Arduino nixie v41 Operating Instructions Manual page 34

One or more of the RGB LEDs will not go out completely
One of the LEDs (either one of the R, G or B channels, or the TICK LED) does not go out
completely when it is configured to be dark or at the darkest point of the "pulse" flash.
Trouble
The FETs which drive the LEDs are very sensitive, and can pick up the stray voltages which
shooting
are carried by excess flux on the board. Carefully clean with a non-scratching instrument
between the three pins on the FET of the affected channel. Alternatively, use a solvent to
remove excess flux.
The MOSFET IRF740 gets really hot.
Try a factory reset. There is a setting about how hard the IRF740 should be driven "PWM
On Time". Perhaps the value has not been set properly. The default value should be OK
Trouble
most of the time, but depending on the tubes and power supply, this might need
shooting
adjustment. The lower the value, the less power will be used and the less hot the MOSFET
will run, but also the less power will be available to drive the tubes.
Check the power supply. If the power supply is too "strong" (too much voltage or too
much current capacity), the MOSFET will have to carry high currents. Try a different power
supply. 9V and 500mA is ideal.
Change the settings for the "PWM On Time". Adjust it to be as small as possible without a
loss of brightness. This also reduces the power consumption of the module: normally it
should not consume more than 3W.