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

[David Broger]

Hi Bob

>*** Try adding a small cap in parallel with C71.  If you don't have
something around 3.3pf to >10pf, you can make a small cap by
>taking two pieces of small gauge insulated wire about 2 or 3 cm long.
Solder one wire to
>ground and the other to the non-grounded
>side of C71. Twist the wires together to make a small capacitor.
>The wires are still insulated from each other, but they are coupled by the
capacitance between >them.  This technique has been used a
>lot in radios to make a small variable capacitor to adjust a critical
circuit.
>The oscillator, Q18,  should be running, even if it's not at the correct
frequency.
>*** Even when the circuit is not working properly, can you get a frequency
indication with the >freq probe at TP1?  It should be
>fairly stable for a few minute or so, even when the PLL is not controlling
it.

I took 12pF the frequency was 8.9MH and stabil!

>>>Check the bias current on the oscillator Q18 by measuring the voltage
>>> across R15 (33 ohms).
>>>Put the red probe on one end of R15 and
>>>the black probe on the other end (it doesn't matter if the reading is
>>> negative or positive).
>>>This should be around 0.05 to 0.3 Volts. Indicating a bias current of 1
to
>>>10 mils.  That's a >broad range but we're just looking for
>>>a very rough value now which indicates that Q18 is operating with some
>>> reasonable level of
>>>bias current.

>> it's only 0.017 volts. It seems to bee too low. But when I tip (not
>> pressing!) with a screw-driver (with a insulator is no! effect) on
terminal
>> 2 of t5 it goes up to 0.044 volts. When I touch t5 with a wet finger, the
>> voltage goes up to 0.067 volts!!!


>This sounds like you need a little more capacitance in this tuned circuit.
The permeability of >the core that T5 is wound on may be
>on the low side, so you don't have enough inductance. This can be made up
by increasing the
>capacitance.

>0.017 V is probably too low.  That's only about 0.5ma of bias, which is
pretty low for a high >frequency circuit.
>0.067 V sounds about right.

>>>The Source voltage of Q18 should be around 1 to 3 volts.
>
>> it's 1.1 volts
>
>With the source voltage of Q18 so low, it should be drawing more than
0.5ma.  I wonder if
>there is something wrong with Q18??  Or
>possibly Q17??

I changed q17 -> no effect

>*** Measure the DC voltage at the base of Q16 and also at the collector of
Q16 without
>touching any of the components with your
>finger.

b u=0.56 volts
c u=1.6 volts

>*** Double check the wires on T5. The turns must go in the direction that's
indicated in fig
>6-16.
>The phase relationship of the windings is critical for oscillator
operation.

it's ok

73 de David