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K2 BFO drift problem

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TheMG .

K2 BFO drift problem

Well, a few weeks ago I finished assembling and testing my K2 S/N 7520 and
the KPA100, KSB2, K60XV, and K160RX options. All went without any problems
at and have had several successful QSO with it.

However, since then I've been noticing quite severe BFO drift. I have to
re-align the filters using CAL FIL almost daily. The low range of the BFO
keeps drifting upward, about 50-100Hz every day, and it hasn't stopped yet
even after two weeks!

Initially (first built and tested), the BFO low range (as measured with CAL
FCTR) was at 4912.6kHz. Today it's at 4913.35kHz.

What I've ruled out so far:

-Temperature. Room temperature is quite stable around 20-22C. Even after
long periods of transmitting with the KPA100 and the entire K2 chassis
getting quite a lot warmer than room temperature, BFO temp-related drift
was only about 30Hz and in the opposite direction of the daily BFO drift of
50-100Hz.

-At first I though maybe I hadn't properly secured L33 and that maybe
mechanical vibrations and stress from moving the K2 around was causing the
inductor to move around, but for the last week I had it sitting on a desk
without moving it at all and the BFO is still drifting.

Also, from my observations, the BFO range is actually constricting, rather
than simply shifting upward. The high range of the BFO has gone from
4917.47 to 4917.15.

Gently tapping or pressing on the RF board doesn't change anything.

Anyone experienced anything like this or having any idea what the problem
might be?
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Don Wilhelm-4

Re: K2 BFO drift problem

Some component (or components) in your BFO (or the generation of the
VBFO signal) is changing characteristics.
It could be the crystals (X3 and X4) or the capacitors (C173 and C174)
or the varactors (D37 and D38), or L33.
It also could be soldering on the R-Pak RP6 or any of the above components.
Since my first guess is that there is a solder connection that is slowly
gaining oxidation, I would re-solder all the components in question
before doing anything else - use a hot (750 degF) iron and add just a
tad of flux from a bit of new solder.

There is also the possibility that the VBFO signal coming from the
Control Board is varying. This is a DC level that tells the varactors
how much to change capacity.
To check that, leave one filter set the same at all times. With that
filter selected, measure and record the VBFO value to as many
significant digits as your DMM will show. Control Board U10 pin 14 is
an easy place to measure it. Then when the BFO shifts frequency,
measure it again (do not change that filter setting). If the voltage
changes, then the problem may be with U10 on the control board.
Re-solder U10 and RP2 and RP3 to see if that stops the change.
On 4/13/2014 1:24 PM, TheMG . wrote:

> Well, a few weeks ago I finished assembling and testing my K2 S/N 7520 and
> the KPA100, KSB2, K60XV, and K160RX options. All went without any problems
> at and have had several successful QSO with it.
>
> However, since then I've been noticing quite severe BFO drift. I have to
> re-align the filters using CAL FIL almost daily. The low range of the BFO
> keeps drifting upward, about 50-100Hz every day, and it hasn't stopped yet
> even after two weeks!
>
> Initially (first built and tested), the BFO low range (as measured with CAL
> FCTR) was at 4912.6kHz. Today it's at 4913.35kHz.
>
> What I've ruled out so far:
>
> -Temperature. Room temperature is quite stable around 20-22C. Even after
> long periods of transmitting with the KPA100 and the entire K2 chassis
> getting quite a lot warmer than room temperature, BFO temp-related drift
> was only about 30Hz and in the opposite direction of the daily BFO drift of
> 50-100Hz.
>
> -At first I though maybe I hadn't properly secured L33 and that maybe
> mechanical vibrations and stress from moving the K2 around was causing the
> inductor to move around, but for the last week I had it sitting on a desk
> without moving it at all and the BFO is still drifting.
>
> Also, from my observations, the BFO range is actually constricting, rather
> than simply shifting upward. The high range of the BFO has gone from
> 4917.47 to 4917.15.
>
> Gently tapping or pressing on the RF board doesn't change anything.
>
> Anyone experienced anything like this or having any idea what the problem
> might be?
>

TheMG .

Re: K2 BFO drift problem

Well, I messed around with it a bit more and I gave L33 and very light
nudge with the plastic tuning tool while looking at CAL FCTR. This
caused the BFO frequency to suddenly jump down from 4913.30 to
4912.42.

