128K: bad RAM repair

first try

remove the logic board

remove the ramchips and reseat them in the same position



reassemble > test

second try

remove the logic board

remove the ramchips and reseat them in the opposite position

(first one (F5) > last, last one > first )

reassemble > test

The suggestion by applefreak to move chips around is an excellent one. The power-on diagnostic routines are not comprehensive, so a "memory error" indication might actually be due to, say, a decoder failure. If the problem does not move with the chip, the memory chip is probably ok. If the problem does move around with the chip, you can feel confident that the chip is indeed bad.

Are all the memory chips socketed in your Mac? If not, then the advice above will be impractical to implement.

LOL exactly, if it's an unupgraded 128K the RAM chips are highly unlikely to be socketed. Tom, can a 256 chip be mixed with a 64? I would think it would function like a 64 without the additional decoding?

Hi all,

thank you for your suggestions - I have since identified the particular "faulty" RAM chip at location F5. Unfortunately, the RAM is not socketed (then again, the same goes for the 512K board I have). I recognise I've little to lose, so I do intend to remove the F5 RAM chip to diagnose the fault - would it be better off to use a solder sucker to attempt to remove the entire chip cleanly, or snip the chip off at the legs and solder the new chip on top?

I can post pics if useful, however the RAM chip is identified as:

128K: 8432; Apple symbol MT 4264

512K: NEC 8428P6734; D41256C-15

I was planning to post this in Conquests, but the 128K was found last week while driving around for work doing house calls (medical), just like the last 128K I found (about ten years ago, a "Macintosh 128K" with carry bag). The one I found is a bonafide "Macintosh" with Imagewriter I printer, and the white plastic case with all original disks, cassette tape and power cables for other countries! Most amusingly, someone cut all the cords off - anything that gets chucked around my way attracts those who sell the cords for scrap metal value :lol: Exterior and interior it shows next to no signs of use, hardly any dust, perhaps sitting covered in a garage for years after the Sad Mac error cropped up. Made for a nice day in a busy week.

JB

Byrd, you would have loved the 128K board that sold on eBay mid week then. It went for around $35, and also had an Xcalibur accelerator card. I was going to bid but forgot all about it.

Heh yeah I saw that 128K board, bidded but no luck! Regarding the Xcalibur board ... I couldn't find a thing online for that, I assumed it was more a RAM upgrade than accelerator - what made you think it was the latter?

As noted, I do have the one good working 128K but hoping to get both going - as keepers, and as a backup for parts in future. The 128K I found years ago is my favourite Mac - and quite amazing that both work (to an extent!) 26 years later!

JB

the photo of the ramchips is one of my 128k macintosh's

it was a mac with 'problems'

i removed the soldered chips and replaced them by sockets

the 'new' ramchips cames out 'the stock' of a dealer who stopped selling macintosh ...

its always good to have some (or a lot) spare parts

his 'stock' moved to ..... my place

generally a dealer is not the place to find working old computers

applefreak, I tend to agree - the best working Macs are those you find for zero cost Second-hand Mac dealers - not that there are many in Australia - charge like a wounded bull and have such a know-it-all attitude I can't be bothered with them.

Pretty impressive soldering job on the 128K RAM sockets, I have to say - thank you for posting the pic. Jaycar (sort of like a Radio Shack/Frys) sell sockets which I'm looking into.

http://www.jaycar.com.au/productView.asp?ID=PI6499&keywords=pcb+socket&form=KEYWORD

My manual soldering skills are up to scratch, but as it's a 4-layer PCB I'm a little wary desoldering the module, but appreciate it'll look a lot better than snipping the legs and soldering something on top.

JB

i don't have the appropriated tools to do such good soldering

but i know where to go,

a friend of me has a electro-shop, and that is the place to be

he has the equipment ... and i make time

The board looked alot like the Levco unit I had installed. From memory, it did look like just a RAM upgrade, you could well be right. I was so tempted to buy it, as it was the original Mac board too!

Having done a couple hundred 128K upgrades, the soldering is not extremely difficult. Cut the legs off the bad chip with a small nose wire cutter, then heat each pin from below and pull it out with needle nose pliers. The corner pins which connect to the power and ground planes inside the board will require a lot more heat than the rest since they are well heat-sinked. Then use a solder sucker or braid to clean the holes. Unless you replace ALL the RAM with sockets, don't socket the replacement chip, because the added capacitance of the socket will make that one chip slightly slower than the rest. One source of 4264 type chips is an Apple //e 80-column RAM card - usually cheap and plentiful. The earlier ones have socketed chips. The 256K chip will also work as a 64K. Good luck.

