Unfortunately, your suspicion is one of two most likely points of failure. The lamp or the high voltage capacitor has failed and rendered the flash system non-functional. Due to the age of this unit no replacement parts are available for repair. Pentax stopped supporting this model around 7 years ago.
Note: Considering the age and present value of this camera, paying the cost of having it repaired , approximately $90, if it were possible (no repair services offer it), really depends on how attached to the camera you are, but in practical terms, is not at all cost effective. The standard "rule of thumb" is if the repair cost is greater than 50% of the value of the camera, it is not worth it. A good used Optio 550 can be purchased for about $15-$35. They show up on Ebay every few weeks. In addition, just about every new camera model in range of the estimated repair cost far outperforms this older model.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well. If the "do it yourself" approach doesn't work out for you and you need conventional repair, please let me know.
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