Elecraft K3 Problem

[LA7NO]

 Hi,

Still working on finding a solution to the below problem.

Elecraft support said:

"Since you are hearing signals, on frequency is presumed, only attenuated
~30 dB,
The SYNTH circuit is not the first place I would look."

Any suggestion wrt where to look first?
I guess it must be somewhere on the RF Board.

Where can I find the PCB layouts with components id's?

73,

Per-Tore
LA7NO

---------------

The K3 suddenly stopped working.
One day all OK, the next day serious problems.

Serial no.: 07818.
It has ATU and 100 W PA installed, and also a P3 connected.
Both K3 and P3 were earlier well calibrated.
Supply: 14,5 VDC.

Symptoms:
- Rx seriously reduced sensitivity, only strong signals to be heard.
- No Tx output. Tuning not successful, no SWR indicated (only the 3
hyphens).
- Same on all bands.
- All displays, controls and menus apparently working normal.
- Mic-gain & ALC normal indication on SSB.

All menu settings seem to be OK.

Connected an XG3 and injected a -73 dBm signal.
It showed up on all bands with signal level about -100 dBm, so around 30 dB
attenuation.

Connected a LP-100A and a 50 ohm dummy load.
Whatever we tried, the LP-100A indicated zero power out on all bands.
Same on both Ant1 and Ant2.
Bypassed the ATU: no change.
Set power to < 10 W, effectively bypassing the PA: no change.

It is clearly something that is common for both Rx and Tx.
It is independent of the selected band.
Apparently not related to ATU or PA.

So, it could be some relay problem, or related to the SWR bridge or
LP-filters.

Hope you can give some helpful ideas on how to proceed.
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[LA7NO]

Measured voltages over the PIN-diodes D26 & D27.
In Rx: 0,72 VDC. over both.
In Tx, the voltages varied a bit around 0,4-0,6 VDC.

While in TX, even with power set to 0,0W, one of the transistors in the LPA
is getting quite hot.
Cools down when switching over to Rx.

P-T


On Fri, 14 Aug 2020 at 17:59, Per-Tore Aasestrand <[hidden email]> wrote:

> Hi,
>
> Still working on finding a solution to the below problem.
>
> Elecraft support said:
>
> "Since you are hearing signals, on frequency is presumed, only attenuated
> ~30 dB,
> The SYNTH circuit is not the first place I would look."
>
> Any suggestion wrt where to look first?
> I guess it must be somewhere on the RF Board.
>
> Where can I find the PCB layouts with components id's?
>
> 73,
>
> Per-Tore
> LA7NO
>
> ---------------
>
> The K3 suddenly stopped working.
> One day all OK, the next day serious problems.
>
> Serial no.: 07818.
> It has ATU and 100 W PA installed, and also a P3 connected.
> Both K3 and P3 were earlier well calibrated.
> Supply: 14,5 VDC.
>
> Symptoms:
> - Rx seriously reduced sensitivity, only strong signals to be heard.
> - No Tx output. Tuning not successful, no SWR indicated (only the 3
> hyphens).
> - Same on all bands.
> - All displays, controls and menus apparently working normal.
> - Mic-gain & ALC normal indication on SSB.
>
> All menu settings seem to be OK.
>
> Connected an XG3 and injected a -73 dBm signal.
> It showed up on all bands with signal level about -100 dBm, so around 30 dB
> attenuation.
>
> Connected a LP-100A and a 50 ohm dummy load.
> Whatever we tried, the LP-100A indicated zero power out on all bands.
> Same on both Ant1 and Ant2.
> Bypassed the ATU: no change.
> Set power to < 10 W, effectively bypassing the PA: no change.
>
> It is clearly something that is common for both Rx and Tx.
> It is independent of the selected band.
> Apparently not related to ATU or PA.
>
> So, it could be some relay problem, or related to the SWR bridge or
> LP-filters.
>
> Hope you can give some helpful ideas on how to proceed.
>
>

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[Mike K8CN]

Hi Per-Tore,

Can you identify which transistor (or other device) on the LPA module is
getting hot in transmit mode?

The drain terminals of MOSFET PA transistors Q4 and Q5 receive +12V
regardless of TX or RX mode, but the gate bias voltages are applied only in
TX mode.  Unless there's a problem with the bias regulator circuit (U1), the
RF amplifier chain on the LPA module should not be getting that warm. It is
also possible that one of the RF power transistors may be getting hot
because it is no longer making good thermal contact with the bottom cover
panel - it would be good to check the tightness of the three screws holding
those transistors against the bottom panel.

Is it possible for you to carefully extract the LPA module from the main RF
board? That requires removal of the rear panel of the bottom cover, and you
may need new thermal pads for the RF transistors if those pads are damaged
by removal of the cover panel.   If you can remove the LPA module, you might
try RX mode ONLY with the Power control set to 0 (to ensure that the KPA3 PA
is bypassed by its relay K1) and see if your receive sensitivity is
restored.  

The output side of the LPA module is nominally connected only in TX mode to
the antenna path via diode D1 on the LPA module, and if the D1 switch is
operating properly one would not expect the high attenuation of signals in
RX mode.  The output of the LPA module is also connected to the input of the
100 W PA module at the same point on the T/R switch circuit, but that module
is bypassed by relay K1 on the KPAIO3 board.

I'm not yet sure how the overheating of a device on the LPA board is
connected to the loss of RX sensitivity.  If the diode switch D1 on the LPA
module has failed shorted (unlikely), then it would leave the output
circuitry of the LPA module bridged across the antenna signal path in RX
mode.

Keep us posted on what you learn!

73,
Mike, K8CN





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