KX1 Please need help with T/R thump

[Jerry W7ANM]

My KX1 has quite a noticeable and irritating "thump" or "plop" going back from transmit to receive.

This "thump" has a  soft nature but seems rather loud and obvious. I have tried different headphones and it doesn't seem to help much. If I set the T-R delay to very slow, the side-tone is fine with no artifacts but when the delay times out and the rig switches back to receive there is the thump. Faster delay just makes it thump the same at each changeover to receive. The changeover from receive to transmit on the other hand is smooth and quiet.

I built this KX1 a while ago and the thump has always been there. I always intended to ask the list about this but never got around to it and I thought I could live with it. After using my other QSK rigs, K2 included, and I go back to the KX1, I am reminded that the KX1 must have a problem and that this is abnormal.

Has anyone seen a problem like this and found a solution? Any ideas on where or what to check?

Any help is greatly appreciated.

[Don Wilhelm-4]

  Jerry,

Check the STL menu parameter.  If that is set to a high value (to
produce a high level sidetone note) you may be hearing a "thump" just do
to the high sidetone level setting.  O have heard that on several KX1s
sent to me for repair.

OTOH, if that is not the problem, you might look at the mute transistors
Q2 and Q3 to be certain they are connected properly (flat side facing up
away from the board), and that R29 is the correct value (22K).

73,
Don W3FPR

On 5/23/2011 12:49 PM, Jerry W7ANM wrote:

> My KX1 has quite a noticeable and irritating "thump" or "plop" going back
> from transmit to receive.
>
> This "thump" has a  soft nature but seems rather loud and obvious. I have
> tried different headphones and it doesn't seem to help much. If I set the
> T-R delay to very slow, the side-tone is fine with no artifacts but when the
> delay times out and the rig switches back to receive there is the thump.
> Faster delay just makes it thump the same at each changeover to receive. The
> changeover from receive to transmit on the other hand is smooth and quiet.
>
> I built this KX1 a while ago and the thump has always been there. I always
> intended to ask the list about this but never got around to it and I thought
> I could live with it. After using my other QSK rigs, K2 included, and I go
> back to the KX1, I am reminded that the KX1 must have a problem and that
> this is abnormal.
>
> Has anyone seen a problem like this and found a solution? Any ideas on where
> or what to check?
>

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[Jerry W7ANM]

Thanks for your thoughts Don.

I checked the STL and It's been at 2. The thump does not really change much on any setting of the STL. I also checked the components you suggested and Q2 and Q3 are installed correctly and in fact are J309s. R29 is also 22k and in the right place. Other associated components are correctly installed.

Rick KL7CW suggested an experiment for me to try. That was to connect a .02 to .1 cap across C9 to see if it made a difference. At first I tried a .1 and the thump was completely eliminated and changeover dead quiet but it made the T-R delay about a second long. (with Menu T-R set to 0). I next tried a .047 and that seemed to do the trick. I could still hear a very slight quiet "poof" at changeover and the T-R delay is slightly longer than before with the thump but still very acceptable. About the equivalent of setting the Menu T-R delay to 30 or 40. Definitely QSK not as fast as my K2 but I'm not sure the KX1 was ever supposed to be fast as the K2.

Still something not quite right? In theory the parallel cap shouldn't be needed but at this point maybe it's a case of component tolerance/anomalies? Any other ideas?

Thanks for all your help and suggestions.

73,
Jerry  

[Don Wilhelm-4]

  Jerry,

Yes, I would look at Q7 and its associated resistor R28.  That
transistor should saturate and prevent Transmit RF from coming into the
receiver.  It sounds like it is not working right and the receiver
saturates during transmit and takes too long to recover.

What you have done by increasing the value of C9 would cover up that
receiver recovery period.
The easiest way to check Q7 is to replace it, but make certain that Q7
base goes to at least 0.6 volts during transmit - if it does not, then
look to R28 and the 6T-2 voltage supply to it, which should be about 6
volts during transmit and zero during receive.

73,
Don W3FPR

On 5/24/2011 9:55 PM, Jerry W7ANM wrote:

> Thanks for your thoughts Don.
>
> I checked the STL and It's been at 2. The thump does not really change much
> on any setting of the STL. I also checked the components you suggested and
> Q2 and Q3 are installed correctly and in fact are J309s. R29 is also 22k and
> in the right place. Other associated components are correctly installed.
>
> Rick KL7CW suggested an experiment for me to try. That was to connect a .02
> to .1 cap across C9 to see if it made a difference. At first I tried a .1
> and the thump was completely eliminated and changeover dead quiet but it
> made the T-R delay about a second long. (with Menu T-R set to 0). I next
> tried a .047 and that seemed to do the trick. I could still hear a very
> slight quiet "poof" at changeover and the T-R delay is slightly longer than
> before with the thump but still very acceptable. About the equivalent of
> setting the Menu T-R delay to 30 or 40. Definitely QSK not as fast as my K2
> but I'm not sure the KX1 was ever supposed to be fast as the K2.
>
> Still something not quite right? In theory the parallel cap shouldn't be
> needed but at this point maybe it's a case of component tolerance/anomalies?
> Any other ideas?
>
> Thanks for all your help and suggestions.
>
> 73,
> Jerry
>
>
>
> --
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[Jerry W7ANM]

Thanks Don,

Maybe were getting somewhere. Q7s base is at zero during receive but on transmit it only goes to 0.1v.
The 6T_2 voltage on the other side of R28 goes from 0v during receive then to 5.63v on transmit. (Normal?)

