SWR Increases

[Patrick DalPorto-2]

I have a K2/KPA-100 and the tuner also.  I have recently tried 160 M. and noticed  that as I operate and the rig heats up from transmitting the SWR begins to increase after the fan has been on for a while.  I have not noticed this before.  Anyone else observe this?  
I made sure the tpa was set correctly.  I recently had the KPA -100 apart and installed new thermal pads.  I wonder if the finals are getting too hot?  SWR comes down as the rig cools again.

I have not retightened the screws on the finals but am going to do that after I send this note.

Any suggestions?

Pat W8FV/5H9PD
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[Don Wilhelm-3]

Pat,

Exactly where does the SWR increase?  If it is on the tuner indication, it
cannot be the KPA100 causing the increase, that would have to be in the
tuner.

Does the SWR indication increase on the K2 display?  If so, then remove the
tuner and operate the KPA100 into a dummy load to see if there is still an
increase.

There are too many variables at the moment for us to venture anything more
than wild guesses.
In any case, it is quite doubtful that the temperature of the PA transistors
is the root cause - a temperature rise in the enclosure may be causing a
morginal solder connection elsewhere to create changes in the SWR reading
reported.  You could check the soldering on the components associated with
the wattmeter and the reporting of SWR - most are located in the KPA100
corner near the antenna jack (T4 and its associated components) and the R26
and R27 trimmers, but the signal flows through U5 and U6 as well with Q10
and Q11 providing scaling when on high power - see the schematic for the
resistors and capacitors associated with these devices.

73,
Don W3FPR

> -----Original Message-----
>
> I have a K2/KPA-100 and the tuner also.  I have recently tried
> 160 M. and noticed  that as I operate and the rig heats up from
> transmitting the SWR begins to increase after the fan has been on
> for a while.  I have not noticed this before.  Anyone else observe this?
> I made sure the tpa was set correctly.  I recently had the KPA
> -100 apart and installed new thermal pads.  I wonder if the
> finals are getting too hot?  SWR comes down as the rig cools again.
>
> I have not retightened the screws on the finals but am going to
> do that after I send this note.
>
> Any suggestions?
>
> Pat W8FV/5H9PD
>

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