Michi HB9WDF had for quite some time a MKU UP 2424 A without any output power in storage and was kind enough to give it to me for analysis and possible repair. Symptoms were only minor current increase in TX (some 240mA vs. 150mA RX) and no output at all.
TLDR: bad solder joint in the p-channel FET at the 28V switching regulator for the final stage.
Long version: All of you who ever dared to open the MKU UP 2424 A will remember the stacked approach of the design with one HF module sitting on top of the final PA (see HERE). As I had nothing to lose (I was expecting a blown final stage), I disassembled the module and opened the lower shielding. Following a rule from my mentor Volker DF7IT(sk):
"Even in RF circuits, most problems are found with a simple multimeter."
I started checking the voltages and sure enough, the 28V for the PA were missing, regardless of the PTT TX/RX state. If you follow the large traces from the final stage, you cannot really miss the two large 2512 case SMD chip resistors (R022=22mOhm) routing the 28V to the PA.
The switching regulator IC is a LT1268B from Analog Devices (former Linear Tech.) and is supposed to have its 12V input at pin 5 but nothing to be seen there. But the FET just above had a funny looking tab..
Careful visual inspection and some gentle pushing with tweezers confirmed that the tab was NOT soldered to the pad below and was moving around. As this FDD4243 is a p-channel FET which provides the TX/RX switching for the 28V of the final amplifier and the tab is the drain (and the only drain connection of this device) the switching regulator effectively was disconnected.
Resoldering with a powerful soldering iron and plenty of flux does the job. Don't even try with a weak soldering iron: the PCB is mounted on a heatsink because of the PA and using a weak tool may result in a solder joint worse than before.
It does not look like it on the picture but the tab is actually now soldered to the pad below AND to the cap next to it, which is the regulator's input cap.
Result: TX current without drive power is now just above 1A and 20W are easily reached when appropriate IF power is applied.
I hope this is useful for anybody with similar problems.
73s Achim, DH2VA/HB9DUN