Looks like I will need to get another Q7?


Jerry

 

[Don Wilhelm-4]

  Jerry,

That voltage would indicate Q7 may be at fault.
If you do not have a proper replacement, but do have a 2N2222A or 2N3904
in stock try one of those as a test.

73,
Don W3FPR

On 5/24/2011 11:48 PM, Jerry W7ANM wrote:

> Thanks Don,
>
> Maybe were getting somewhere. Q7s base is at zero during receive but on
> transmit it only goes to 0.1v.
> The 6T_2 voltage on the other side of R28 goes from 0v during receive then
> to 5.63v on transmit. (Normal?)
>
> Looks like I will need to get another Q7?
>
>

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[Jerry W7ANM]

Don,

I did extract the old Q7 without any damage or problems.

I was looking at the spec sheet for the 2N3904 after getting your last suggestion and after ordering the 2N4124 from Elecraft. Do you think the 2n3904 is a superior replacement for the 2n4124? Any reason to suspect it would not work or why the 2n4124 was chosen for that circuit?

73,
Jerry

On Wed, May 25, 2011 at 5:13 AM, Don Wilhelm-4 [via Elecraft] <[hidden email]> wrote:

  Jerry,

That voltage would indicate Q7 may be at fault.
If you do not have a proper replacement, but do have a 2N2222A or 2N3904
in stock try one of those as a test.

73,
Don W3FPR

On 5/24/2011 11:48 PM, Jerry W7ANM wrote:

> Thanks Don,
>
> Maybe were getting somewhere. Q7s base is at zero during receive but on
> transmit it only goes to 0.1v.
> The 6T_2 voltage on the other side of R28 goes from 0v during receive then
> to 5.63v on transmit. (Normal?)
>
> Looks like I will need to get another Q7?
>
>

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[Don Wilhelm-4]

  Jerry,

All that transistor has to do is remain in an open state between the
collector and emitter during receive and provide a path to ground
between collector and emitter during transmit.  Any transistor that has
a low saturation voltage will serve in that capacity - it is just a
switching transistor.  I do not know why the 2N4124 was chosen other
than that there are other 2N4124s in the KX1 design, and it is easier if
most of the transistors are the same.

Try the 2N3904 and see what happens.  The worst thing that can happen is
if you have to clip it out, you will lose the transistor, but they are
quite inexpensive.

73,
Don W3FPR

On 5/26/2011 1:34 PM, Jerry W7ANM wrote:

> Don,
>
> I did extract the old Q7 without any damage or problems.
>
> I was looking at the spec sheet for the 2N3904 after getting your last
> suggestion and after ordering the 2N4124 from Elecraft. Do you think the
> 2n3904 is a superior replacement for the 2n4124? Any reason to suspect it
> would not work or why the 2n4124 was chosen for that circuit?
>
> 73,
> Jerry
>

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[Jerry W7ANM]

Don,

Installed the 2n3904 for Q7. Transmit voltage now normal 0.65V going to zero when switched to receive. 6T_2 voltage at R28 still 5.63v for TX and 0v at RX.  Unfortunately thump still there unchanged.....

Jerry


"Jerry,

Those voltages do look better.  Did the thump change any at all?
I would suggest you look for an incorrect value component somewhere in muting transistor area.
Is there any chance that you mounted the muting transistors upside-down?  I have seen that, and yes it causes a thump.  The flat side of the transistors should face away from the board.

73,
Don W3FPR"  


Don,

Thump is the same. It does not seemed to have changed any. If anything the T-R turn around is snappier is all (.047 cap across c9 has been removed). Thump has same intensity and duration.

I have checked the muting transistors and they are not installed upside down. The other components associated with the muting transistors are the correct ones. In fact I swapped out c9 just in case it might have been open.... No improvement.....

73,
Jerry


"Jerry,

OK, but I have run out of ideas.  All I can say is that it is abnormal - a properly operating KX1 does not do that.

Are you operating into a dummy load or an antenna?  If that is into an antenna, try a dummy load.  You could have some form of RF feedback from an antenna, and the results of that condition are unpredictable - but the place to fix that situation is in the antenna system.

73,
Don W3FPR"


Don,

It is the same with an antenna or dummy load.

What would be the effect of a slow or strange MCU Mute signal? Anything to look at with a scope?

Is a normal KX1 thumpy to any degree?


Jerry


"Jerry,

I would not say the KX1 is "thumpy", but the sidetone note is not the cleanest shape in the world.
Can you determine whether it is the return to receive that creates the thump or is it something on the tail end of the sidetone?  You might try turning the STL parameter down as low as it can go for a test.  Several that have come into me for repair have the sidetone cranked up to max and they really sound terrible.

73,
Don W3FPR"


Hi Don,

It is definitely the return to receive that creates the thump. The way I determined that is to set the Menu T-R to something really long like 400 or 500 or so. Also I can set the STL to 0 or 1 (or even 2 or 3). Then with these parameters you can key the KX1 and you would hear the side tone very much separated from the change-over from T to R and the thump.

The sidetone sounds good. Not distorted or strange but, as above, after you have stopped keying the sidetone, when the Menu T-R times out , you hear the thump. I always run the sidetone at 1 or 2 normally. You can hear this with an antenna or a dummy load connected, using batteries or power supply, any STL, and using any headphones.

Please try this on one of your KX1s and let me know what you hear in your normal one.

Oh, I did do some more voltage checks this am. Q2  and Q3 voltages are in spec. U1s mute line is the proper voltage also.

73,
Jerry