Ricoh, after Hoya, to reduce costs on units for Pentax cameras, switched to other component vendors out of Japan.

Eventually, most K-30 K-50 K-70 K-500 K-S1 K-S2 camera models (employing plunger type aperture interrupter) got affected after a new Chinese solenoid utilized which fails due to coil copper/alloy ratio, magnetic properties, surface finish and plastic housing quality issues.

This part is not the green PET type $7 solenoid made by several Chinese manufacturers which costs less than 50 Cents.

We sell original Alps/Matsushita (based on batch) solenoid plunger made in Japan lastly used in K-r and K-x series and a few K-30/50 cameras obtained from the former Colorado Service Center. 

This is the only working DIY replacement to fix costly aperture opening issue.

- Modifications like sanding or lubricating the green solenoid will only last for a while.
- Buying another green solenoid, for sale by many Chinese here, comes with tools, at quite attractive prices, will defer the issue temporarily, and waste your time.

If you want to get the job done right for the first time, this is the part you need. Compare the price from Pentax service with this DIY solution. 

Thanks and good luck on your repairs!