Z dont move, just a little burst... (no replies)

Hello,

I test my steppers, without any mechanicals: just arduino, ramps 1.4 and 3 steppers X/Y/Z (and 3 endstops also).

I use Marlin_V1 and pronterface (on linux).

X/Y are ok. Z don't move. When I send an Z order, Z stepper just do a very little rotation (not really in fact just a 'glitch'), and block.
There is also a high-pitched sound, as do sometime power supplies...
If I click on Stop stepper in pronterface, the Z stepper is 'released' and sound is stopped.

I try to invert X/Y and Z steppers => same thing: Z not OK
Same thing with X/Y and Z pollolu: Z not OK, others axes OK

I try also delta configuration (X/Y/Z are configured the same way): Z alway not OK.
I test different value of VRef on Z pollolu (currently 0.9V), test with no micro-steps: same problem.

Do you have an idea of the problem origin ?
What should I test now ?
(I have only a voltmeter, no scope)

Thanks

a+
Vicnet