Re: Rev 4 mobo disk trashing mystery

["Michael J. Mahon" <mjmahon@aol.com>]

RedskullDC wrote:
> Hi Schmitd,
>
> "schmidtd" <schmidtd@my-deja.com> wrote in message 
> 1177263826.143384.84240@d57g2000hsg.googlegroups.com">news:1177263826.143384.84240@d57g2000hsg.googlegroups.com...
>
> > I picked up a rev 4 B&H machine the other day.  Integer ROMs, raised
> > power key, dark green slots, serial 27xxx something with 7930 date.
> > It's temperamental as those machines tend to be - and I have some
> > keyswitch rebuilding to do as well.  One problem is truly vexing me:
> > it trashes track zero of any disk I feed it when booting.
> >
> > I have tried different power supplies, different disk controllers,
> > different ROMs (I plugged in a set of Applesoft ROMs on a switchable
> > card), and even swapped out every chip from a known-good mobo.  Still
> > I get the same behavior: booting with the disk card in slot 6 causes
> > disks to lose their minds.  They become unbootable, uncatalogable, and
> > have to be reformatted (not just re-imaged) in order to be reused.
> >
> > What on earth might be causing this?
>
>
> Process of elimination:
>
> 1. Try a different drive.
>
> If this fixes the problem, I would be inclined to look around Q2 or the 
> ULN2003
> chip on the drive board.
> If the 2003 chip has failed, it may be turning the Erase head on when it 
> shouldn't.
> = Random trashing of the diskette, but no permanent physical damage.

The 74LS125 is even more likely to cause this problem, since it is
directly connected to the cable, and vulnerable to ESD and misplugging.

> 2. If Problem persists, try a different controller card.
>
> Similarly, check the 74LS05 and 74LS174 on the controller card for shorts, 
> particularly where
> they control the WRREQ and WDATA lines.

He's eliminated the controller card.

> 3. If problem persists, try card and drive together in a different 
> motherboard.
>
> If you get this far, check the rails coming out of the power supply 
> (particularly 12V)
> when it is under load.

That's one that I'm wondered about, too, but he's tried different
power supplies...

-michael

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