17 Jan
2005
17 Jan
'05
12:47 p.m.
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