The illumination of the three buttons failed and then returned during a few days of on-off cycling, now the blue illumination does not return on the right three.
The blue illumination still glows very dimly on these three, and the right-most one of the three intact ones is a bit dimmer than its two mates, suggesting that the light function for all six buttons is two light sources (micro bulbs?) that channel their output to the six buttons and maybe it is one of two sources (bulbs?) that has burned out or the power to it has been interrupted.
I don't know this though, as to get it out from the cabinet it lives in and take the cover off means working in an awkward place and unplugging everything on the back, and I'd like to do this repair all in one fell swoop if possible.
Does anyone know the nature of these illuminated buttons and what part it might be that has failed?