I agree that the FET is not fully conducting, but that was pretty much what I measured on my Adafruit clock. I believe this is normal behavior and is sufficient to drive the filament on the majority of tubes, although occasionally a tube will exhibit the dim digit problem.PhilD13 wrote:John I went off the fact that when the FET was reversed the display on Frank_tt's clock worked ok without missing segments. Although that could be just the body diode conducting, I would think that would more or less rule out an issue with the processor or VFD chip. The voltages measured would indicate the FET is not fully conducting for some reason especially since the processor is pulling the gate to 0v.
Since all segments use the same filament, the problem should be similar across all segments if low filament power is the problem. Nearby segments should fail similarly because they share a filament, but that isn't the behavior described. The problem must involve the grids and segments, which are controlled by the MAX chip and the microcontroller.
I do not mean to suggest that microcontroller or VFD driver chip are bad. As you noted, reversing Q3 did solve the problem. But I was trying to provide an explanation of how an oscillating, partially-on Q3 could be affecting the behavior of one of the chips. Reversing the diode would cause conduction through the body diode, which should reduce or eliminate the oscillation that I suspect is generating too much power noise, and causing dodgy chip behavior.