K2 Low transmit power after shut down

[Mick Hall]

Hi

Working on my K2 with a long wire antenna and externalATU and my K2 shut down. Only working at 5 watts and an SWR of 2.1:1.


 
I didn’t smell anything initially so I powered it backup. At first I couldn’t hear anything on any band and slowly the sound cameback, which was a bit strange.


 
The lights on the ATU no longer indicated any output sostarted checking a bit more when I got a faint smell of fried component.


 
Before removing any covers I started checking basicthings. S meter stopped registering any received signals, although all bandscould be heard at normal levels.


 
The ATU can only handle 15 watts so I tested up to thisand found that when I dialled in up to 10 watts, It was indicating 0.5 watts. Icould hear the KPA100 switch in at 11 watts but anything up to 15 watts showeda max of 6 watts.


 
I bypassed the KPA100 by powering it from the base K2power socket and the I had the same results.


 
I immediately thought that the finals have blown but whythe S meter issue?


 
Here are some voltages as I worked through the transmittersignal tracing.


 
P13 Basic voltagechecks

step 4.

Key down volts at D6 2.6v and D7 7.9v

step 5.

Power output  0.5W


 
Page 14 Tx mixer,Buffer, Band Pass etc.

Step2

Xmit Mixer Output 2.12vrms

Step 3

Buffer Output 1.61vrms

Step 4

BP filter output 5.65vrms

Step5

T-R switch #1 output 5.6vrms


 
Pre-driver, Driverand PA

Step1

Pre-driver output 4vrms

Step2

Driver Input 1.26rms

Step3

Driver Output 8.2vrms

Step 4

PA input Q7 1.77vrms

Step5

PA input Q8 3.56vrms

Step 6

RF detector input .38vrms

Step 7

DMM set to Diode mode

Q7          Base1.125

                Collector1.12

                Emitter0

               

Q8          B 1.12

                C1.12

                E 0


 
Something else I noticed was that when I turn the RF gaindown fully then back up to max, the sound comes back slowly and not at the samerate you turn the dial, there is a lag.

These last results I think indicate the finals, but whereto start looking for the cause?

Any help would be much appreciated.


 
Mick


 

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[Don Wilhelm-4]

Mick,

You cannot properly troubleshoot a problem like that with too many
variables involved.  In your case, you have the base K2, the KPA100
(even when bypassed), your antenna tuner, and your antenna involved -
the problem could be with any one of them.
The solution is to reduce that number down to one at a time.

One thing your lack of receive tells me is that you are likely looking
for an intermittent connection somewhere.  The challenge is to find out
where.

First, physically remove the KPA100 from the base K2 and set it aside.
Then connect the base K2 to a known good 50 ohm non-reactive dummy load.

When that is done, see if you have good power output (the K2 display
will be close to correct with the dummy load).  Check at a range of powers.
If it OK, then we can move on to the KPA100 for the next step - tell me
that is the case and we can continue with that.

If the power output of the base K2 has problems (I presume it will have
low power at some power setting), then set the power knob to 5 watts and
proceed to make the measurements of RF voltage at the points shown in
Appendix A page 14, starting midway down the left column.
Make the measurements in order.  If the RF voltages are *higher* than
those listed, that is *not* a problem.
Tell us the first point where the RF voltages are substantially *lower*
than that listed.  That is the output of the failing stage. We don't
have to know the voltages at the prior stages, just the one that is
lower than expected.
Once we have that information we can help you localize it to the failing
component.

73,
Don W3FPR

On 6/28/2015 8:04 PM, Mick Hall wrote:

> Hi
>
> Working on my K2 with a long wire antenna and externalATU and my K2 shut down. Only working at 5 watts and an SWR of 2.1:1.
>  
> I didn’t smell anything initially so I powered it backup. At first I couldn’t hear anything on any band and slowly the sound cameback, which was a bit strange.
>
> The lights on the ATU no longer indicated any output sostarted checking a bit more when I got a faint smell of fried component.
>
> Before removing any covers I started checking basicthings. S meter stopped registering any received signals, although all bandscould be heard at normal levels.
>
> The ATU can only handle 15 watts so I tested up to thisand found that when I dialled in up to 10 watts, It was indicating 0.5 watts. Icould hear the KPA100 switch in at 11 watts but anything up to 15 watts showeda max of 6 watts.
>
> I bypassed the KPA100 by powering it from the base K2power socket and the I had the same results.
>
>
>  
>

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