query on power readout

[chris meagher]

Hello there from VK,

I am investigating severe loss of TX power on my K2.
With 5W set, I get almost no power, the RF detector input measured with
the RF probe/DMM shows 0.009V.
however, the display reads approx. 5W out.
The reading on pin 2 of U8 on control board is 0.55V.

This would suggest to me that U8 is trying to deliver maximum power,
which is what it ought to do if only 0.009V is seen at the detector.
All detector components seem to be OK.

What I don't understand is how the LCD is reading 5W on transmit.

Other TX RF measurements (with SSB board still installed, rig set on CW
mode)
Q6 base = 0.021V
Q6 collector = 0.88V
Q7 base = 0.089V
Q8 base = 0.108V
(would have expected Q7 & 8 to be the same)
Receive and menu functions all appear normal.

Any ideas welcome, or advice as to where to look next.

Chris  K23666

Chris Meagher
VK2LCD
PO Box 61
Woodburn NSW  2472
Australia

Nearest town: Evans Head
HF Rig: Elecraft K2
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[Don Wilhelm-3]

Chris,

When the K2 is attempting to deliver maximum power, Control Board U8 pin
2 will be near 5 volts.  The 0.55 volts that you measure there indicates
that it is controlling at a much lower power level.

If you do not have either the KAT2, the KPA100, or the KAT100 installed,
the K2 does not display the actual power output on the LCD (the
requested power is displayed briefly when the power control is changed).
  What is the indication on the LED bargraph during transmit?

 From your RF voltage measurements, the voltage at the base and
collector of Q6 appear to be about correct for about a 3 to 4 watt
level, but the RF voltage at the bases of Q7 and Q8 is much lower than
typical for that level.  Check T2 as your first action.  Be certain it
is wound just like the illustration in the manual (particularly the
dressing of the green wire), be certain the leads are well tinned and
properly soldered, and that T2 is mounted off the board by 2 to 3 mm.

Also check the soldering at the RF detector and its associated voltage
divider resistors.

The behavior of the K2 for power control is such that if it does not
detect power output equal to the power detected on the VRFDET signal,
then the microprocessor will attempt to increase power until the power
is equal to the requested power.  If that is not happening, then the
detector driving the VRFDET line may have a problem, or the VPWR and
VALC signals (control Board ALC components) may not be working properly,
or the BFO injection to the transmit mixer (RF Board Q24 and its
associated components) may not be working properly.

73,
Don W3FPR


chris meagher wrote:

> Hello there from VK,
>
> I am investigating severe loss of TX power on my K2.
> With 5W set, I get almost no power, the RF detector input measured with
> the RF probe/DMM shows 0.009V.
> however, the display reads approx. 5W out.
> The reading on pin 2 of U8 on control board is 0.55V.
>
> This would suggest to me that U8 is trying to deliver maximum power,
> which is what it ought to do if only 0.009V is seen at the detector.
> All detector components seem to be OK.
>
> What I don't understand is how the LCD is reading 5W on transmit.
>
> Other TX RF measurements (with SSB board still installed, rig set on CW
> mode)
> Q6 base = 0.021V
> Q6 collector = 0.88V
> Q7 base = 0.089V
> Q8 base = 0.108V
> (would have expected Q7 & 8 to be the same)
> Receive and menu functions all appear normal.
>
> Any ideas welcome, or advice as to where to look next.
>
> Chris  K23666
>
> Chris Meagher
> VK2LCD
> PO Box 61
> Woodburn NSW  2472
> Australia
>

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