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

[David Broger]

-----Urspr�ngliche Nachricht-----
Von: David Broger [mailto:dabro@besonet.ch]
Gesendet: Dienstag, 19. August 2003 22:50
An: Masleid, Michael A.
Betreff: AW: [Elecraft] K2 - VCO-Test (Alignment an d Test, Part II)


Hi Michael

>Quiet sound on the DMM?  Most don't make noise.  Mine does, because of the
>auto range relays.  Is that what you are hearing?

Normally it does not. I heard it the first time.

>>Next intresting point: I measured the voltage between ground and connector
4
>>of t5 - its 0.07 volts (touching t5 has no effect) but the frequency goes
up
>>to 8.8 MHz!! When I touched the connector 4 with a insulator or
metal-piece
>>the frequency goes not to 8.8 MHz.

>Nice.  The DVM is a high impedance capacitance to an antenna - same as your
finger
>really.  But terminal 4?  This is nasty.  This should be at 2 to 3 volts.
Does this
>voltage stay at 0.07 volts even when the frequency is up to 8.8 MHz?

I measured once again: I got 0.05 volts. The frequency is at 8.8MHz and
touching t5 has no effect (voltage and frequency).
Connector 3 has 0.8 volts resp 1.1 volts (with touching). There is no effect
at the frequency
Connector 1 has 0 volts, the frequency is low (1.2MHz). Touching increasese
the frequency to 8.8 MHz and the voltage ist 0 volts.
Connector 2 has 0 volts and the frequency is 0Hz (touching has no effect)

>The voltage is
>supposed to be 2 or 3 volts to hold Q18 near cut off so that the VCO
doesn't oscillate
>at too much amplitude.  Make sure that C60, 100 uF is not backwards
(shorted).

c60 is ok

>(I might try to grounding Q17 Gate through a 1K resistor in order to cut
off Q17,
>then voltage on C60 should rise and shut down Q18, but you might not want
to do that.)

>Now, a little capacitance on terminal 2 might add a little more feedback
into Q18 and
>wake it up.  A little more capacitance on terminal 4 should not do much.

>What is the voltage on Q18's Drain?

8.1 volts (touching has no effect)

>What we know so for, I think:  T5 is good.  K13-K15 and capacitors are
good.  Feedback
>path and ALC path around Q18 is bad.  Q18 is being pushed too hard.  Might
be parts
>around Q18, might be Q18 is bad?  If the voltage at Q18 Drain is low, then
Q18 is
>probably OK, but something else in the Q18, Q17, Q16, LT1252 loop is bad.

>Just had an odd thought:  It could be that your finger is loading down the
oscillator
>enough to stabalize it.

>Do check C60!

C60 is 100uF and the polarisation is correct.

>If ALC doesn't control Q18, then D13 could conduct and quench oscillation
>on Q18.  Rather like a super-regenerative receiver.  Very noisy RF.  The
bursts of 8 MHz
>RF would fool the counter into thinking it saw some fractional part of
that.  It would
>help to check this with a 'scope but any receiver antenna near T5 should
pick it up.
>Look for a very ragged signal around 8-10 MHz.

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


73 de David, HB9RVS