Hi Mike
Wow! That is a very thoughtful piece of work, and I imagine it will be useful
for helping people solve future KPA3A problems.
I spent a fair bit of time last weekend reading about other KP3A issues. As a
result, I had a close look at the plug/socket pins. I have a mixed of gold and
tin. I cannot see any discolouration that may indicate excessive current and I
cannot recall seeing the 12V ERR message; but I operate the radio remotely from
the other end of my house so, I cannot rule that out. I used some contact
cleaner to clean the pins and tested installed performance again but did not
see any meaningful change. But there may be deterioration inside the sockets.
Replacing the tin plugs/sockets may be something I should do.
I did review the signal loss behaviour by noting what happens on the P3. I
could not reproduce the signal loss I had seen before. I may not have looked
closely enough the first time round. Or maybe the contact cleaning changed
something. I note someone used their XG3 to examine signal loss behaviour. I
can look at doing the same. For now, I am assuming there is no drop in receiver
performance above 12W. This may revert thinking about the diodes as the problem.
I did measure using K3S meters the current consumption, and output power using
LP-100A, with the following results:
Set 10W (measured 9.8W),current 4.32A
Set 15W (measured 0W),current 3.08A
Set 25W (measured 0W),current 3.7A
Set 30W (measured 0W),current 4.13A
Set 45W (measured 0W),current 5.5A
Receive current at all power levels was 0.98A
So, where to from here. I read that someone used ribbon cables to allow them to
extend the KP3A out of the radio chassis. I could look at that but am worried
about high current.
I am looking to next measure changes in installed voltages 7R, 7T below and
above 12W, ie to see if they are changing appropriately.
I read that someone had noticed that the problem goes away if the K3S RF Board, KPAIO3A,
KPA3A "sandwich"is compressed. I will look to try that in case there is a
socket/plug seating issue but I cannot see one at the moment. I note there is an Elecraft
modkit E740267 that adds a bracket to hold the KPAIO3A down. I do not have that mod. I
assume it was devised in response to unseating issues, but I could not find an
explanation of why it was introduced.
Thanks for your ongoing interest in helping me solve this one.
Cheers
Mike, VK1OO
Message: 1
Date: Thu, 8 May 2025 16:55:46 +0000
From: Michael Carter<[email protected]>
To: Elecraft Reflector Reflector<[email protected]>
Subject: Re: [Elecraft] K3S KPA3A No Power over 12W
Message-ID:
<bl3p223mb03850f4f1e54394696d69c84e4...@bl3p223mb0385.namp223.prod.outlook.com>
Content-Type: text/plain; charset="iso-8859-1"
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, ? 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
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