16 May
2009
16 May
'09
1:32 a.m.
Some things developed today on this front.
The UPS man came with another load of stuff from the donor of the 4051
and 4907.
This batch of packages included a bunch of documentation, including --
an original copy of the 4907 Service Manual! So, the documentation
problem is solved.
So, I spent some time reading through it. It says that the most common
cause of the FAULT light being on is a RAM failure. Unfortunately, to
diagnose it, a special fixture is required, which is essentially a front
panel for the system, including address and data toggle switches,
examine/deposit/start, stop, reset, and various other controls, as well
as LEDs for display of the address and data busses. Finding one of
these is probably pretty unlikely, at least in a timely fashion.
Building one would be a bit time consuming. I don't have a logic
analyzer, so trying to track down DRAM failures with just a scope would
be tedious. Thus, I'm more prone to doing some component substation to
see if I can just eliminate the cause.
With the service manual saying that that the FAULT light being on is
most likely indicating a RAM problem, I started looking at the datasheet
for the MK4027 chips used in the 4907. I don't have any of these (or
equivalents) on hand, but I then looked at the datasheet for the MK4116
(a 16K x 1 DRAM), which, I happen to have a bunch of. The only
difference between the two devices is that the MK4116 replaces Pin 13
(~CS) on the MK4027 with A6. All of the other pinouts are identical,
and the voltages and general timing specifications are the same between
the two chips. What I'm wondering is if I could just pull all of the
MK4027's and replace them with MK4116's. In the schematic for the RAM
section of the 4907 controller shows the ~CS pin on all of the MK4027's
is wired to ground. This would have the effect, with a 4116 plugged in
place of the 4027, of forcing the A6 address line 0 at all times, which,
if my brain is working properly (there are days when I wonder), should
not have any effect on the 4116's ability to serve as a replacement for
the 4027s.
Any thoughts or guidance? It might be a quick solution to the problem
without having to find replacement 4027s. Of course, this all assumes
that the support logic around the DRAMS (refresh, address multiplexing,
parity generation & detection, etc.) is all operating properly, which
also could be the cause of the problem.
Thanks to all who have responded.
-Rick