Troubleshooting an IBM 5155 - (portable personal computer)

[Tony Duell]

> 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