Success!
Replaced R73, R74 & R75 with new 150K 1/2 W resistors as per jpdylon's suggestion and voila! Red! Out of the circuit, R73 reads open; in circuit it was variously 200-300 ohms, which I thought was just bad contact between the probes and the leads but I guess not. Moral: it's hard to analyse components in-circuit.
Now, how hard is it to take the chassis out of the cabinet? The chassis needs cleaned (fuzz!) and I'd like to refinish the cabinet.
And thanks, everybody.
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Being cheap isn't a character defect -- it's an art form.
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