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K3 repair - deafness after HI RFI warning

 




K3 repair - deafness after HI RFI warning


M0VFC
Thanks for sharing!

This was  3 years ago  


I also found the K3 total deaf after HI-RFI warning.

Need to have the K3 service manual opened  with PDF reader ( I prefer Sumatra for PDF)

In my case, D25 was fine, but the signal is stopped in the KXV3 by D5, which is a BAP64-4 .

The issue was caused by transmitting on an antenna very close to the K3 antenna on the same band. After checking the RX preamp on the main board, I found that preamp Q4 and the post-preamp were undamaged and still sensitive to hand effects. Connecting a temporary antenna to the 80m band filter produced normal expected signals, and receiving through the sub-receiver AUX port was also fine. However, the rear SO239 antenna inputs were completely dead. The antenna signal travels a long path in the K3: entering via SO239 1/2 at the KAT3 ATU, passing through to P70 on the main RF board, going through the SWR bridge where the ANT signal is grounded by T4, then through the bandpass filters, over the LPA10 and KPA100 antenna switch circuits, and into the KXV3 before reaching the Z3 band-stop circuit, then preamp Q4, adding 10dB gain to mixer Z1. The IF signal then goes to the Q8/Q9 post-IF amplifier (double NE46134), where I measured 0.5V across R75 (6R2), indicating 80mA. The high bias heats the amplifier, and the collector pad is slightly discolored.  


Update 7 August 2023
I used a signal generator and rf detector to trace the very long antenna path through all K3 units to reach preamp Q4
I found that the RX signal in the end must pass the KVX3 board over P66 pin 15 and should come out at P66-pin 13
P66 is shown at first picture the 16 pin connector pin 15 is 2nd from left pin 14 is open and 13 is connected to D25
But no signal was present at P66-13
The RX/TX signal way is not easy to find in the diagrams I had to combine the 3 XV3 sheets to get it a bit clear

In my K3, reception was blocked in the KXV3 option transverter unit, which is not easy to access. A short between pins 15 and 13 on the P66 main RF board brought the RX back to life. The suspected culprit is pin diode D5 in the KXV3 main unit, a BAP64-04 type 3-port SMD. By default, when powered on, the XV3 relay contacts are closed and D5 conducts from port 1 to 3, allowing P66 pins 15 and 13 to pass the antenna signal to Z3, the bandstop filter, and then to preamp Q5. With such a long “wire antenna” between the ANT input SO239 and the preamp, unavoidable noise and spurious signals are picked up along the way. The tiny PCB of the Z3 bandstop filter is likely engineered to block unwanted internal signals from reaching the preamp.


P66 on top, pin 16 on the left, RX enters at pin 15, then passes through the KXV3 unit and comes out at pin 13, then over D25 to Z3 filter PCB  below    (pic M0VFC).


D3   U1B U1A  is the RF detector that detects  the  Hi RFI  situation.

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