden email]>
  To: Don Ehrlich<mailto:[hidden email]> ; [hidden email]<mailto:[hidden email]>
  Sent: Thursday, September 30, 2004 7:21 AM
  Subject: Re: [Elecraft] K2 Frequency Counter Sensitivity


  Don,

  I did not see an answer to your question on the reflector, so I pulled the
  left side panel from my K2 and took some readings.

  The peak to peak voltage at my TP3 is about 2 1/2 volts with the probe
  attached and the K2 in CAL FCTR mode, so yours is within reason.  This level
  signal is amplified and shaped by Control Board Q9 and Q10 to a clipped and
  shaped 4 volts p-p at the collector of Q10 (you can read the voltages at RP4
  pins 1 thru 4).  Q9 and Q10 are amplifiers rather than flip/flops - with
  this front-end amplifier on the counter, it is quite sensitive and should
  read input levels as low as 0.1 volts or so.

  Check out the soldering on the Q9/Q10, RP4/RP5 area and you should find th
  problem.
  Also, check to see that the frequency probe is connected to the Control
  Board properly (the shield should be nearest the left side panel).

  73,
  Don W3FPR

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[lizlarry]

Hello all,

Don Brown is right.  I had the same problem, and it turned out Q9 was extremely weak - even though the E B C voltages measured fine.  The Scott at Elecraft said to use a 2N2222 in its place 'til the replacement parts arrived.  I did and it worked like a champ.

The guys at Elecraft are great.  The K2 kit is a biggun to tackle, so it's quite a comfort to know you won't be left with your buns in the breeze if you encounter trouble...

73
Larry Rickard, Jr. KT4G
K2 4431

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