Yet more on -8/m Mem problems - test-drb@ccmp.vtda.org

22 Nov 1999

...
   I had the
following results when depositing 7777 to the memory.

 Memory locations: xxx0 - xxx3 don't save anything
 Memory locations: xxx4 - xxx7 save 7760 
OK, on the G227 card there's a pair of 8251 decoders at locations E46 and
E39 that decode the lowest 3 address lines, along with the Read H signal.
E46 handles locations xxx0-xxx3 and E39 handles xxx4-xxx7 (at least if 
Unfortunatly I won't be able to look into this until I get off work tonite,
or tomorrow before work.  If nothing else I'll try and hit the books tonite
and get some of the preliminary research on this done.

BTW, does anyone know of a way to read the TIFF's on Highgate on a UNIX box,
for when I don't have the paper copies of the Maintenance manual handy, but
have a chance to do a little reading?  It would be nice if XV can handle
them, but it only seems to be able to display the first page.

...
 > Also step 6 tells me to check the resistance of
diodes FSA2501.  Um,
> someone mind explaining how to do this exactly?  Beware, I don't have the 
...
 Then for each diode pack, check for diode action [1]
between pin 1
(common cathode) on the chip and pins 3,12,2,5,8,7,11,9 separately (8
tests). Then do the same between pin 14 (common anode) and
3,12,2,5,8,7,11,9 (8 more tests). 
Terrific!  This is exactly the info I was looking for!  I'll have to double
check tonite, and redo my readings, but I think the core board is OK.

...
 HOWEVER. Diodes are non-linear components. It's not
meaningful to talk
about the forward resistance unless you specify the current that was used
to measure it. Different meters (and even the same meter on different
ranges) can give very different results. My guess is that the figures
given in 4.7.1 in the manual relate to a particular type of meter (the
Triplett Model 310 mentioned in Table 4.1?), and you might get very
different results on a working board. 
This and the pinout above look to explain my problems with checking the
FSA2501's.  I'm showing about .8 MegaOhm's one direction, and infinite the
other direction.  I'm using a Fluke 77III Digital Multimeter to take the
readings.  That's so far off from the ranges given in the manual, that I was
fairly sure I was doing something wrong.  After all I think the manual says
something like 25 Ohms, while I'm getting .8 MegaOhm's.  I still find that a
little hard to believe.

This morning I went so far as to try and check out ALL the X-Y lines and 
Sense-Inhibit lines running through the core.  I'm fairly sure the core plane 
itself is intact.

Thanks for all the great info Tony!  I'm starting to think I might just be able
to get this fixed.

			Zane