Intermittent RXsensitivity...

[Gerald Eberle]

Hi all
I notice an intermittent rx sensitivity on my K2, Serial-Nr.
488, after changing band eg. from 20 to 40 and vice versa. After
pushing tune or transmitting a couple of dots the sensitivity
returns
on the activ band and is gone on the other band. This happens with
and without the KAT2.

Where shall I start searching?

73 Gerald, HB9CEY
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[Don Wilhelm-3]

Gerald,

That certainly sounds like it might be hard to isolate.
I may be wrong, but from the behavior it sounds like you have a relay with
points that are not making good contact - when you put a bit of RF through
it, it becomes OK for a while.  Please be aware that this is just
speculation on my part.  You may have to work with it a lot to determine if
it is really fixed.  Intermittents are very difficult to troubleshoot.

Before you do anything drastic, check for unsoldered connections in the
bandpass and lowpass filter areas and the T/R switch too.  Sometimes
unsoldered connections will work just fine for a long time but afterward
they begin to develop oxidation which makes the connection first
intermittent and then perhaps fails completely.  The connections for these
areas are under the heat sink, so be prepared to remove it for your
soldering checks.

If you can induce the failure and then have it continue while you remove the
heat sink, you could also check the relay points with a continuity test
between the pins.  Check the schematic key for the realy chart and the
schematic itself for the appropriate relay points and pins.

You should be able to receive with the heat sink removed, but be certain you
do not attempt to transmit until it is replaced - WORK CAREFULLY if you
attempt power on with the heat sink removed.

73,
Don W3FPR

> -----Original Message-----
>
> Hi all
> I notice an intermittent rx sensitivity on my K2, Serial-Nr.
> 488, after changing band eg. from 20 to 40 and vice versa. After
> pushing tune or transmitting a couple of dots the sensitivity
> returns
> on the activ band and is gone on the other band. This happens with
> and without the KAT2.
>
> Where shall I start searching?
>
> 73 Gerald, HB9CEY
>

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[Geoffrey Mackenzie-Kennedy-2]

Don W3FPR wrote


> Gerald,
>
> That certainly sounds like it might be hard to isolate.
> I may be wrong, but from the behavior it sounds like you have a relay with
> points that are not making good contact - when you put a bit of RF through
> it, it becomes OK for a while.  Please be aware that this is just
> speculation on my part.  You may have to work with it a lot to determine
> if
> it is really fixed.  Intermittents are very difficult to troubleshoot.

 <snip>

> If you can induce the failure and then have it continue while you remove
> the
> heat sink, you could also check the relay points with a continuity test
> between the pins.  Check the schematic key for the realy chart and the
> schematic itself for the appropriate relay points and pins.

-----------------------------------------------------------------
Gerald,

I agree with Don about a possible problem with relay contacts, I had a relay
problem with my K2 some time ago which caused the receiver's sensitivity on
40m to decrease but which could be restored by either changing the band from
40m and back again, or transmitting a couple of dits. The cause of my
problem was K1's contacts which switch in the 40m bandpass filter,
specifically those of K1A.

With all due respect to Don, making a continuity test might fool you unless
the relay contacts have failed and are not just 'dirty', because the current
from a DMM etc when reading low ohms might be enough to 'clean' a dirty
contact just as a little bit of RF would, but certainly worth trying and
would expose a more severe problem.

If the problem is a 'dirty' contact sometimes the slightest vibration or
movement of the rig / relay can 'cure' the problem - for a very short time.

Since I am obliged to QRT for a several hours, please let me know if you
would like to hear more.

73,
Geoff
GM4ESD




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