Hi all,

H3NRY thanks for your advice - today I desoldered a 64K chip and put in place a 256K chip from the spare 512K board I have here, using solder sucker and lots of cursing

Fired it up: usual beep, then Sad Mac error code now reads 040080

... indicating a faulty RAM chip at F12.

... little to lose, back I go!

JB

First, carefully double-check your repair. Use plenty of light and a magnifying glass, as well as an ohmmeter as necessary.

Also, what is pin 1 of the DRAMs connected to (I haven't looked at my 128K schematic and I don't remember the answer off the top of my head)? If it's floating, I recommended grounding it. The 4164s don't use it, but it's the higher-order address line for the 41256, and shouldn't be left floating.

And as I mentioned earlier, the memory self-test is very crude. It's not able to distinguish between a true memory failure and one that's due to other causes. Since it is very rare for multiple DRAMs to die spontaneously, you should suspect something else if, say, one more memory chip replacement doesn't fix your problem.

Good luck!

Thanks for the tips, tomlee - like yourself I was suspecting something additional than the RAM to be at fault. Having tested the first chip, I wasn't confident that replacing another would result in a great deal of success.

However, I have some good news, having replaced a second chip - it's working! Once the board was in, it fired up straight away off the boot floppy stuck in the drive, to System 1.1g. It booted so fast by the time the CRT had warmed up it was almost at the desktop!



Picture 1: alive, perhaps after 20 years of no use



Picture 2: Circled RAM chips replaced with those from donor 512K motherboard

My very rough guide to removing RAM chips off 128K/512K motherboard:

- Using a solder sucker and iron (@ 350 deg C), remove as much solder as you can from the underside pads of the RAM chip. By the time you are done most pads should have a hole going through to the other side of the board.

- Gently, slightly bend the legs back to the centre of the hole (they are usually angled slightly diagonally, making it harder to pull the chip out if you don't do this)

- Holding the RAM chip firmly with some clamping pliers, heat the pads on the underside of the board using a mini butane torch (mine is a cheap DealExtreme torch/soldering iron, it was set to +++ full). Run the torch slowly up and down the pads, pulling the chip firmly as you do - it's tricky, but if hot enough it should slide out slowly and cleanly. It's daunting, but the hotter the flame better, I found - even if it did cause some slight cosmetic damage to the board itself.

- Once RAM chip is removed, clean up pads/holes again with solder sucker

- Replacement RAM chip should slide straight in, solder in.

JB

I just use a heatgun. heat underneith the board, and pull em streight out.

if the braid wont remove the solder, those cheapie radio shack solder suckers, do exactly that, SUCK.....

but i use a tiny 1/64th drill bit thats in a pack of dremel bits, and make holes through any solder that hasnt been removed. CAREFULL.... but it works for me.

having replaced a second chip - it's working!

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Interesting. All my Macs have returned an error code which indicated more than one chip at fault. But Tom Lee is correct, it's hardly a comprehensive diagnostic. However, I have found more often than not, that if one RAM chip has failed due to bit-rot (a symptom that seems exacerbated by extended periods of disuse), that it is usually not alone. Perhaps RAM is not unlike the human brain, if it sits idle for long periods of time, it begins to lose its ability to store and recall memories.

I think this repair is interesting because I've never had such precise diagnostic error codes displayed on any computer I've repaired before - for a mid-80's computer to tell me exactly what is at fault is highly impressive. Even though from what others have posted, I appreciate that it usually doesn't work this way But old Apple designers would be pleased to note it did for me, many years later. In addition, having two chips fail is seemingly quite rare as well (although if a stick of RAM you'd just chuck it and move on), so I'm quite chuffed at the repair and how well it worked out, just following the Sad Mac codes.

Having fired it up - my suspicions about it not being used in recent times were wrong! Would you believe it was only decommissioned in 2000 - apparently having been used by a wealthy stock investor to keep tabs of his dividends, bills and letters. The (original) MacWrite disk is jammed hard in the floppy drive, so much so I can't even eject it using the sliding mechanism - the grease must have dried up to a solid. The good thing is the floppy drive (and disc) work fine, so I'll be extra careful freeing up the mechanism to bring it back to life again.

