GS ROM-3 RAM Fault found !

["Mark Cummings" <NOTfigjams@primus.com.au>]

I finally have a ROM-3 :> Fixed by replacing 1 of the DRAMs in location UE8
(FAST RAM). If interested in how I found it read on...

background...
I picked up this complete ROM3 unit in a shop about 9 months ago, for $50
AU. It came absolutely full of Applied Engineering interfaces. I tested it
at the time and it didn't boot, although most of the interfaces did work,
and the screen was fine, it was still one of the best bargains I think I've
ever found. With all the cards removed and powering on, all I got was a
screen full of garbage, no beep, no response from the keyboard, except RESET
(sort of). The only thing I could try was the battery, and the CPU. Neither
altered the fault. Everything else was soldered to the board. Anyway it
looked too hard at the time, so disappointed I placed it on the too hard
shelf and checked out all the cards. One of the cards was a GS RAM ULTRA,
which I wouldn't work in my ROM-1 (as I found out later), and no-one I knew
had one to test it out for me, so it sat on the shelf with the ROM-3.

looking for the fault...
About 3 months ago I decided to have a go at the ROM-3 again, this time
armed with circuit diagram and logic probe, I begain to check signals around
the processor. I was getting clocking pulses but eventually found that the
NMI (or IRQ, I have it written down, just forgot at the moment) was stuck
on. Thinking I had found the cause of the fault, I started to trace the line
on the diagram, only to find that it could have come from several PLCC
chips, like the mega-2 etc. Now there was no easy way to isolate it without
hacking the board, which I hate to do, so I came to the conclusion that this
was becoming all too hard, and it looked like this one was heading for the
spare parts pile. So I put it back on the shelf again until 2 days ago.

how I found it...
I was working on a IIe RAM fault, only in the past week, as some of you may
have read. Anyway that was fixed a couple of days ago, and I realized that
the fault symptoms looked very similar to the ROM-3 fault, so I got a bug
under my skin, and decided to have another crack at it. And yes 6 of the
RAMs were the same type as the IIe, so I thought I would be looking at one
of those. 4 are in the Standard RAM slots, I removed those first, and tested
them in the trusty IIe, recently fixed and temporarily fitted with sockets
for exactly this purpose. All 4 worked. Then I saw 2 more of this type in
the Sound RAM sockets, and 8 different RAMs in the Fast RAM sockets.
Thinking that this was a manufacturers bad chips thing, I continued with the
Sound RAM first, and it was fine. Damn, I was running out of luck, so moving
onto the Fast RAM, I had a ROM-1 set up with a 4MB GS Juice Plus card, along
with the RAM test program. Made sure it worked, by removing a chip, and
running the program pointed directly at the hole. OK, I had a working test
jig, now onto the RAM. Not wanting to cause too much damage to the board,
because of the way the legs are folded over, I continued to remove just one
DRAM at a time, place it into the test-jig, and when that passed, I just
continued to the next DRAM. Then I finally struck gold when I hit number 4
(of 8), and it failed the test. So double checking I moved it in the test
jig, only to find the bad one follow this chip. So I finally had it, but
were the rest faulty ???, and should I continue ?
I have been taught to assume that there is only one fault in a system, so I
decided to put all the good ones back, and got one off my GS Juice Plus,
which was later replaced.

testing...
After cleaning up the circuit board, I moved onto testing. Placed good
battery in, and no cards fitted, or drives connected, not even RAM, it
booted up. Followed by a loud "Yee haaaar" (me not the GS !). Moving on to
testing, I set the clock in the control panel, and changed a few setting to
my preference. Powered off, rebooted and so far so good, all settings
retained. Then fitted the 4MB GS Juice Plus, and re-visited the control
panel, yep, its ~5MB (cool). Re ran the RAM tests on the GS Juice Plus, but
the program hung. Hmm, put the GS Juice Plus into a ROM-1 and it works, back
in the ROM-3, no go on the test, but it is right in the control panel.
Booted into System 6 from the floppy, and it looks fine. So I can only
conclude that the test program will not work on a ROM-3.

in closing...
Anyway, I finally have my ROM-3 that I've wanted for over a year, and better
still it didn't cost me a thing. Just a loss of hair !!!
BTW, the GS RAM Ultra did work also, so I'm over the moon. It's only got
2MB, but maybe I can fill it to 4MB if I can find some 44C256's (4bit x 256K
=20 pin). These could be a bit hard to locate, but I havn't looked too much
yet. Any ideas ?
Oh yeah, if anyone has a good method of locating faulty RAMs without having
to remove the bloody lot, I'd be interested, just in case it happes again.
One thing that Apple should have left out of the self test was RAM testing,
it's just a waste of ROM as far as I can see. If the RAM corrupts the data
bus, how is the CPU going to do anything ? It would have been far better to
fit IC sockets IMHO.

   -------------
  |  ---------  |
  | |Apple//GS| |
  | |         | |
  | |  ROM3   | |
  |  ---------  |     ____
  |_@___________|    |====|
 _ -+---------+-     ------
{ |             |   | _--_ |
 }|_@___________|~~~|  --  |
{ +------------/+    ------
 _---------------_~~~~~~
|  # # # # # # #  |     }
| # # # # # # # # |    [-]
 -----------------     [_]

Mark