Meanwhile I have 6 Japanese made solenoids here (including a faulty K50) on my workbench to make sure I don't come to conclusions just by diagnosing just one single unit.
I took a Pentax K50 apart and, checked it with a lens, it was obvious, the solenoid didn't function.
I then unscrewed the green solenoid and then unsoldered the two wires (which I prefered to unsoldering the wires from the printed circuit board)
Then resoldered those wires to the japanese made (origin Pentax K100D), calm hands and fingers and secure handling of a fine-tip soldering iron required here! I have a dentists magnifying lamp which helps.
I checked the K50 with the same lens: Wonderful, it works perfectly fine. Reassembled the K50... Great, my first DSLR taken apart and repaired.
All works like it should.
Now I investigated the difference of those solenoids:
- Coil resistance identical!
- magnetic force seems to be similar!
Then I moved the "plunger" (horseshoe) of both solenoids. I checked play through tilting them slightly when they were hold in place by the magnet and found out that the plunger (horseshoe) of the
green solenoid has some play! Compared to the Japanese solenoid when unlocking the plunger from the magnet via tilting it, it took force to get it out of this hollow body with the coils. I did not find the magnetic force to be different but there was a difference getting the plunger off, which I think is linked with the play. This gave me an idea.
The solenoid moves the lever connected to a spring, you can see how it functions very well on this video:
This lever brings a force from the right side to the plunger (horseshoe)! This force tilts the plunger!
If there is more play, the metal plunger might "scratch" the plastic material it move in and out from and leave traces or changes it structure.
Anyway, I took photos of the two solenoids next to each other.
The first photo shows the two solenoids with the plunger in its place (held by the magnetic force)
The second photo shows the plunger tilted. For this I inserted a tiny piece of wood (from a match) inside the right hollow part.
Here one can see very well how much more play the plunger has, the gap between the sidewalls of the hollow part is much larger.
I am positive that this causes the problem.
Also clearly obvious:
The hole of the chinese plunger is uneven, it looks almost like the surface detoriated
The surface of the Japanese plunger is way smoother!
I believe that sanding off the corners of the plunger does work (at least for a time) but it might have an effect over a longer period on the
lever (force of the spring etc.)
I don't believe soldering solder onto the plunger is a good solution! This might work but the plunger and thus the lever are now in a different position which could cause further problems!