Elecraft K3, no output power

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 This is actually the second time I have had this happen. Imagine getting down to shack, to work that ATNO dx, just to find out that your rig does not put out any rf. Last time, it took some investigating to get back on track. I determined that the LPA was not working (low power amplifier). The LPA is used when power setting is below 12W, so it should not be the 100W PA that had failed, because it could not get any power out on the lower settings either. Forum posts advised to look for driver PA mosfets heating, replace driver mosfet etc. I could hear a faint signal in a separate receiver when transmitting so I ordered replacement mosfets, figuring I was going to replace them all. Upon diassembly, I got a bit lazy and just gently wiggled the connectors of the LPA, reinstalled the KPA 100 and tested the transmitter, and lo and behold, it worked.

Oxidation or bad contact between the LPA and main board caused this.  My K3, serial 796, does not have gold plated pins on the LPA board, but from what I understand, later models do, so this issue should be rare in those K3´s. Please advise me if that is not the case.



 Last time, the fix lasted for about 3 months, and now the issue returned. This time I figured I could make it better, but I still did not feel like removing the low power amplifier and reseating the mosfets. (I´m out of heatsink grease). First, I made sure I could hear the synth in a separate receiver.

I set my KX2, without antenna,  beside the K3, tuned it to the same frequency, and transmitted. A faint signal could be heard on the KX2. That means that we are generating some rf, ready to be amplified.

I removed the top panel (9 screws) and disconnected the speaker connector.


The LPA sits beneath the KPA 100. The KPA 100 is removed by removing 3 screws and the circuit breaker. I found it easier to loosen the circuit breaker instead of trying to pull the spade connectors from the breaker. Just loosen the plastic captive nut on the back panel and slide it out. Carefully lift the KPA 100 from the main board.


The LPA sits in the main board, with it´s 3 mosfets bolted to the bottom cabinet, which acts as a heatsink. This time I loosened the screws a couple of turns, and  carefully lifted the connectors a few millimeters using a bent picking tool. I then trickled a drop of IPA (not the beer) down each of the header pins and pushed the connectors down. I repeated this 2 times, and reinstalled everything.

Before testing, I let the radio sit, to let any fumes evaporate, and then turned it on. I set the power to 12W (no 100W PA used on this setting) and transmitted with the KX2 monitoring. An S7 signal was heard, confirming that I once again had output power. Turning up the power to 100W indicated 100W on my Bird 43.



I hope that this cleaned the contacts better than last time, and that this fix holds up longer. If not, I will have to remove the LPA, and perhaps replace connectors. 

Thanks

//Tommy SA2CLC

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