K2 VCO Voltage Instability

Posted by John Huffman-3 on Nov 19, 2009; 2:05pm
URL: http://elecraft.85.s1.nabble.com/K2-VCO-Voltage-Instability-tp4031967.html

Guys -

This topic was KPA100 Behaving Badly, but at Tom's N0SS suggestion, I
tried the KPA100 on my working K2 and all was well.  I also tried the
control board of the sick K2 in my working K2 and all was well.  So,
it's not the KPA100 Behaving Badly, it's the K2.

I'm back to the original problem -  watery CW signals on receive,
frequency shifts on receive and loss of all signals.

I suspect the VCO is not getting the correct consistent voltage.  I
attempted to do the VCO Alignment in Alignment and Test - Part 2.  I
tuned to 4mhz and easily adjusted L30 for 6.0v as shown on R30 on the RF
board.

But, as I tuned toward 3.5mhz, the voltage decreased steadily until
about 3.8mhz where it dropped to zero.  Tuning back up to 4mhz, it
remained at 0.  Turning the radio on and off while tuned to 4mhz
restored the voltage to 6v, but the same thing happened when I tried to
tune below 3.8mhz.

Turning the radio on while it is set for 3.5mhz give a reading at R30 of
6.8v.  Tuning higher increases the reading to 8v where it stays at all
higher frequencies.  Tuning back down, the voltage goes to 0v around
3.8mhz and remains there at all frequencies.

The voltages on these pins of U6 (RF Board) remain constant - pin 5 -
4v, pin 6 - 4v, pin 8 - 8v.  Would a bad U6 explain these problems?

It there somewhere else I should be checking?

Any and all help greatly appreciated.

73 de K1ESE
John
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