Palindrome sez:

As I and others have written - the diode didn't burn up because of
transient energy. There is a supply problem, somewhere.

The pcb had failed 'lytic caps (ends pushed out), so that could have added to
the problem. Or, being beyond a certain age, the 'lytic problem may lie in
the PS as well. I'll see about 'scoping the PS voltages in the machine.
--
John English