I removed the 8-pin DIP op amp and tested one of the amps as described here:
https://www.circuitbread.com/ee-faq/what-are-the-golden-rules-of-op-amps
I used the schematic at the very bottom of the page as the test setup.
The result of the test was that the non-inverting input voltage (4.5V)
did not match the inverting voltage input (0.7V) and according to one of
the golden rules of op amps:
"In a circuit with negative feedback, the potential difference between
the inverting and non-inverting inputs is zero...."
That means the op amp is defective.
By the way, many of the voltage +/- inputs for the quad and dual op amps
when measured in-circuit were amiss. In other words, the non-inverting
input voltages were not equal to the inverting voltages but perhaps
there was no feedback loop? (too many pc traces for me to keep track of.)
In summary, I'm going to replace both chips. I think I checked every
semiconductor, resistor and capacitor on the board and I'm fairly
confident that my diagnosis is correct.