Wow...I must have a marginal batch of 7420s. Geoff had a problem with one of his 7420s too. Geoff, was yours functionally good but having out of spec logic outputs?
You can use any 20, like a HC20, LS20, L20, etc. You might be able to find one in another piece of hardware.
If you can't locate another one immediately you might be able to use a pull down resistor to bring the low voltage into spec?
Grant
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