Troubleshooting an IBM 5155 - (portable personal computer)

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Jan 16 17:14:52 2005

> Well, I did this and learned something VERY important. The disk only
> formatted as 180K. So, it doesn't see that it is a double sided drive. The
> same diskette put back in the working drive formatted to 360K. So, how do
> these drives tell if they're dealing with SS vs. DS meadia? Does it just
> look for a signal from the second head? Is there another sensor that
> indicates "yep - double sided". This is how it works with 8" drives. The
> index hole is in a physically different spot.

I've got the schematic in front of me. The head selection circuitry is
quite qimple, and the rest of the read/write chain is common to both
heads, so presumably it's working correctly.

The heads are selected by U4 (75451 dual AND gate/driver), the outputs of
which are pulled to+5V, and slo drive the bases of Q7 (Head 1) and Q6
(Head 0), PN2222 transistors via restors. One input of each of the AND
gates comes from the power-OK circuit, built round Q9 and Q10 (as this is
common to both heads, it's working correctly), the other input comes from
the sides select signal on the connector (U4b, for head 1) or from that
signal inverted by U8b (7414) on U4a, for head 0. That should give you
some components to check.

As normal, the head switching involves diodes, in this case CR1-CR3 for
head 0 and CR4-CR6 for head 1. The heads plug into the connectors A1-A7
(Head 0) and B1-B7 (Head 1), the pinout being :

1,2 : ground
3 : Erase drive
4,6 : Head select (other end of erase coil, centre-tap of read/write coil)
5 : One end of the read/write coil
7 : Other end of the read/write coil.

You can check the head for continuity, but don't pass too much current
through it, and if possible demagnetise it (a tape head demagnetiser is
fine) after measuring it.

-tony
Received on Sun Jan 16 2005 - 17:14:52 GMT