Strange thing is, only a few minutes after nudging L33, the BFO
frequency had already drifted 100Hz up and still drifting. I give it
another nudge and it drops down again but then eventually resumes
drifting upward.

I also measured the VBFO voltage, as you suggested, before and after,
and there is no change even as it resumes drifting.

I had already removed and re-soldered L33 and the 1/8W resistor
holding in place about a week ago. I have also put a couple pieces of
heatshrink tubing on the leads of the resistor to ensure they aren't
shorting on L33's turns. This did not make any difference. I also
confirmed the solder connection is good by measuring the resistance
across the pads of L33: 0.3 ohms.

Starting to look like L33 might be defective? Still puzzling why
giving it a small nudge would cause it to behave temporarily again.
I'm not even touching the windings just gently pressing down a bit on
the part of the core that isn't covered in turns.

I also used the tuning tool to apply a bit of pressure to the other
components associated with the BFO, as well as their soldered pins,
and couldn't cause any change by doing that, nor did reflowing any of
the solder joints do anything.

I wish I had an LCR meter, I'd remove L33 and hook it up to the meter
overnight looking for any drift in its inductance. Unfortunately that
is an instrument I do not have.

Thanks for the help.

On Sun, Apr 13, 2014 at 11:43 AM, Don Wilhelm <[hidden email]> wrote:

> Some component (or components) in your BFO (or the generation of the VBFO
> signal) is changing characteristics.
> It could be the crystals (X3 and X4) or the capacitors (C173 and C174) or
> the varactors (D37 and D38), or L33.
> It also could be soldering on the R-Pak RP6 or any of the above components.
> Since my first guess is that there is a solder connection that is slowly
> gaining oxidation, I would re-solder all the components in question before
> doing anything else - use a hot (750 degF) iron and add just a tad of flux
> from a bit of new solder.
>
> There is also the possibility that the VBFO signal coming from the Control
> Board is varying. This is a DC level that tells the varactors how much to
> change capacity.
> To check that, leave one filter set the same at all times. With that filter
> selected, measure and record the VBFO value to as many significant digits as
> your DMM will show. Control Board U10 pin 14 is an easy place to measure
> it. Then when the BFO shifts frequency, measure it again (do not change
> that filter setting). If the voltage changes, then the problem may be with
> U10 on the control board. Re-solder U10 and RP2 and RP3 to see if that
> stops the change.
>
> On 4/13/2014 1:24 PM, TheMG . wrote:
>>
>> Well, a few weeks ago I finished assembling and testing my K2 S/N 7520 and
>> the KPA100, KSB2, K60XV, and K160RX options. All went without any problems
>> at and have had several successful QSO with it.
>>
>> However, since then I've been noticing quite severe BFO drift. I have to
>> re-align the filters using CAL FIL almost daily. The low range of the BFO
>> keeps drifting upward, about 50-100Hz every day, and it hasn't stopped yet
>> even after two weeks!
>>
>> Initially (first built and tested), the BFO low range (as measured with
>> CAL
>> FCTR) was at 4912.6kHz. Today it's at 4913.35kHz.
>>
>> What I've ruled out so far:
>>
>> -Temperature. Room temperature is quite stable around 20-22C. Even after
>> long periods of transmitting with the KPA100 and the entire K2 chassis
>> getting quite a lot warmer than room temperature, BFO temp-related drift
>> was only about 30Hz and in the opposite direction of the daily BFO drift
>> of
>> 50-100Hz.
>>
>> -At first I though maybe I hadn't properly secured L33 and that maybe
>> mechanical vibrations and stress from moving the K2 around was causing the
>> inductor to move around, but for the last week I had it sitting on a desk
>> without moving it at all and the BFO is still drifting.
>>
>> Also, from my observations, the BFO range is actually constricting, rather
>> than simply shifting upward. The high range of the BFO has gone from
>> 4917.47 to 4917.15.
>>
>> Gently tapping or pressing on the RF board doesn't change anything.
>>
>> Anyone experienced anything like this or having any idea what the problem
>> might be?
>>
>

Vic Rosenthal

Re: K2 BFO drift problem

Could the core be cracked?

On 4/13/2014 11:45 AM, TheMG . wrote:
> Starting to look like L33 might be defective? Still puzzling why
> giving it a small nudge would cause it to behave temporarily again.
> I'm not even touching the windings just gently pressing down a bit on
> the part of the core that isn't covered in turns.

