Here we are, back with another project that would have been much
more useful if I had undertaken it 15 years earlier.
If it's not already abundantly clear, I spend quite a lot of
my time working with old computers. I do it for The
Hackery, I do it for other collectors, and I do it for fun.
But given that most of the stuff I'm working with is a minimum
of 10 years obsolete, it can be quite the challenge to source
the needed parts. Sometimes that means I have to make my own,
as is the case today.
One of the joys of working at the Hackery is getting access to
weird and wonderful things which people donate for recycling.
Today's project, like many of mine, was inspired by one such donation.
Amongst a load of gear from a point of sales company was a large
antistatic bag filled with 64 megabyte 72 pin RAM SIMMs. For the
uninitiated, this was the standard RAM used by PCs, Macs, Amigas,
etc, through most of the '90s. And though sticks as large as 128mb
exist, anything beyond 32 is vastly harder to find. So I was quite
happy to get enough 64s to keep me stocked into the foreseeable
But then I noticed something interesting about the sticks. Though
there were only memory chips on one side of the stick, both sides
had solder pads. This suggested to me that the stick was wired
to support up to 128mb. Given how rare 128mb sticks are, this
was an exciting thought. And with so many to play with, it would
be no great loss if I broke a few testing this theory. So that's
what I did.
To start with I sorted them all out. There were two types, ones
with Micron chips and ones with Spec Tek chips. I started with
a pair of Micron-based sticks. From there I looked up the datasheets
for the RAM chips and the little
voltage regulator in the center. The
datasheets told me that these were EDO chips, 4K refresh. It also
said they ran at 3.3v, which explained the onboard regulator.
Most computers which used 72 pin simms ran at 5v. Now, running
3.3v chips in a 5v system isn't ideal, even with a voltage regulator,
because the IO voltages will still be a little bit off-spec. This
is probably why the Micron-based sticks are actually wired to
get ~4.1v from the regulator instead of 3.3v. Again, a bit hokey,
but they worked in all the machines I tested them in, so I went
I didn't take any pictures of the first attempt. I hope you'll
forgive me. But as you'll see in a moment, you didn't miss much.
With my hot air pencil, I de-soldered all the memory chips, the
regulator, the capacitors and resistor from one Micron stick.
Then I soldered them onto the backside of another Micron stick.
After washing the flux off and inspecting all the solder joints
beneath my microscope, I threw it into a Mac
Quadra 605. This was a machine I knew to support 128mb simms.
Unfortunately, while it had worked fine with the 64s, it gave
of death when I turned it on with my home made 128mb stick
installed. Clearly something was wrong.
First thing I did was pull the stick out and go over it extra
carefully beneath the microscope. I could see nothing wrong. No
unconnected pins, no shorts, no nothing. So I re-cleaned the contacts
and put it back into the machine. Again, the chimes of death.
But this time, when I went to pull it out, I discovered that the
voltage regulators were very hot. I knew from the earlier tests
that the 64s didn't draw enough to heat up the regulator so quickly,
and though I'd doubled the number of chips, I'd also doubled the
number of regulators. It shouldn't have been a problem. It seemed
there was a short somewhere.
So for the next half an hour or so I studied the now blank PCB
I took the chips off of, to see if perhaps there was a manufacturing
defect that caused the regulator on the backside to be wired incorrectly.
It was during this examination that I discovered the odd wiring
of the regulator. Rather than having its adjust pin tied to ground
to give 3.3v, it was instead tied to its output pin, so that it
produced something more like 4.1v. This seemed strange to me,
until I considered the misaligned IO voltages that would result
from putting 3.3v chips in a 5v system. And indeed, the absolute
maximum rating for VCC in the Micron datasheet was +4.6v. So I
figured that wasn't the cause of my problem.
