[Elecraft] [XV432} LO Instability Problem

[Ian Greenshields]

Thanks for all the help and comments received so far, on and off reflector
- I have to say the responsiveness and quality of help on this reflector is
second to none!

To summarise:

1. I have tried detuning L4, along with L3 and L5. L4 is very sensitive as
is probably to be expected, L3 and L5 somewhat less so. The most stable
point of L4 does seem to coincide with the peak in voltage at TP1, and
detuning L3 and L5 doesn't seem to make much difference. The problem is
that the voltage at TP1 can easily reach 4v or higher, and the LO
is definitely less stable if I detune it to keep the voltage below 3v.

2. Is a voltage well above 3v at TP1 a problem? It seems to be just a
detector diode circuit on the output of the LO, so I presume a fair amount
of variation is to be expected.

3. The crystal seems mechanically stable in that tapping it doesn't send
the frequency all over the place, but it is highly sensitive to being
touched, which I assume is heating effect,

4. The biggest single improvement was a suggestion to ground the can of the
crystal. This is something very familiar to any K2 builder. I grounded the
crystal can, K2 filter style, to the oven -ve pad, which was the closest
ground point. Huge improvement. The frequency jumping has gone, although
there is still significant drift, even long after warm-up.

5. I then tried readjusting L3, L4 & L5 to see if detuning the voltage to
about 2.5v at TP1 helped now that the oscillator seemed more stable. This
seemed to be a backward step as the LO lurched down about 10kHz in
frequency, started to drift back up, but then actually stopped oscillating.
Restoring L3, L4 and L5 to the point where the voltage at TP1 peaks - about
4v - seems to be the most stable point & the LO starts up fine. From switch
on it drifts about 3kHz low, then as it warms up, it starts to drift HF
until about 3 or 4kHz above the switch-on point.

6. If my unit is normal, the oven would seem an essential. As I want to use
the transverter on SSB, I can't really live with a drift range of 7kHz!

7. What is the view on grounding the crystal can? It does seem to help
tremendously with the stability (and I mean the 'crystal mode' stability,
just to differentiate from drift).

I think my next step is to order the oven (I'm going to have fun fitting
that now I've grounded the can!) & possibly another crystal.

73 Ian G4FSU


On 25 January 2013 14:01, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> Ian,
>
> Two possibilities occur to me.  First, detune the LO inductors L3, L4 and
> L5 a bit to give you 3 volts maximum at TP1 and see if it stabilizes.
> Secondly, try adjusting L4 a bit.  It may be that the adjustment of L4
> puts the crystal right on the 'hairy' edge of oscillation which would make
> the frequency unstable.  The voltage at TP1 should remain between 1.7 and
> 3.0 volts.
>
> 73,
> Don W3FPR
>
>