Thanks for the 5V regulator advice. That was one of the first things I checked since the unit had the early 78M05 regulator, but the output was ~4.95VDC so I didn't worry about that yet. I've ordered a 7805A and some replacement electrolytic caps to keep the unit happy for a few more years. And I replaced the 0.22uF film cap at the input of the 5V regulator, which I read may also be problematic.