mesa 7i76e stepgen failure

Probably a lost cause, but just in case ...
Turned on my CNC and the Z axis did not move.
After swapping out the stepper driver found that the stepgen no3 on the mesa was not outputting any step pulses as shown on my oscilloscope. Direction pin was changing as expected.
Changed it to number 4 and all is ok.

Assume just a hardware / output driver issue.
Is this a common fault on these cards. It is at least 5 years old. It lives in a pretty protected environment.

At this point I only need 4 stepper outputs so can live with it as is.

Failed stepgen outputs are most likely due to a problem with one of the
the output driver chips. For stepgens 0..3 these chips are a 74ACT541 (for the + outputs)
and a 74ACT540 (for the - outputs)

Does the  step3- output work? If not, it may be a deeper hardware issue
(PCB or soldering issues) These kinds of issues are rare.

Bad step/dir outputs caused by driver chip failure  are not rare but usually
traceable to accidental contact  of the step/dir pins to a voltage outside
the 0 to 5V range perhaps during initial wiring.

Firstly made a minor mistake on the numbering i.e. 3rd stepgen therefore stepgen.2

This is a small cnc in my home so pretty clean environment using DM556 drivers.
Scoped it with the driver disconnected and hooked between gnd (first pin) and step+.
Figured that since the dir pin toggled when I jogged the Z up and down, that all programming was connect.
Scope looked fine on stepgen.3 and Z now works on that channel.

Also another thing, is that I have changed the stepper driver to a new one as I thought initially that might be the problem.

*possible* that the original is faulty and took down the output. Not prepared to test it on another channel though (: