[Elecraft] Need some help please

[W3FPR - Don Wilhelm]

Kevin,

>From the resistance readings on Q7 and Q8 you appear to have shorted final
transistors.  I'm not certain how it happened, they are quite hardy
transistors.

If you havn't already, repeat the resistance measurements with the heat sink
removed on the chance that you just have a short to the heat sink.  Then
remove the transistors to confirm that they are indeed shorted - the board
resistance readings should be restored to about normal if the finals are
indeed the problem.

The 'too high' RF voltage readings that you obtained at the transmit mixer,
buffer etc. are not a problem - that is the normal reaction of the K2 when
it does not sense output power, it just increases the level in an attempt to
boost the output higher.

You likely do not have a failing D9 - it just does not have any RF to
detect, but having an extra on hand is not a big deal.

The real challenge will be to determine why they failed - there may be
something in the KPA100 input that is not right, or you did not have the
base K2 heat sink in contact with the tabs of Q7 and Q8, or ??, just guesses
here - but excessive heat is normally the killer of solid state devices, the
question is 'how did it get so hot'.

73,
Don W3FPR

> -----Original Message-----
>
> Hello all.
>
> I have been working on K2/100 #4684.
> It worked beautifully in a QSO with my brother on 40m last weekend until
> the power out went to 0.
> Doing some troubleshooting with the procedures in the manual this is
> what I came up with.
>
> I started with the receiver and those measurements were within reason.
> I moved on to Transmitter Basic Voltage checks and recorded these
> voltages.
> These are all on the basic K2 with the SSB adapter removed and jumpers
> for J9 and 10 and C167 installed.
>
> D6 Anode on tx - *.25V*
> D7 Anode on tx - *7.9V*
> Actual Power Output - *0.3W*
>
> I moved on to the ALC voltage checks.
>
> Power Control Test - *4.97V    *Not even close.
> ------------------------------------------------------------
> Transmit Mixer, Buffer, Band-Pass Filter, T-R Switch
> *
> *Xmit Mixer Output - *.212 Vrms*
> Buffer Output - *1.9 Vrms*
> Band-Pass Filter Output - *0.056 Vrms
> *T-R Switch #1 Output - *0.057 Vrms*
>
> All way too high.
> ----------------------------------------------------------------------
> Pre-Driver, Driver, PA.
>
> Pre-Driver Output - *0.170 Vrms*
> Driver Input - *0.024 Vrms*
> Driver Output - *4.07 Vrms*
> PA Input - Q7 - *.94 Vrms
>                  *Q8 -  *1.08 Vrms
>
> *RF Detector Input - *0.0 Vrms *   Not good at all! I checked D9 and it
> is a 1N5711 and it is installed correctly.
> ------------------------------------------------------------------
> ------------------
> PA Transistor Tests. Both flunk with very low resistances.
>
> DMM positive lead on Bases I get about *.65 Ohms* to the collector and
> emitter.
> DMM negative lead on Bases I get about *1.7 Ohms *to collector
> and emitter.
>
> I checked the hardware and thermal pads for both Q7 and Q8 and they're
> all in great shape. No mis-alignments of shoulder washers or thermal
> pads. No torn  thermal pads etc.....
>
> Which component or components are bad.? I suspect I'm going to be buying
> new finals possibly a driver and 1N5711 diode. The voltages are all
> haywire after the pre-driver Q5.
>
> Thanks for your help.
>
> --
> R. Kevin Stover	ACØH
>
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