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

[tony duell]

> 
> 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

I think it is very likely you have damaged the LB1620, and maybe the other ICs too.

> 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

This IC was originally designed to operate the head drum motor in a VCR, and at least in the 
UK some old-establised TV spares companies still have it. I have no idea if you can find it
in Italy.

What _I_ would do is to trace out the schematic of your spindle motor board. You will probably 
have to dismantle the mechanical part to see the windings and hall effect sensors. There may
be a wiggly PCB track round the outside of this area, just inside the rotor. This is the FG 
(frequency generator) track, it produces an AC signal that is fed to the TC9142 chip to 
measure the motor speed. 

Most of the time you can temporarily remove the TC9142 and associated components (I 
notice an 8 pin IC on the board too, that is probably a single or dual op-amp), and run
just the commutation IC. The motor will run far too fast, but if it can be got to run then
the windings, etc, are OK. 

Yes, if you replace the LB1620 then the new one might fail too. But you'll not get it working
without trying things.

-tony