--
73,

Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/

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Don Wilhelm-4

Re: K2 BFO drift problem

In reply to this post by TheMG .

That indicates that you should concentrate on L33 and things around it.
"Nudging" things and finding significant changes indicates a bad
connection somewhere and/or possible shorted turns.

Did you flush cut the leads on the RF board under it?
Is it mounted on the rubber bumper?
Did you cut half of the tip on the bumper off?
The resistor is effectively an insulator, and also provides mounting
stability.
After soldering the leads to the board (and to the L33 wires), you
should push the resistor body down into the center of the toroid to
secure L33 tightly.

Resolder the resistor leads and the leads of L33 again to see if that
helps, If not, request a replacement L33 and the resistor to mount it.
It does sound like there is something amiss with L33 - or the soldering
of the crystals under it. Before mounting a new L33, re-flow the
soldering below it.

73,
Don W3FPR

On 4/13/2014 2:45 PM, TheMG . wrote:

> Well, I messed around with it a bit more and I gave L33 and very light
> nudge with the plastic tuning tool while looking at CAL FCTR. This
> caused the BFO frequency to suddenly jump down from 4913.30 to
> 4912.42.
>
> Strange thing is, only a few minutes after nudging L33, the BFO
> frequency had already drifted 100Hz up and still drifting. I give it
> another nudge and it drops down again but then eventually resumes
> drifting upward.
>
>

TheMG .

Re: K2 BFO drift problem

Leads under L33 are flush cut as flat as possible and it is mounted on
the rubber bumper with half the stem cut off exactly as per the
instructions. The resistor was pushed down into the center to keep L33
secure and to prevent the resistor body from being pressed on by the
bottom cover.

No luck on resoldering L33 or the crystals underneath it. However I
did notice that when I pull up on the resistor so it isn't pressing
down on L33 (the inductor is at this point completely loose and free
to move around. I have the K2 upside down with the bottom cover
removed and it has been running this way for several hours with
absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
0.01 of that as it warms up.

It's like there is a couple of turns on the inductor that are ever so
slightly shorting together and only when it is being held/pressed down
onto the rubber bumper. Looks like it may be time to obtain a new
L33...

On Sun, Apr 13, 2014 at 1:46 PM, Don Wilhelm <[hidden email]> wrote:

> That indicates that you should concentrate on L33 and things around it.
> "Nudging" things and finding significant changes indicates a bad connection
> somewhere and/or possible shorted turns.
>
> Did you flush cut the leads on the RF board under it?
> Is it mounted on the rubber bumper?
> Did you cut half of the tip on the bumper off?
> The resistor is effectively an insulator, and also provides mounting
> stability.
> After soldering the leads to the board (and to the L33 wires), you should
> push the resistor body down into the center of the toroid to secure L33
> tightly.
>
> Resolder the resistor leads and the leads of L33 again to see if that helps,
> If not, request a replacement L33 and the resistor to mount it. It does
> sound like there is something amiss with L33 - or the soldering of the
> crystals under it. Before mounting a new L33, re-flow the soldering below
> it.
>
> 73,
> Don W3FPR
>
>
> On 4/13/2014 2:45 PM, TheMG . wrote:
>>
>> Well, I messed around with it a bit more and I gave L33 and very light
>> nudge with the plastic tuning tool while looking at CAL FCTR. This
>> caused the BFO frequency to suddenly jump down from 4913.30 to
>> 4912.42.
>>
>> Strange thing is, only a few minutes after nudging L33, the BFO
>> frequency had already drifted 100Hz up and still drifting. I give it
>> another nudge and it drops down again but then eventually resumes
>> drifting upward.
>>
>>
>

Don Wilhelm-4

Re: K2 BFO drift problem

Get a replacement L33 and see if that takes care of it. You might wan
to request a replacement mounting resistor as well.

73,
Don W3FPR

On 4/13/2014 9:23 PM, TheMG . wrote:

> Leads under L33 are flush cut as flat as possible and it is mounted on
> the rubber bumper with half the stem cut off exactly as per the
> instructions. The resistor was pushed down into the center to keep L33
> secure and to prevent the resistor body from being pressed on by the
> bottom cover.
>
> No luck on resoldering L33 or the crystals underneath it. However I
> did notice that when I pull up on the resistor so it isn't pressing
> down on L33 (the inductor is at this point completely loose and free
> to move around. I have the K2 upside down with the bottom cover
> removed and it has been running this way for several hours with
> absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
> 0.01 of that as it warms up.
>
>

TheMG .