One other question - where is the Shut Down command for System 1.1?

JB

I don't think there is one. I use a System 2.0 disk in my Mac Plus, and there is no menu option, you just have to flick the switch.

In addition, having two chips fail is seemingly quite rare as well
One other question - where is the Shut Down command for System 1.1?

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Actually failed RAM chips is relatively common. Others have rejected my theory, but I find this to be the most common on Apple-branded chips. Have a stack of bad boards with these chips to repair sometime. It's called bit-rot.

And there is no shut down command on 1.1. So be careful that you quit everything on the disk, given that it is stuck and you will not be able to eject before you turn it off, which could result in disk corruption.

Very interesting about the Apple-branded DRAMs exhibiting unusually high incidence of bit rot. I've not seen that myself, but your data certainly trumps my non-data. I'm just trying to figure out how two could fail together (assuming that this is what happened), or what type of failure of one DRAM could cause the death of another.

It is true that US memory manufacturers didn't do such a great job of manufacturing and testing DRAMs back in the 80s (which is why we got our backsides kicked by the Japanese). Perhaps you're seeing the longer-term consequences of poor QA/QC.

In any case, I'm glad that the OP got it working -- it's nice when a compact Mac is saved from the scrap heap!

What's even more puzzling is that going off the documents saved to the floppy drive, this 128K was used for nigh on fifteen years before apparently failing - two DRAMs together, or something along those lines.

Hoping to test out the Imagewriter I printer next.

JB

Byrd, this is pure speculation, and certainly others may know more than I, but my guess is the 512K was functioning properly when it was mothballed, in favor of a new computer. It was right around 2000 where the Internet had progressed to a point where many of my 68K Macs were rendered useless, by more sophisticated websites. In particular BBS and other text-based sites, the kind on which stock quotes could be found, had begun to be discontinued.

But bit rot strikes me as something that happens when the RAM sits idle. Though since I don't know what causes it, I can't be sure, but there are a number of stories here and elsewhere of someone putting their working 128K/512K into storage for only a year or so, to find a failed RAM error code upon attempting to put it back into service. So a Mac that sat idle for over a decade would certainly not be uncommon to develop bit rot.

Tom Lee may certainly be onto something with Quality Control. In my experience there is no particular set of chips that fail. Granted the fact it is usually two, not one, or three or more, is certainly strange. But it does always seem to be the Apple variety. What has been mentioned before is that Apple bought their chips from many manufacturers and screened their logo onto them. If this is the case, I would still suggest that I am seeing boards from one or more manufacturers with poor QC. In which case, it is not all Apple-branded chips but Apple chips are the most likely to experience bit rot failure.

bit rot strikes me as something that happens when the RAM sits idle...

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I'm not saying this to start a fight, so... don't freak out.

I'm somewhat curious where you found this definition of "Bit Rot", IE, how this nickname applies to physical failure of a DRAM. The standard definition of "Bit Rot" refers to corruption of *data* stored in "static" memory technologies, most often EPROM/Flash chips, but also SRAMs storing data for long periods without refresh. In all these cases it describes the data loss that results when the stored charge denoting a bit "leaks" enough to change a "1" to a "0". The chip itself may not actually be damaged but the data has become corrupted and the only way to fix it is to write a new copy of it back to it. (A nontrivial process in the case of an EPROM.)

(Reference: A description of Bit Rot in EPROMS)

The term is also applied to data corruption on disk, tape, and other magnetic storage devices due either to mutual interactions of the magnetized bits causing it "erase itself" over time, or because the media itself literally degrades, or "rots". (The physical degradation definition also applies to, for instance, data stored on aging CD and DVD media.)

