[Elecraft] [K1] VFO Drift Problem

[Don Wilhelm]

  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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