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I have a mortal fear of directly connecting any independently mains powered equipment to the 0V line of the MK14, because so many switch-mode PSUs have tingle-inducing low-current 110VAC superimposed on their 0V output.
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Very wise, SH. I had troubles with an analogue circuit a while back. It turned out that a SMPS I was using was injecting mush into the system ground. The result was that, to my circuit, I, my bench, my 'scope, and everything even remotely earthed appeared to have mush on it.
Keep up the good work.
It has come back to me now: the Mk14 cassette interface does not have stringent requirements on pulse width. The crude ASK modulation scheme of the published cassette interface alters the pulse widths anyhow. I think there's a threshold of around 15 millisec. So long as long and short pulses comfortably straddle this threshold, the Mk14 should decode them okay.