[Elecraft] K3 LPA failure, again

[Martin Sole]

Hi,

So I had some success in fixing this and thought I would write it up 
here briefly for future reference.

Short version
C6 on the K3REF module had become a 12 ohm resistor.

Longer version
Finding the 7T line only going to 3.5 volts or so was suspicious. 
Measuring the resistance to ground of that line showed about 20 ohms 
where the 7 volts is generated at U11 on the RF board, very suspicious. 
The first step then was to remove any removable item that has a 
connection to the 7T line. That's most modules in the K3. Checking the 
resistance of the 7T line each time I removed a plug in module showed 
the fault to be somewhere on the K3REF module. The 7T line on this board 
goes to Q3 via R16, R8 and R9 and also to Q2 and U3 via L5. Measuring 
either side of L5 showed  lower resistance on the Q2 and U3 side, now 
around 12 ohms as opposed to 14 or so at pin 3 of P75. Focus then was 
directed at Q2 and U3 as likely culprits being active devices likely 
more prone to failure but removing Q2 showed no change nor did very 
carefully lifting pin8 of U3 and confirming no connection to L5. That 
left C6 as the remaining component on that line and its removal restored 
a much healthier 2.7k resistance to ground on that line as formed by 
R16, R8 and R9 around Q3 noted above.

I don't have any of the microscopic 0.1uF capacitors used in this 
location, are they 0402? but I have a good selection of 1206 parts and 
being smaller than the chokes on this board I considered it a suitable 
change. The board mounts very close to the chassis so size is critical.

All restored and the rig is back to normal working order. I don't know 
if these capacitors are prone generally to such failures but at least it 
was on a plug in board so easy to narrow down.

So that was a fun Saturday afternoon for an hour or so, hopefully the 
radio will keep on working well now.

Martin, HS0ZED