[Elecraft] [K1] VFO Drift Problem

[Matt Scholz]

Don,

Thank you for the very detailed response/analysis - that was helpful. I
definitely will heed your advice and replace one component at a time.

73,
-Matt W6ZBA

On Thu, May 5, 2011 at 11:37 AM, Don Wilhelm <w3fpr at embarqmail.com> wrote:

>  Matt,
>
> That takes only one of the varactors out of the circuit.  You could remove
> both C2 and C7 and that would remove both varactors from the circuit.  But
> that still does not eliminate the supply voltage because Q8 will still
> change in response to a change in the 6 volt regulator output.
>
> Other possibilities are that D17 is changing characteristics, or Q9 is
> changing a bit which can pull the oscillator.  Thermal drift in R7 could
> cause changes in the base circuit of Q7.  Q8 itself could be another
> probable cause.  In cases like this it is a matter of replacing one thing at
> a time and monitoring the results. Work methodically - wholesale replacement
> of parts may or may not correct it.
>
> If you play air from a hair dryer on the VFO area and note if the rate of
> the frequency change increases substantially, then you may be able to
> conclude it is temperature related (the core of L1 is then the most likely
> suspect).  Beware of drawing false conclusions if attempting to raise the
> temperature - that WILL change the frequency somewhat even in the best of
> times, so the real question becomes a guess as to whether the rate of change
> is greater than anticipated or not.  If you can find another K1 to compare
> your results while applying the same exposure to the normal K1, then you can
> draw better conclusions.
>
> 73,
> Don W3FPR
>
>
> On 5/5/2011 11:52 AM, Matt Scholz wrote:
>
>> Thanks Don.
>>
>> So here's another question. My first step was to remove C2, which
>> effectively removes the varactor (D3) from the equation. I was still
>> observing an excessive amount of drift. Doesn't this rule out the voltage
>> regulators as the cause of this problem?
>>
>> 73,
>> -Matt W6ZBA
>>
>> On 5/5/11 7:19 AM, "Don Wilhelm"<w3fpr at embarqmail.com>  wrote:
>>
>>   Matt,
>>>
>>> I would first suspect the stability of the voltage regulators.  That VFO
>>> is tuned with a varactor, if the voltage changes, the frequency will
>>> change.  If that does not settle it down, I would wind L1 on a new core
>>> on the suspicion that the present one is more temperature sensitive than
>>> usual.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>> On 5/5/2011 1:40 AM, Matt Scholz wrote:
>>>
>>>> Hi -
>>>>
>>>> I am trying to reduce my K1's VFO drift to an acceptable level. The VFO
>>>> frequency drifts up at a rate of about 30Hz/minute (after a 5 minute
>>>> warm
>>>> up). Even after one hour, the drift is still about 15Hz/minute. This is
>>>> too much, obviously.
>>>>
>>>> I have been trouble shooting this with the help of Gary, AB7MY. So far I
>>>> have done the following:
>>>>
>>>> * Removed C2 to isolate D3 from the rest of the VFO circuit. No change.
>>>> * Replaced all three VFO poly caps (one at a time). See above.
>>>>
>>>> I was wondering if anybody had any ideas on how to further trouble-shoot
>>>> and resolve this issue...
>>>>
>>>> Thanks in advance and 73,
>>>>
>>>> -Matt / W6ZBA
>>>>
>>>>
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