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Mac IIsi rescue project

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Found this machine on Craig's about 20 minutes from my house - for free.  Picked it up today.  It's a basket case.  One amazing thing is that the power supply appears to be in working order.  I get all the proper Voltages yet the board doesn't turn on.  Going to order the capacitors and soak the board in isoproply in the hopes that I can get most of the green funk off.  The floppy looks wasted - I'm not sure if a restoration can bring it back but electrically it "tries" to mount a floppy.  The hard drive spins up and down every 5 seconds (anyone seen this problem?).  And so far this machine obviously doesn't boot.


I'm planning to do the power supply and the logic board in the hopes that it will boot.  Not sure from there but figure it *could* be worth saving from the trash pile.  Does this look too far gone to you guys?





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Logic board doesn't look too bad, really—at least there's no battery bomb to deal with. You'll definitely want to give it a good clean to make sure the cap goo is gone. I can't tell by the photos if the FDD is repairable, but give it a clean and lube and see if it reads (a disk you don't care about losing). Alternative is to invest in a FloppyEmu, if you've got other vintage floppy-dependent Macs as well, or plan on acquiring more.

That case... looks like it was sitting on its front side down in a puddle for a while... can't tell if that's algae or what :lol: but that would explain the FDD rust.


The intermittent power sort of sounds like the small daughter board inside the PSU has leaky caps (that's one symptom of that anyway). But could always be the HDD has other issues: age, water ;) 

One other thing of note is your IIsi has ROM SIMM populating the slot... not very common if it's the IIsi ROM SIMM. Could also be that the previous owner has popped in another ROM, like perhaps a IIfx ROM. Interested to see, once this is up and running, if it's a 32-bit clean ROM.

Edited by jessenator
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Definitely worth giving the logic board a bath. Some people get testy about boards going in the dishwasher, but I've been very pleased with dishwasher results (please no soap or other dishes) . Scrape that orange barcode sticker (if you feel the need to keep it) in the board corner off if you do any bathing, as it'll likely disintegrate. 


Washing that case top and bottom (no components when washing them) and carefully wash/clean/lube the floppy drive would give me a lot of hope.


And sometimes washing actually rehydrates the capacitors enough to bring it (temporarily) back from the dead.  Techmoan shows this phenomenon with his Mac Classic in this YouTube video.

Edited by gogopuffs
oh you know some bits and bobs
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The drive heads could be stuck and that's generally a pretty easy fix.  I had a similar Conner hard drive - if you're careful and take the torx screwdriver to open up the top in a relatively dust-free environment, then plug it in and power it up, you may see that the head is stuck.   You can see an example of this on YouTube here.

It could also be stiction, which there are other little tutorials.  Would be neat to try and revive the guy. And naturally replace with a SCSI2SD or RaSCSI if you want!

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Thanks goodness a Tadiran PRAM battery was used. The most reliable brand and least prone to leaking.


@jessenatorEvery Mac ROM from the IIci (September 1989) on was 32-bit clean, so also the IIfx (March 1990) and the IIsi (late 1990).


This MIT document refers not specifically to 32-bit cleanless but to their support policiy at that time, considering SAP and Open Transport. These in turn required 68020/030 processors and 32bit-addressing. The IIsi was probably simply forgotten to metion.


@PotShotScott If these are really IIfx ROMs, can this be a IIsi prototype? Did you check the serial number and compare the board layout and silk screening?

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A picture-search on Google for "Mac iisi logicboard" gives lots of examples to compare. A quick look gives me the impression that most boards have a socketed ROM, one site mentions a soldered one. (Example: https://www.bigmessowires.com/romdrv/rominatorii-instructions.pdf ) This hints that you may have a normal production model.


To my astonishment ROMs are quite interchangeable. The IIfx is a truly different beast compared to a IIsi but ROMs may be compatible. (At least I hope so. For example, there is a new third party ROM produced by BMOW claimed to be compatible with Macintosh IIx, IIcx, IIci, IIfx, IIsi and SE/30 (see pdf linked before).


I did not look up how to identify ROM-versions. You are sure it's a IIfx ROM in your Mac? Both the IIfx and IIsi are designed to be 32-bit-clean.

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IIRC only the early units had a socketed rom, but all of them kept the socket and ability to switch from he soldered Rom to the socket one. (probably for late upgrades that never came)


I have had a few units, and only recently came across a board without soldered Roms (also unfortunately missing the IIsi Rom Simm) so useless without.


At some point this Rom simm was sought after to convert a SE/30 (dirty) to 32-bit-clean, so many were removed for this purpose.



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I just very recently learned that both soldered ROMs and a ROM socket can coexist on some boards. Probably the jumpers mentioned in the support PDF of BMOW (see my previous posting) take care of this.


17 hours ago, bibilit said:

At some point this Rom simm was sought after to convert a SE/30 (dirty) to 32-bit-clean, so many were removed for this purpose.


Yep. I recently complaint to a blogger who deprived his little IIsi of his ROM for upgrading an SE/30. I don't have any affiliation with BMOW, but since third party ROMs are now available there is no more excuse for this gruel treatment [;)] .

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Okay - so I might have spoken too soon.  I recapped and cleaned my board today.  I do not have a PRAM battery so first question is - will the IIsi start without one?  I get no chime.  Second question - what PSU Voltages should I be seeing?  I have the following on the ten pins as I have the machine turned upside down with it "powered up".


3 ground pins

(1) 4.66 VDC

(1) 3.69 VDC

(1) -12.17 VDC

(3) 4.97 VDC

(1) 12.18 VDC


The 4.66 and 3.69 pins seem off.  Any thoughts here?

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