I FOUND AND SOLVED THE PROBLEM
I'm tallking about MAM MB 33 2 (which transmits potentiometer movements as control change MIDI messages)
The problem was a faulty 4051 mux/demux.
In the board we have two of these ICs, one to multiplex the 6 signals coming from potentiometers (env pwm, cutoff, reso, env mod, accent, decay, distortion) before converting it in digital (thru the ADC0804) and sending it to the micorcontroller (8051), the other to demultiplex 8 signals coming out form the 8051 and translated in analog thru the TLC7524 digital to analog converters. These 8 signals are the previous 6 plus two new used to drive the pitch CV.
During statrtup the 8051 lowers the pitch loweing these two voltages, and monitors the pitch through the square wave produced by the LM393. (this square wave is obtained from the 555 oscillator sawtooth wave compared with a reference voltage in the LM 393 that is a dual voltage comparator; one side for our ears the other for the microprocessor pin 2)
When the 4051 demultiplexer (near MIDI connectors side) is broken, the CPU can not perform the tuning at powerup, so stays forever with MIDI light on doing nothing.
So, do replace the 4051 (both is better), and your MAM MB 33 will work again (in the most cases)