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

[Brent Hilpert]

On 2015-Mar-01, at 9:56 AM, Enrico Lazzerini wrote:

> Thanks Tony for your suggestions. What I means is:
> 
> It happen in two distinct times every time when I used the drive to read
> through a PC with the software IMD to create images of floppy disks 8 ". Not
> having the correct power connectors i power the drives in this way: + 5vdc
> and ground coming by a PC power supply and +24vdc and ground from a Chinese
> power supply. I must have certainly reversed this last one even for a few
> seconds exchange +24vdc with the ground. Now both spindle motor supplied
> also with only +24vdc start smoking in the vicinity of the chip LB1620 and
> cause of cooling you can not touch it as hot it is. If I try to measure the
> resistance between pins  +24vdc and ground of each spindle motor drive i get
> 1327ohm on FD0 while i measure 19ohm on the spindle motor drive FD1. There
> are no visible damage nor to the components nor the PCB wires. 
> In both pcbs anyway the chips LB1620 sunburnt from not being able to keep a
> finger on it. 
> I searched on ebay and the only place that has LB1620 seems to be:
> http://www.ebay.it/itm/LB1620-INTEGRATED-CIRCUIT-IC-BOX-74-/390955452872?pt=
> LH_DefaultDomain_0&hash=item5b06c2c9c8
> 
> The input's pins of the pcb are : +24vdc, ground, and the rest could be a
> control.
> 
> The point is that I have no experience on these components. 
> Replace the chip could make it burn again for my inexperience.
> 

I notice in one of the photos you have written 15V for the zener D102, and on both boards they look like they have been replaced.

D102 and R112 are a simple regulator to reduce the +24V down for the chip supplies (looking at the photos to trace the circuit).
+24 is otherwise used directly only for the motor windings, in part via Vcc1 on the LB1620.

The LB1620 datasheet specifies the Vcc2 range for the 1620 to be only 8.5 to 14V, not 15V.

Check the Vcc2/zener level to see whether the zeners are good and what voltage they are.
I expect Vcc2 (and the zener V) is supposed to be 12V or 9V.