Parts arrived yesterday and I fitted the 40 Pin socket and 8051 uC replacement.
Unfortunately Pin 9 is still receiving the reset signal and Pin 5 is at a steady 5V or so.
However, I do have an oscillation on pin 18 and 19, so that's a win.
Pin 30, ALE looks as depicted. What's with that? Looks like 2 superimposed signals...
Is that a pointer to a defective part?
I switched to 1:10 attenuated probe after the pictures were taken.
Unfortunately my old scope keeps blowing its fuse, so can't use that at the moment...
I desoldered the 2 capacitors connected to the oscillator, as well as c890 and measured their capacitance and they are all within 10% of their nominal spec too.
I checked the diodes in the circuit around the comparator, D890 and D891 and they are fine.
Since the uC kept getting the reset signal I replaced S890/LM139 but the outputs of all 4 comparators inside it remained unchanged.
Both the 1012 transistor and the 5V regulator remain almost at ambient/room temp when the DME is powered, another positive change.
All other IC's receive steady 5V supply and are grounded.
Nonetheless the reset signal keeps being sent. Where do I look into next?