Troubleshooting an IBM 5155 - (portable personal computer) - test-drb@ccmp.vtda.org

17 Jan 2005

...

 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.
  Are you sure U4a drives Q6 and U4b drives Q7? Tracing the board shows them
on Q7 and Q8. Yikes. Any chance you could scan the schematic and email it?
1bpp tiff, bmp or pcx would do. The higher the resoultion the better, but I
could probably make due at 150dpi.

You could even send it in seperate parts. I could then stitch them together.
Making some preliminary measurements, it's beginning to look like a head
problem. I am reluctant to actually load the heads until I have to. I still
have not "cleaned" the top head thouroughly, although it seems clean
examining it through a loop.

I may just install a pair of known good drives in the machine and
troubleshoot these two independantly. At least I would have matching drives
in the machine until I can figure out what is wrong here.

Thanks,
Kelly