Instead I assumed that one of the smaller components, a filter
cap or similar, had shorted. It seemed plausible given the amount
of heat that the parts were exposed to during desoldering and
resoldering. And being the lazy bastard that I am, I preferred
to not desolder everything and check them one at a time. So I
used my bench power supply to apply 3.3v power after the regulator,
then let it run for a moment to allow whatever was shorting to
heat up. After that it would be a simple matter of feeling around
to find the hot spot.
To my surprise, there was no single spot which got hot. Instead,
all 8 chips which I had soldered on were warming up, while the
ones which came from the factory were not. Had I really killed
Well, yes, as it turned out, but not by overheating them during
soldering. Instead it turned out there was a simpler explanation.
Unlike most RAM sticks I can think of, these 64s were wired up
so that the chips on the backside needed to be mounted upside
down compared to the chips on the front. And having not considered
that possibility, I went ahead and soldered them on right side
up. As a result, the power polarity was reversed, causing the
chips to bake themselves. Oops.
Here I've removed one of the chips I had soldered on to test
it out of circuit. There was no doubt about it, I had installed
it upside down and killed it. So I decided to call it a day and
went to sleep.
The next day...
The following morning, having learned from my mistake,
I decided to have another go at building a 128mb stick. And this
time I decided to take more pictures during the process as well.
I started with Spec Tek sticks this time. Again,
the first step was to strip all the components off with the hot
Then I soldered the smaller components to the backside
of another stick using a traditional soldering iron.
I had tried a few different methods of attaching
the RAM chips during my previous attempt. It would have been very
easy to put them on if I had access to soldering paste, stencils,
and a reflow oven. But sadly this was not the case. And though
there was just enough space between the chips for me to get the
tip of my soldering iron, I couldn't quite touch the PCB pads
with it, only the pins. That sounded like a recipe for bad joints
So what I ended up doing was a sort of half-assed
version of the solder paste method. Instead of using a paste,
I used traditional solder to heavily tin each pad on the PCB.
Then I brushed on a thick layer of rosin
flux and set the chips on top. Careful nudges were needed
to position the chips on top of the bulging solder blobs, instead
of allowing them to slip off and sit between them.
But despite the jury rigging, a short pass with
the hot air caused the chips to settle neatly onto their pads
and adhere. A quick check beneath the microscope revealed joints
which were perhaps a little crooked, but electrically sound.
So with the method proven, I proceeded to tin, flux,
populate and solder the rest of the stick. With the exception
of one chip that went sideways half way through, it was all smooth
sailing. Then I used a can of flux
remover to blast most of the sticky rosin off, revealing the
nice clean stick beneath.
And here it is, on the left. I think I voided the
warranty... Can you tell which chip shifted sideways?
Despite the crooked chip, all the joints themselves
looked good. And that meant it was time for the test. I grabbed
the Quadra again and socketed the chip in. I flipped the switch
and watched carefully for signs of smoke. The mac bonged, and
then was silent. This was good, no chimes of death. But nothing
else happened either. The monitor remained black, there was no
hard drive activity. I started to feel around the system to see
if anything was getting hot. This time the regulators and RAM
chips felt fine. I was just about to assume something had gone
wrong when suddenly the monitor lit up. A moment later, the happy
mac icon appeared. Yes! It was booting!
Turns out that the RAM test which occurred on every
cold boot was simply taking far longer than usual due to having
so much RAM to check. And being a mac instead of a PC, there was
no way to tell how much RAM was detected until the system had
reached the OS. I had to wait impatiently all the way through
the test and boot process, wondering if the RAM was actually being
addressed correctly. I half expected the system to crash at any
Instead, it worked! All 128mb, plus the 4mb on the
motherboard were detected. Success!
So there you have it. If you need some big 72 pin
simms for an old computer, all you need to do is find a hot air
pencil, a high end soldering station, a Canadian POS company that's
retiring old gear, and several free hours over 2 days to solder
and troubleshoot with. Then you too can make something that's
often available on ebay for $30 in your own home.
Page created October 20th 2013
Memories, memories, sweet