"Bit Rot" is a great term to apply to DRAM chips failing over time, but again, I'm curious if there's actually some electrical/chemical/technical phenomenon you're describing with it. (A link would be great.) Most of the references I can find on aging effects on semiconductor memories suggest that said devices have a very flat degradation curve once they're past an initial "breaking-in" period. An implication derived from that is that it's possible the memories that "fail" in Macintosh 128ks haven't actually "failed", but were simply marginal in terms of output response to begin with. If that were the case then perhaps there is something else in the support circuitry on the Mac motherboard that *is* more prone to aging that is now failing to work with said marginal devices. (Top candidates would probably be filter capacitors or parts of the power supply. Capacitors in particular are something that seem to suffer from periods of idleness.) It would be an interesting experiment to try the "bad" chips in something else that takes 64k DRAMs, say an old PC expansion memory card, and see if they fail RAM tests there as well. (It's also possible that aging in other components is subjecting the mediocre RAM chips to out-of-spec voltage surges and "killing" them, in which case it may only a matter of time before the more robust replacements or other chips on the board start to fail.)

Well, I can invent scenarios that could explain the observations; whether they're correct is a different matter altogether.

For example, suppose these DRAMs aren't truly hermetic. Over time, moisture could seep in and corrode things (say, a bond wire or bondpad). Or if gold bondwires were used, it's possible for the dreaded "purple plague" to work its nefarious magic.

These problems would show up over time in powered or unpowered units but if poor hermiticity were the problem, storage in a damp environment would certainly be a bad thing. An operating machine would also be a hot machine, perhaps keeping moisture somewhat under control.

Without some failure analysis on the dead chips, one can only speculate. But mechanisms that could plausibly explain a couple of dead DRAMs in one machine do exist.

Corrosion of the bonding wires hadn't occurred to me... that could well be a possibility. (Although I suppose you'd have to *very carefully* crack open the case of a dead DRAM and look at it under a microscope to really say if that's what is going on.) If it *is* some sort of internal corrosion that's killing these units it would seem that "Chip Rot" might a better name for it than "Bit Rot", given the conflict with the widely-used definition for the latter.

(IE, "Bit Rot" describes the data loss from aging or physical degradation, rather than being a technical term for a specific type of physical degradation process affecting the hardware. The only way the standard definition of "Bit Rot" applies to DRAM is the corruption of data in a "live system" due to soft errors, IE, bits flipped due to static discharges, alpha particles, cosmic rays, etc. Since DRAM doesn't retain data when powered off by design it cannot *itself* suffer from "Bit Rot".)

Anyway, seriously, I was curious if the term was being used specifically here to denote some manufacturing or process weakness widespread in chips of this age, and if there were a reference one could point to when "diagnosing" a failure of this type as "Bit Rot". Otherwise it doesn't seem like it's useful to diagnose these chip failures as such.

(I vaguely remember a term similar to "chip rot" being used in an article I read *many moons ago* in an 80's computer magazine to describe an age-related failure in early PROM chips, PROMs being programmable memories which store bits essentially by "blowing fuses" instead of retaining a charge like an EPROM. PROMs were widely used in early S-100 and other low production volume computer systems, and I *seem* to recall that the problem was traced to a "Tin Whiskers"-type phenomenon inside the packaging... but it's been far too long for me to remember specifically.)

Another perfectly valid possibility is that it's a not-too-unlikely coincidence. If one chip could fail completely on its own in a 10-year window, another chip could also fail completely on its own. Simply square the probability of the first one failing and you have the probability of both independently failing.

Multiple chip failures are fairly common. What kills one is likely to kill another from the same batch, ie. they are likely to have the same bad spot on the die as the mask is stepped across the wafer. As for dying in storage, it's common for the analog section to drift as aluminum capacitors lose their oxide dielectric when not carrying a charge. They need time under bias to reform the oxide and for leakage to decrease. That's why old tube radios are usually gradually awakened on a variac. Anyhow, it's pretty likely that +5V is a bit lower than it used to be, and those 2 chips need a full 5 Volts to work. Like an overclocker raising his RAM voltage to run his memory faster than it's rated speed. I can think of a couple more similar explanations, but I have noticed the same effect, and the Apple-branded RAM failed fairly commonly when it was new. Most of it was made by Micron, and their early chips were flaky. Flaky chips are good when they are potato chips, but not the silicon variety. (Micron was founded by an Idaho potato millionaire.)

My math is slightly off in my previous post.

Most of it was made by Micron, and their early chips were flaky. Flaky chips are good when they are potato chips, but not the silicon variety. (Micron was founded by an Idaho potato millionaire.)

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Hey now, electronic television was invented by a farm boy from Idaho (not to mention some other cool things, like the only fusion reactor that qualifies as a science fair experiment), so no hating on Idaho when it comes to technology.