Phoenix CD 2.0/Phoenix_CD.cdr/24b/mem_help.zip/MEMORY2.DOC

Text File | 1988-05-03 | 4KB | 76 lines

In the last message on memory, I talked a lot about memory arrays  and
how they are arranged. You should go scrounge that mesage up and  read
it first, before you dig into this one.

MAKE  SURE you take all precautions against static damage.  Touch  the
unpainted metal insides of the machine for a second or two before  you
touch  anything electronic. Don't hand a card to someone else- put  it
down, have them, touch top of the power supply, and pick it up.  Don't
let  anything  electronic get near regular white  styrofoam  or  white
packing  peanuts,  or  platic  bags  of  any  kind.  Use  "pink  poly"
antistatic bags, 3M dark gray antistat bags, or black d-stat bags. Use
common sense, that helps a lot.

I  assume  you have already figured what column of 9 chips (or  8,  if
parity is not used) contains the bad chip. See previous message.

The next problem is of course to figure out which chip in that  column
is  the baddie. Most computers have a bios that gags on memory  errors
but  doesn't  provide much useful information. It might give  you  the
address  of the failing chip, necessary information, but  that's  only
half  the story. You also need the failing bit number, a value from  0
to  7, or Parity Bit. You might get lucky, and have the power-on  self
test  die  on the bad chip. Then it should tell you the  failing  bit,
well most POSTs tell you that. Better to get a good memory tester  and
let it cook for a few hours. There is a real good one floating  around
as shareware called RAMTEST3 and if you can find it, use it. It  tests
EMS  ram,  conventional, expanded too. If you can't  get  it  anywhere
else, join the EXEC PC BBS at 414-964-5160. If I uploaded this message
to your BBS, chances are I uploaded RAMTEST3 as well- look around.

Anyhow,  a ramtest should give the bit number of the failure.  What  I
often  do  if  the bit number is uncertain or if it seems  to  be  the
parity  bit is to shift the entire column up one chip and see if  I've
identified  the right column. The error should move to another bit  if
so.  If the error bit is erratic or moves around between  tests,  then
it's likely the parity chip is bad. Or several chips are quite  seated
snugly  in their sockets. Try pushing them all down into  the  sockets
firmly.

If your memory test gives a bit number, that should be the chip within
the row thats bad. Often, they are numbered 0 1 2 3 4 5 6 7 P, so  the
first  one  is 0, etc. Which end is which is anybody's  guess;  memory
board makers can lay out a board any way they like. Look for hints  on
the  board itself, sometimes the bit numbers are silkscreened  on  the
board in tiny white letters, or green ones.

If  your memory test gives the bad bits as a bit pattern it will  look
something like this:
0101 1010 WRITTEN
0101 0010 READ
0000 1000 XOR      although many don't give this line. As you can see,
7654 3210 (bit 3)  the bit that's different is #3. Replace the  fourth
                   chip  in the column and test again. If a  different
bit is implicated each time, it might be the parity chip, or maybe the
address  decode  or refresh logic on the board has gone  south.  These
chips  are almost certainly soldered in, so buy a new board or  get  a
good scope and and Electrical Engineering degree.

Things you should try before anything else: tamp all the memory  chips
down  into the sockets. Don't flex the board to much-- it will  break.
Or a marginal trace will let go and you will have a real flakey  board
on your hands.

Take  all the boards out of the computer one at a time and  check  the
gold fingers at the bottom. Any that look real grungy should be attack
with  a clean pencil eraser. Don't touch the fingers-- skin oils  make
them  oxidize. Sometimes just pulling the card and looking hard at  it
will fix it. Really.

Good luck,
Thomas M. Peters
Stevenson's Office Equipment
4517 N. Oakland Ave
Milwaukee, WI  53211
414-964-6330