Quick update to close the loop. I went through the components related to the NE555 timer and concluded that it was, indeed, a dead IC. I injected a 20.48 kHz signal at the output of the NE555 and the display came to life. I replaced the IC and its working great. Calibration was pretty much spot on as far as i can tell with my limited testing. Thanks again for all the feedback. Another TM504 lives along with the PS503A, FG502 and of course the DM502 that came with it. The fourth slot has a storage box that had an unused temperature probe & manual and a variety of test leads, banana jumpers and some odd push-in BNC jumpers with molded plastic ends that fit into a female BNC and will accept a male BNC on the back side of the plug. Looks great along side the TM503 . . .
Rich
Richard Peterson