It could be that your multimeter is not reading right - I had to recalibrate mine against my osciloscope when I saw the same problem building my first tuner kit. Regulators can actually deliver up to 10% over or undervoltage. I would be expecting 4.2 - 4.6 V for a 5V rail. 3 odd Volts is a bit low - check your meter calibration.
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One of the few remaining Mk1 owners... #00015