[Elecraft] K3S KPA3A No Power over 12W

[Michael Carter]

Hi Mike,

Sorry for the extended delay in responding - I have not given up on you!

I've done some more thinking about your measurements, and I did some experiments of my own, although not on my KPA3A module. I was curious about the difference in using 'diode test' and 'resistance' measurement methods in-circuit.  I use a Fluke 115 DMM for most of my DC circuit testing, but I suspect most DMMs have similar methods of executing these two test modes. Bear with me here as I explain my experiments.

I tested a known good 1N4004 PN diode in 'diode test': 0.54V forward, 'OL' overload in reverse.  I then measured its resistance: about 1.4 M_ohms forward, 'OL' in reverse.  The resistance result for forward bias was puzzling, but after reading a bit more I realized that the auto-ranging DMM applies a constant current source to the DUT starting with a low value and then steps up to progressively higher current until the DUT resistance is within range.  A more indicative resistance test for a diode would use a higher constant current source value to bring the diode into full conduction.  I may try that again this morning and force a manual resistance range selection.

I then paralleled the diode with a 10 k_ohm, ¼ watt resistor.  This mimics, although without the fixed 100 ohm series resistors and DC inductor resistances, the R11 parallel path to diodes D5 and D8 in the KPA3A T/R switch.  In 'diode test' mode, the DMM reads 0.54 V forward, 1.94 volts reverse.  In resistance test, it measures approximately 10k_ohms with both probe polarities.

Out of curiosity I measured the 10k resistor alone in 'diode test' mode, something I've never considered doing until now: 1.94V.  The Fluke DMM steps up the applied voltage in 'diode test' mode until sufficient current is registered, then stops ramping up the voltage beyond 2.0 V and just indicates 'OL'.  Since I knew that the diode was good, the result for the paralleled diode and resistor in 'diode test' mode makes sense - the diode is not seen by the DMM with reversed probe polarity, but the resistor allows enough current to flow to 'trick' the DMM and register the same voltage in 'diode test' mode as the resistor in isolation presents.

So, what does this suggest about your T/R switch diodes D5 and D8?  If either diode failed shorted, that would have been evident in both the 'diode test' mode and the resistance measurements you made. If either diode failed open, that would not be evident in the in-circuit resistance tests, but it would have been evident in the 'diode test' mode, and your diodes had forward voltage drops in the correct range instead of 'OL' indications.  My sense is that both D5 and D8 are OK.

So, for me it's back to the beginning of your posts when you checked the two diodes in the +12V input bus.  The 1N5408 is a reversed supply polarity protection diode, and the BZW50-15 is a TVS diode, effectively a Zener that protects against a supply voltage in excess of +15 V DC.  The diode test voltages you measured for those diodes are affected by other circuit elements attached to the +12V bus, but I haven't yet looked at those details.

73,
Mike, K8CN