Re: K2 BFO drift problem

Wow, you'll never believe what I just found out!

There is absolutely nothing wrong at all with L33. It turns out it's
the rubber bumper that is at fault!

I found this out by pressing down on the stem of the bumper with the
plastic tuning tool, instead of pressing on the inductor. Guess what?
Frequency started to drift!!!

On a hunch, I decided to get my multimeter and measure on the bottom
of the rubber bumper. It's slightly conductive! Starts at about 3
megohms and if I hold the multimeter probes in place the resistance
slowly starts to drop down to 1 megohm and below. After a couple
minutes holding the probes in place about 1/4 inch apart on the bottom
of the bumper, the resistance got down to 500k!

Since the solder pads for X3 BFO crystal are underneath the rubber
bumper, it is effectively like putting a resistor in parallel with the
crystal, which obviously has undesirable effects on the operation of
the BFO oscillator.

The slowly decreasing resistance as the meter probes are held in
contact with the rubber is consistent with the fact that the BFO
drifts over time.

I suspected contaminants on the surface of the rubber, but after
cleaning it with iso alcohol, there is no change. Something in the
rubber is causing it to be conductive.

I wonder if somewhere out there, there are hams with K2s experiencing
BFO drift related to the same cause? If I got a conductive rubber
bumper in my kit, it's highly likely someone else has also.

Anyways, it's getting a little late right now. I'll communicate my
findings to Elecraft tomorrow (in case they aren't already reading
this).

73,
Mikael
VE8MT

On Sun, Apr 13, 2014 at 9:50 PM, Don Wilhelm <[hidden email]> wrote:

> Get a replacement L33 and see if that takes care of it. You might wan to
> request a replacement mounting resistor as well.
>
> 73,
> Don W3FPR
>
>
> On 4/13/2014 9:23 PM, TheMG . wrote:
>>
>> Leads under L33 are flush cut as flat as possible and it is mounted on
>> the rubber bumper with half the stem cut off exactly as per the
>> instructions. The resistor was pushed down into the center to keep L33
>> secure and to prevent the resistor body from being pressed on by the
>> bottom cover.
>>
>> No luck on resoldering L33 or the crystals underneath it. However I
>> did notice that when I pull up on the resistor so it isn't pressing
>> down on L33 (the inductor is at this point completely loose and free
>> to move around. I have the K2 upside down with the bottom cover
>> removed and it has been running this way for several hours with
>> absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
>> 0.01 of that as it warms up.
>>
>>
>

Johnny Siu

K2 BFO drift problem

Very good to let the mail listing know. I would think Elecraft support will send you a new rubber bumper. 73 Johnny VR2XMC

________________________________
寄件人︰ TheMG . <[hidden email]>
收件人︰ [hidden email]
副本(CC)︰ [hidden email]
傳送日期︰ 2014年04月15日 (週二) 2:15 PM
主題︰ Re: [Elecraft] K2 BFO drift problem

Wow, you'll never believe what I just found out!

There is absolutely nothing wrong at all with L33. It turns out it's
the rubber bumper that is at fault!

I found this out by pressing down on the stem of the bumper with the
plastic tuning tool, instead of pressing on the inductor. Guess what?
Frequency started to drift!!!

On a hunch, I decided to get my multimeter and measure on the bottom
of the rubber bumper. It's slightly conductive! Starts at about 3
megohms and if I hold the multimeter probes in place the resistance
slowly starts to drop down to 1 megohm and below. After a couple
minutes holding the probes in place about 1/4 inch apart on the bottom
of the bumper, the resistance got down to 500k!

Since the solder pads for X3 BFO crystal are underneath the rubber
bumper, it is effectively like putting a resistor in parallel with the
crystal, which obviously has undesirable effects on the operation of
the BFO oscillator.

The slowly decreasing resistance as the meter probes are held in
contact with the rubber is consistent with the fact that the BFO
drifts over time.

I suspected contaminants on the surface of the rubber, but after
cleaning it with iso alcohol, there is no change. Something in the
rubber is causing it to be conductive.

I wonder if somewhere out there, there are hams with K2s experiencing
BFO drift related to the same cause? If I got a conductive rubber
bumper in my kit, it's highly likely someone else has also.

