[Elecraft] power output

Tue May 28 23:38:01 2002

Hi Earl,

One of two significant problems with my K2 is presenting the same
symptoms.

Could be either oscillating or the pwr ctl is calling for max all the
time.  Is the output on the indicated frequency?  If not, oscillating
PA.  The gurus at Elecraft tell me the likely culprit is poor T/R
diode isolation. Not my problem...  Check 8T for 0-8 volts Rx to Tx. 
Also, 8R for 8 to =>0.25 volts Rx to Tx.
Another troubleshooting possibility.  Clip W6 to break the RF chain
and check for 0.6 volt drop across D1-D7 that tracks switching Rx to
Tx to Rx.

The problem could be on the control input or output.    The RF
sniffer, D9 cathode, should be 4-5 volts at 15 wts.  If there is a
short/load after R66, you might expect about 2 1/2 volts instead. 
After R66 this signal is "VRFDET".  The signal is fed to U6 on the
control board to be compared to the power level requested.  To control
PA output, U6/U8 send a signal (VPWR) to U10A to develop VALC.  My
VALC is at 2.25 volts/Tx, calling for max power.

If VALC is low, the controller is trying to back-down the amp and
can't (probable osc).  If VALC is high (in my case 6.5 v), pwr ctl is
calling for max power.  This indicates a problem in the power control
circuitry.

All symptoms pointed to U6, pin 2 being the problem.  With U6 inserted
normally, VRFDET (in Tx) at pin 2 was 0.24 volts.  With U6 inserted
with pin 2 bent out (no contact), VRFDET rose to 3.95 volts.  ...Pin 2
loading the RF sense line.  I am now waiting for a new U6 to confirm
the diagnosis.

73 & gud luk

Woody

Earl Ritchie wrote:
> 
> Just puchased a K2 kit and put it together. I am now working through the
> voltage chaecks but briefly the problem I have is that it shows 15 w out
> regardless of the power control setting.  Wonder if anyone else has had
> this problem
> 
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