[Elecraft] K3S KPA3A No Power over 12W

[Mike Walkington]

Hi

I purchased a K3S/100-K ie K3S with a KPA3A Rev A in August 2015.
Recently, the KPA3A ceased providing power over 12W. Receive also falls off as power is turned past 12W.

Relay clicks as I turn up the power.
 From information on the email Elecraft reflector, I have done the following:
Checked all radio settings and run power calibration. Power calibration worked OK for 5W. For 50W it aborted with message about not getting power required.
I replaced R5, the 100ohm resistor with higher wattage version. Re ran power calibration but same result, ie aborted the 50W calibration.
I checked the 4 big red coils. I cannot see any breakages, and multimeter indicates they have conductivity through them from pad to pad.
I cannot see any burnt parts.

With the KPA3A removed from K3S and not powered, and using multimeter in diode test position:

I have checked D7 (1N5408) as follows:
Forward Bias - multimeter’s red (positive) lead to the anode of the diode., multimeter’s black (negative) lead to the cathode of the diode.
Voltage climbs to 1.68V
Reverse Bias - reversed multimeter leads
Voltage 0.51V

I have checked D10 (BZW50-15 ) as follows:
Forward Bias - multimeter’s red (positive) lead to the anode of the diode., multimeter’s black (negative) lead to the cathode of the diode.
Voltage climbs to 1.68V
Reverse Bias - reversed multimeter leads
Voltage 0.508V

I am not sure how to interpret the diode readings ie as good or bad. It seems that other components are having some effect. Probably charging C37 and C38.

I'd appreciate advice on diode measurements and suggestions on what to do / try next.

Cheers
Mike, VK100