[Elecraft] K2 - VCO-Test (Alignment an d Test, Part II)

[Masleid, Michael A.]

Hello David,

I checked mine at home - I'm using a DMM that measures to 0.001 ohm, but
I have to subtract out lead resistance of 187 to 205 milli ohms.

>>Terminal 1 on T5 is grounded.  Touching it with the DVM can't change =
the
frequency.
>>The ground connection is on the top side of the board.  Why is this =
not
connected
>>through the plated through hole?

>The resistance between terminal 1 and ground is 0.1 ohm

I get perhaps 1/1000 ohm here.

>>Notice that L30's metal can has a ground connection right next to =
Terminal
1.

>the resistance is 0.2 ohm

I get perhaps 5/1000 ohm here.

The thing is, if the DVM only reads to 1/10 ohm, it could show exactly =
what
you see, even though the read value is much smaller, since lead =
resistance may
be enough to show 0.1, and adding another 5 milli ohms might make it to =
two.

At terminal 4 I get 30/1000 ohms - mostly the resistance of the coil on =
T5.

>> Try soldering a jumper wire on the bottom side of the board between
Terminal 1 on T5 and the ground at L30.

>no effect

Strange and stranger still.  Remove the jumper eventually.

>>>What is the voltage on Q18's Drain?

>>8.1 volts (touching has no effect)

>>So Q18 is not conducting??  It would be tempting to change Q18, but =
the
ground
>>on terminal 1 is more interesting, since what is happening there is =
not
possible.

>D	8.1 volts
>S	1.1 volts
>G	-0.13 volts

I would expect more current through Q18 with -1.23 Volts Gate to Source.

>>>At 9MHz to 9.1 MHz there is more or less a strong signal

>>So in terms of RST, how do you rate the tone?

>596(7)

Note exactly a pure tone.  So the oscillator isn't running right.

>When I repeat the voltage-measurment at terminal 4 of t5 I get now 1.2 =
volts
>(the frequency goes up to 8.8MHz). The wet finger reduces the voltage =
to 1.1
>volts

>When I touch terminal 2 with the probe-tip (the second tip not =
grounded),
>the frequency goes up to 8.7 MHz. When I take a screwdriver it goes up =
to
>8.9 ore 9MHz and the voltage at r30 to 6 volts - so strange!!!

I'm figuring that this provides extra feed back to Q18.  I could be =
wrong, since
some of the evidence points to too much feed back to Q18.  (Negative =
voltage on
the gate, poor oscillator tone.  Finger loading down T5.)

I think Q18 is sick.  I would try soldering a 4.7 or 5 pF capacitor =
accross C68,
which should be 10 pF now, so that it is closer to 15 pF.  If that fixes =
things,
I would figure on replacing Q18 and put C68 back to 10 pF.  If that =
makes no
difference, double check C68 again.  It might be marked 10 or 100, for =
10 pF or
for 10 pF X 1, but it must not be 101, for 10 pF x 10, or 100 pF.

73 de Michael AB9GV