I removed the cover on the IC that's on the print head PCB, here's photos
74C9TNTG4
SN105117BPZP
1525-0036...

Today a new problem appeared where my battery wasn't being charged with charger plugged in:
I used my laptop on battery for couple hours, then plugged it to charge, and hours later when it should already be full I noticed that the battery is still at 51% and has not charged, and the charging light wasn't even on!
I unplugged and replugged the charger, and Windows recognized when I unplugged and replugged the charger, but the charging light on the motherboard didn't come on.
So the battery just would not charge.
Then I unplugged and replugged the battery after which it started...

I have received the new DCIN mosfet B20P03, but the problem of the charge light staying on with charger unplugged has not come back at all, so for now I'm not changing the mosfet.

One related problem that I mentioned initially does still exist though, occasionally the charging light on my laptop starts flashing green and red alternately.
Replugging the battery makes it stop, until it randomly starts again.

I reassembled the printer, it was difficult because it has been months since I disassembled it so I didn't remember how to assemble it but I got it done and this disassembly video helped me https://www.youtube.com/watch?v=hgiCGBt-C3U

Those ribbon cables are a nightmare, they are so thin and you have to push them in which makes them buckle, very bad design. I put silicone spray on one of them to try and make it go in easier.
I sanded the cable and the plastic tab that fell off of two of them and superglued it back on.

I powered up the printer and guess what, same...

I removed the three electrolytic caps on the print head board and they measure good.

print head board photos
2004-HPDC
C9050-60037
CC47200PTJG
ELCN A.18
C9050-80003-54...

With this printer design the cartridges are the print head.
I cleaned the cartridges and their contacts on the printer already before disassembling the printer, I also tried other cartridges which made no difference.
That's why I think there's a problem on the motherboard, or the print head board that the cartridges connect to.

I have now removed all electrolytic caps on the motherboard to check their capacitance to rule them out, and they all measured close to their value apart from the largest cap which is 50V 330uF and measured 316uF but I assume that's still okay so...

with the motherboard powered up there is a very faint buzzing sound coming from the board from the side of the board where the power input area is, I can't pinpoint which component is making the noise, I only hear it bringing it close to my ear.
does that mean anything?

I removed this diode to measure it out of circuit in diode mode, it is 0.235V one way, OL the other way.
is that okay?...

I removed this 10V 330uf capacitor to check its capacitance with my multimeter and it read 350uf so I soldered it back....

Thanks stj,
The LR8726 mosfet was the problem indeed, I wasn't ready to remove it before since I only had a soldering iron but now I have hot tweezers so I removed it and soldered a new one and now the LED lights up and the variable speed trigger works.
I also resoldered the 3 thin black, blue and red wires that go from the trigger to the control board since the wires fatigue snapped from the control board from bending them.
The LED and trigger behavior wasn't reliable though so I opened the trigger to clean the sliding contact which didn't help then I opened it further and saw...

Oh no all my photos in this thread have been lost.

I added a 6.3V 2200µF10x12mm capacitor.
I connected the capacitor to a ground and battery rail point I found on the camera's PCB, and added a 150ohm resistor I found in series with the capacitor to limit inrush current when plugging in the battery:


First I taped and glued the capacitor to the lens housing but the cap was too high so the case didn't fit:


Then I moved the capacitor and attached it lower, right on the PCB:

But there was one protruding feature on the camera...

With power connected to the motherboard, this is the DC voltages I measured on the 16V NPN BJT, black lead was on a ground.
16V on collector, 0V on base and emitter.
What does it mean?...

oh no all my photos have disappeared from this thread.

I noticed that when powering the printer's motherboard this black 50V 10µF electrolytic capacitor I circled in red has 13.55V measuring across its pins, and if measuring from ground to its negative side 31.7V and from ground to its positive side 45.47V.

Is that normal? the power adapter provides 16V on one wire and 32V on the other to the motherboard, so where is the 45V coming from?

That capacitor is not connected to ground. the negative side of that capacitor is connected directly to the 32V BJT which...