Altair 8800b + drive alignment

From: Joe <>
Date: Wed Feb 4 09:17:56 1998


At 12:13 AM 2/5/98 +1100, you wrote:
>I recently purchased an Altair 8800b from the original owner. He told me
>that last he used it (1984?), he was having trouble reading from the disk

  FWIW if it's just an alignment problem you should be able to write data
to a disk and read it back with no problem. But you will not be able to
read a disk written on another machine. If you can't write data and read
it back from the same disk then there is something else wrong. The head
material on the very old drives wasn't as hard as the stuff they use now
and it's possible that the heads are worn out.

>drive, and he was sure that it needed alignment. Apparently an alignment
>disk is required.
>I suspect an oscilloscope as well.

   Yes and a way to set the drive to a specific track. Usually a special
test box but the are utilities for the PC to do that. Perhaps you can
connect your drive to a PC long enough to align it. If you have the pin out
of the drive interface and you understand how the interface operates it
would be easy to make a test box. In addition, you will have to know the
specific procedure that your drive uses. Also the procedure usually
requires an alignment disk with special patterns at specific locations. I
don't know (but I doubt) that all alignment disks are the same.
>I have two questions;
>1) Can someone explain the process of aligning a disk drive, what is out of
>whack, and how tricky it is for a rank electronics amateur like myself

   Basicly you insert the alignment disk, step the heads out to a specific
cylinder and look at the *analog* data coming from the head(s). The
alignment disk has a two SIN waves recorded on it that are 180 degress out
of phase, one is recorded on each side of the alignment track. When you
look at the output of the read amplifier, the two SIN wave will be
superimposed over each other and should be the same amplitude. If the
drive is out of alignment, one SIN wave will be greater than the other.
You then have to adjust the head radially to equalize the SIN waves. If
the head is out of position more than a few ten thousands of an inch, you
will see only one SIN wave or none. Then you have to step the head in or
out to try and find out in whickh direction the alignment track is on the
disk. Once you find it you know which way to move the head so then you set
the haed to the proper track and move the head to find and align the SIN
ways. If you have a double sided drive you have to repeat this procedure
for the other head. You need the drive information to know where the test
points are to look at the read amplifier outputs. And you have to know
which track(s) on the alignment disk contains the alignment pattern. Of
course, this is the simple version and assumes that the track step distance
is fixed so you only have to align the reads at one position. In some older
systems you have to adjust the head position at several points in it's travel.

>2) Where can I obtain an alignment disk, if required.

   I have an 8" one.
>Oh, I forgot to mention, the drive is an 8", the Altair model that has the
>same basic case and look of the Altair itself. This repair is one of the (I
>suspect) many that will be required to bring the Altair back to life.

  What model disk drive is it? I have a manual for a Shugart SA 400 (5
1/4"). It's very similar to the Shugart SA 800 (8"). It doesn't have the
alignment procedure but it does expalin how the drive operates and it may
be helpful.

  BTW does anyone know what kind of 8" drives are used in the Intel MCS 80
developement system? I have four of those drives and I wonder if they're
suitable for this?

>I've managed to get it (almost) firing up during the self-test stage, but
>one of the address lines doesn't behave (the light doesn't come on on A2
>when I flick the switch that should light all of them, yet the light is
>operational at other stages). I'll get to that one later.
>Looks like a long, slow process. But worth it, no?

  I hope so. I have to do all this with mine too. Good Luck.

Received on Wed Feb 04 1998 - 09:17:56 GMT

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