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[K1] VFO Drift Problem

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Matt Scholz

[K1] VFO Drift Problem

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

K1 #2978

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Jessie Oberreuter-2

Re: [K1] VFO Drift Problem

IIRC there are two cascaded voltage regulators before the VFO which
may also be good candidates for improvement. Mine's been more drifty
since I blew the first one; I probably should have replaced the second one
at the same time.

On Wed, 4 May 2011, 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
>
> K1 #2978
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html
>

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Don Wilhelm-4

Re: [K1] VFO Drift Problem

In reply to this post by Matt Scholz

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:

Matt Scholz

Re: [K1] VFO Drift Problem

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" <[hidden email]> 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
>>

Michael Babineau-2

Re: [K1] VFO Drift Problem

In reply to this post by Matt Scholz

Very interesting problem Matt. The VFO in my K1 is stable once it has warmed up for about 5 minutes,
as long as the rig isn't out in the sun. I have observed some significant drifting due to physical warming
while operating portable so I now know to keep it shaded.

Sometimes I wish for a K1.5 with a DDS VFO as my KX1 is rock solid from -25C to +30C (and yes I have operated portable in those condtions ;-) ) .

Michael VE3WMB

P.S. Once you have figured this out please let us know what the solution was.
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David Gilbert

Re: [K1] VFO Drift Problem

In reply to this post by Don Wilhelm-4

I'd also suspect the voltage regulator first, but another possibility is
the varactor itself. Many years ago in another universe I was the
product engineer for tuning diodes at a large U.S. semiconductor
manufacturer. Varactors are simply reversed bias diodes with a junction
profile optimized for a particular capacitance-versus-voltage curve.
Accomplishing that inherently requires wafer processing that makes them
less than ideal for other parameters, such as leakage. Varactors are
normally pretty stable, but as with anything else exceptions can occur.

I have a Kenwood TS-130V with an unstable varactor-tuned RIT as proof
(culprit verified).

73,
Dave AB7E

On 5/5/2011 7:19 AM, Don Wilhelm 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
>>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

Don Wilhelm-4

Re: [K1] VFO Drift Problem

In reply to this post by Matt Scholz

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"<[hidden email]> 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
>>>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

Matt Scholz

Re: [K1] VFO Drift Problem

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 <[hidden email]> 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"<[hidden email]> 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
>>>>
>>>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>>