How to save two Mitsubishi M2896 drive (spindle motor smoked!)

[Brent Hilpert]

On 2015-Mar-17, at 4:08 PM, Enrico Lazzerini wrote:
> I wish to update about my work. I received finally a couple of LB1620 from
> Arizona. Italian custom got them for a lot of time. Well I desoldered it
> from the pcb and I checked between +24vdc and ground for not have any
> shortcircuit. Good thing. I algo put away the circuit with a resistor and a
> zener diode and insert a new 7809 ic to give a good power of 9vdc to pins:
> 19,9 and 12 of the lb1620. powering with +24vdc the spindle motor its not
> rotate. As you can see from the its pics from here: 
> https://www.dropbox.com/sh/p12w71v69tjgfz7/AAAIReql4Se5ZCkLClPoAVOPa?dl=0
> there is a tirdh input pin in the connector: what is it use? Is goes to pin5
> of the C358C as input of the operational IC. On each pin of the engine I
> misured a +4,6 vdc respect to the gnd. 
> I have also a scope but what have I to checK?

Looking at your photos, I think I can trace the 3rd connector pin to (LM358).5 as you say, the other input looks like it comes from a voltage divider, the output looks like it goes to a transistor and then on to (TL9142).11, which from the datasheet is the play/stop control. So it looks like that 3rd pin is a motor on/off input, using an op-amp as comparator for sensing the control input. You could try pulling the 3rd connector pin up to +5V through a (say) 10K limiting resistor, to see if it turns on, but we won't really know what the trip voltage is without knowing the voltage divider resistor values.

If you'd like continued assistance, I have to ask you to take a couple of better-resolution photos of the top and bottom of the board(s), so we can make a decent schematic. Then we might be able to figure out some voltages or signals to test for, to check whether any of the other semiconductors were  damaged by the previous bad voltages.

I'm also curious about D101 - is it really a diode? it sort of looks like a resistor in the photo - and what is the color code? From tracing it looks like it raises the GND leg of the 9142 IC, which would lower the supply voltage to that IC.