Anyways, it's getting a little late right now. I'll communicate my
findings to Elecraft tomorrow (in case they aren't already reading
this).

73,
Mikael
VE8MT

On Sun, Apr 13, 2014 at 9:50 PM, Don Wilhelm <[hidden email]> wrote:

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vk2rq

Re: K2 BFO drift problem

In reply to this post by TheMG .

That's very interesting. I'm in the middle of building a K2 myself, although I haven't yet reached the stage where I mount L33. Out of interest, I took out my rubber bumper, and tested it with my DMM set to the 20M range -- the meter would only read infinite, showing no trace of any conductivity at all. So, if your is somehow conductive, then replacing the bumper may well resolve the drift problem.

73, Matt VK2RQ.

On 15 Apr 2014, at 4:15 pm, TheMG . <[hidden email]> wrote:

> Wow, you'll never believe what I just found out!
>
> There is absolutely nothing wrong at all with L33. It turns out it's
> the rubber bumper that is at fault!
>
> I found this out by pressing down on the stem of the bumper with the
> plastic tuning tool, instead of pressing on the inductor. Guess what?
> Frequency started to drift!!!
>
> On a hunch, I decided to get my multimeter and measure on the bottom
> of the rubber bumper. It's slightly conductive! Starts at about 3
> megohms and if I hold the multimeter probes in place the resistance
> slowly starts to drop down to 1 megohm and below. After a couple
> minutes holding the probes in place about 1/4 inch apart on the bottom
> of the bumper, the resistance got down to 500k!
>
> Since the solder pads for X3 BFO crystal are underneath the rubber
> bumper, it is effectively like putting a resistor in parallel with the
> crystal, which obviously has undesirable effects on the operation of
> the BFO oscillator.
>
> The slowly decreasing resistance as the meter probes are held in
> contact with the rubber is consistent with the fact that the BFO
> drifts over time.
>
> I suspected contaminants on the surface of the rubber, but after
> cleaning it with iso alcohol, there is no change. Something in the
> rubber is causing it to be conductive.
>
> I wonder if somewhere out there, there are hams with K2s experiencing
> BFO drift related to the same cause? If I got a conductive rubber
> bumper in my kit, it's highly likely someone else has also.
>
> Anyways, it's getting a little late right now. I'll communicate my
> findings to Elecraft tomorrow (in case they aren't already reading
> this).
>
> 73,
> Mikael
> VE8MT
>
> On Sun, Apr 13, 2014 at 9:50 PM, Don Wilhelm <[hidden email]> wrote:
>> Get a replacement L33 and see if that takes care of it. You might wan to
>> request a replacement mounting resistor as well.
>>
>> 73,
>> Don W3FPR
>>
>>
>> On 4/13/2014 9:23 PM, TheMG . wrote:
>>>
>>> Leads under L33 are flush cut as flat as possible and it is mounted on
>>> the rubber bumper with half the stem cut off exactly as per the
>>> instructions. The resistor was pushed down into the center to keep L33
>>> secure and to prevent the resistor body from being pressed on by the
>>> bottom cover.
>>>
>>> No luck on resoldering L33 or the crystals underneath it. However I
>>> did notice that when I pull up on the resistor so it isn't pressing
>>> down on L33 (the inductor is at this point completely loose and free
>>> to move around. I have the K2 upside down with the bottom cover
>>> removed and it has been running this way for several hours with
>>> absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
>>> 0.01 of that as it warms up.
>>>
>>>
>>
> ______________________________________________________________
> 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
> Message delivered to [hidden email]

Don Wilhelm-4

Re: K2 BFO drift problem

In reply to this post by TheMG .

Mikael,

If you can, please do a test. See if you can loosen L33 from the rubber
bumper just a little by raising the bumper. then slide a strip cut from
paper stock (like a business card) between the board and the bumper.
See if the BFO still drifts.

If it stops, then your assumption is correct and you should get a new
rubber bumper.

Good troubleshooting!

