Originally posted by Photogoof It's a Vivitar 2800. I followed the link for compatibility, and it said, "No?"
Hmmmm.
The 2800 most likely has too high a trigger voltage - you can quickly check it if you have a VOM meter. Simply turn on the flash, let it charge fully, and check the voltage between the center pin and the metal piece under the edge of the mount. It it's higher than roughly 15-20 volts (this figure varies by who you ask and what you read) it should NOT be used with a Pentax DSLR. For comparison, my old Vivitar 285HV measures 6.7 volts (safe to use) and 2 older Vivitar 285's measure 6.8v and 7.1 volts respectively (both safe to use).
Another thing to check - is it "just" a 2800? Or is it a 2800D? If a "D" model, it has a dedicated module - could be for Canon, Nikon, Pentax, etc - should say somewhere on the unit the mounting foot is on. If it's dedicated to something other than Pentax, it won't work, regardless of the voltage reading.