It's not your fault the seller bought & sold a defective camera, if it came through the auction site, I'd INSIST on returning it. It wasn't as described.
Originally Posted by luke_h
For all you know he was aware the camera was defective. Believe it or not there are some unscrupulous folks on the bay.
1) Don't send it to the guy in Kansas!
2)The shutter does have to be removed from the chassis to be adjusted but the good thing is, once adjusted they stay that way for a long time.
If the spring lost tension, it's not age or use that caused the fault you see, it's more likely been dropped or bumped & lost a tooth or two of tension. A little bit goes a long way.
3)It does look like the Nikon and your Pentax share some flaws but the Pentax is the easier to cure. If you refer to Chan Tran's post that's the cure for the Pentax, a simple travel time adjustment.