73,
Don W3FPR

On 4/15/2014 2:15 AM, TheMG . wrote:

> Wow, you'll never believe what I just found out!
>
> There is absolutely nothing wrong at all with L33. It turns out it's
> the rubber bumper that is at fault!
>
> I found this out by pressing down on the stem of the bumper with the
> plastic tuning tool, instead of pressing on the inductor. Guess what?
> Frequency started to drift!!!
>
> On a hunch, I decided to get my multimeter and measure on the bottom
> of the rubber bumper. It's slightly conductive! Starts at about 3
> megohms and if I hold the multimeter probes in place the resistance
> slowly starts to drop down to 1 megohm and below. After a couple
> minutes holding the probes in place about 1/4 inch apart on the bottom
> of the bumper, the resistance got down to 500k!
>
> Since the solder pads for X3 BFO crystal are underneath the rubber
> bumper, it is effectively like putting a resistor in parallel with the
> crystal, which obviously has undesirable effects on the operation of
> the BFO oscillator.
>
> The slowly decreasing resistance as the meter probes are held in
> contact with the rubber is consistent with the fact that the BFO
> drifts over time.
>
> I suspected contaminants on the surface of the rubber, but after
> cleaning it with iso alcohol, there is no change. Something in the
> rubber is causing it to be conductive.
>
> I wonder if somewhere out there, there are hams with K2s experiencing
> BFO drift related to the same cause? If I got a conductive rubber
> bumper in my kit, it's highly likely someone else has also.
>
> Anyways, it's getting a little late right now. I'll communicate my
> findings to Elecraft tomorrow (in case they aren't already reading
> this).
>
> 73,
> Mikael
> VE8MT
>
> On Sun, Apr 13, 2014 at 9:50 PM, Don Wilhelm <[hidden email]> wrote:
>> Get a replacement L33 and see if that takes care of it. You might wan to
>> request a replacement mounting resistor as well.
>>
>> 73,
>> Don W3FPR
>>
>>
>> On 4/13/2014 9:23 PM, TheMG . wrote:
>>> Leads under L33 are flush cut as flat as possible and it is mounted on
>>> the rubber bumper with half the stem cut off exactly as per the
>>> instructions. The resistor was pushed down into the center to keep L33
>>> secure and to prevent the resistor body from being pressed on by the
>>> bottom cover.
>>>
>>> No luck on resoldering L33 or the crystals underneath it. However I
>>> did notice that when I pull up on the resistor so it isn't pressing
>>> down on L33 (the inductor is at this point completely loose and free
>>> to move around. I have the K2 upside down with the bottom cover
>>> removed and it has been running this way for several hours with
>>> absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
>>> 0.01 of that as it warms up.
>>>
>>>
> ______________________________________________________________
> 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
> Message delivered to [hidden email]
>

Phil Wheeler-2

Re: K2 BFO drift problem

In reply to this post by TheMG .

Good job of troubleshooting, Mikael. It's really
good you found this rather obscure problem. Who
would have guessed?

Perhaps there was a bad lot of rubber with
contaminants that you particular bumper was made from.

Mainly you now have the problem identified and can
correct it with a new part.

Phil w7ox

On 4/14/14, 11:15 PM, TheMG . wrote:

TheMG .

Re: K2 BFO drift problem

In reply to this post by Don Wilhelm-4

I've put a piece of paper under the bumper and re-installed L33 held
tightly down against the board (pushing the resistor down so it holds
it tightly).

No observed drift in frequency after a couple hours other than the
normal temperature-related drift (about 10Hz downward).

This does seem to confirm that the bumper conductivity was in fact the problem.

Looks like I will need a new rubber bumper. I'm not sure what happened
to this one to make it conductive, but it's definitely not something I
did as I did not use any cleaning chemicals when initially building
the K2 (not even alcohol).

I'm also considering just securing L33 in place with a dab of
electronics-grade silicone and leaving it at that. Although Elecraft
does not recommend it I don't see how it would be a problem if the
proper type of silicone is used that is non-corrosive, non-conductive,
designed specifically for electronics use. If it's good enough for use
in aircraft electronics... it ought to be fine for this. Of course, it
would make it slightly more difficult to remove L33 in the future if
for some reason it was ever necessary to remove it, but not
impossible. I'm not too worried about that.

73,
Mikael
VE8MT

On Tue, Apr 15, 2014 at 7:20 AM, Don Wilhelm <[hidden email]> wrote:

> Mikael,
>
> If you can, please do a test. See if you can loosen L33 from the rubber
> bumper just a little by raising the bumper. then slide a strip cut from
> paper stock (like a business card) between the board and the bumper.
> See if the BFO still drifts.
>
> If it stops, then your assumption is correct and you should get a new rubber
> bumper.
>
> Good troubleshooting!
>
> 73,
> Don W3FPR
>
>
> On 4/15/2014 2:15 AM, TheMG . wrote:
>>
>> Wow, you'll never believe what I just found out!
>>
>> There is absolutely nothing wrong at all with L33. It turns out it's
>> the rubber bumper that is at fault!
>>
>> I found this out by pressing down on the stem of the bumper with the
>> plastic tuning tool, instead of pressing on the inductor. Guess what?
>> Frequency started to drift!!!
>>
>> On a hunch, I decided to get my multimeter and measure on the bottom
>> of the rubber bumper. It's slightly conductive! Starts at about 3
>> megohms and if I hold the multimeter probes in place the resistance
>> slowly starts to drop down to 1 megohm and below. After a couple
>> minutes holding the probes in place about 1/4 inch apart on the bottom
>> of the bumper, the resistance got down to 500k!
>>
>> Since the solder pads for X3 BFO crystal are underneath the rubber
>> bumper, it is effectively like putting a resistor in parallel with the
>> crystal, which obviously has undesirable effects on the operation of
>> the BFO oscillator.
>>
>> The slowly decreasing resistance as the meter probes are held in
>> contact with the rubber is consistent with the fact that the BFO
>> drifts over time.
>>
>> I suspected contaminants on the surface of the rubber, but after
>> cleaning it with iso alcohol, there is no change. Something in the
>> rubber is causing it to be conductive.
>>
>> I wonder if somewhere out there, there are hams with K2s experiencing
>> BFO drift related to the same cause? If I got a conductive rubber
>> bumper in my kit, it's highly likely someone else has also.
>>
>> Anyways, it's getting a little late right now. I'll communicate my
>> findings to Elecraft tomorrow (in case they aren't already reading
>> this).
>>
>> 73,
>> Mikael
>> VE8MT
>>
>> On Sun, Apr 13, 2014 at 9:50 PM, Don Wilhelm <[hidden email]> wrote:
>>>
>>> Get a replacement L33 and see if that takes care of it. You might wan to
>>> request a replacement mounting resistor as well.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>> On 4/13/2014 9:23 PM, TheMG . wrote:
>>>>
>>>> Leads under L33 are flush cut as flat as possible and it is mounted on
>>>> the rubber bumper with half the stem cut off exactly as per the
>>>> instructions. The resistor was pushed down into the center to keep L33
>>>> secure and to prevent the resistor body from being pressed on by the
>>>> bottom cover.
>>>>
>>>> No luck on resoldering L33 or the crystals underneath it. However I
>>>> did notice that when I pull up on the resistor so it isn't pressing
>>>> down on L33 (the inductor is at this point completely loose and free
>>>> to move around. I have the K2 upside down with the bottom cover
>>>> removed and it has been running this way for several hours with
>>>> absolutely no drift. BFO range is 4912.31 to 4917.85 and stays within
>>>> 0.01 of that as it warms up.
>>>>
>>>>
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>>
>

Don Wilhelm-4

Re: K2 BFO drift problem

Just get a new rubber bumper. Not only does that glue make a mess of
things, but it has potential for changing the inductance.

73,
Don W3FPR
On 4/15/2014 5:35 PM, TheMG . wrote:

> I've put a piece of paper under the bumper and re-installed L33 held
> tightly down against the board (pushing the resistor down so it holds
> it tightly).
>
> No observed drift in frequency after a couple hours other than the
> normal temperature-related drift (about 10Hz downward).
>
> This does seem to confirm that the bumper conductivity was in fact the problem.
>
> Looks like I will need a new rubber bumper. I'm not sure what happened
> to this one to make it conductive, but it's definitely not something I
> did as I did not use any cleaning chemicals when initially building
> the K2 (not even alcohol).
>
> I'm also considering just securing L33 in place with a dab of
> electronics-grade silicone and leaving it at that. Although Elecraft
> does not recommend it I don't see how it would be a problem if the
> proper type of silicone is used that is non-corrosive, non-conductive,
> designed specifically for electronics use. If it's good enough for use
> in aircraft electronics... it ought to be fine for this. Of course, it
> would make it slightly more difficult to remove L33 in the future if
> for some reason it was ever necessary to remove it, but not
> impossible. I'm not too worried about that.
>
>