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Macintosh Plus - Repair Help


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Hello,

 

I purchased this Macintosh Plus knowing there was some Bad Things™ going on. I describe the steps I've taken below but I need suggestions from more experienced folks. Before we begin, I want to point out that I did not recap the logic or analog board — they appear pristine. I've recapped other compact macs in the past so I am reasonably familiar with what capacitor failure looks and smells like. That being said, I did not remove and test the capacitors on either board.

 

IMG_1707.thumb.jpg.26638fbe1e5ad87ebaefa92616465145.jpg

 

On page 59 of The Dead Mac Scrolls they describe a scenario like below, which I would say matches my scenario.

Quote

On a Mac Plus there is no startup bong. The raster shows tightly spaced horizontal lines and is peppered with random dots. There is no desktop. The computer is unusable

  1. The recommendation is to replace SN74LS38N on the analog board, which I swapped from a donor board to no effect. It's possible both chips are erroneous but seems somewhat unlikely.
  2. The following page on the same topic recommends checking the reset button — which I found some continuity across. I removed the button wholesale and tested with no changes.

 

Moving on from the unsuccessful recommendations of The Dead Mac Scrolls, I searched for other threads with early compact macs displaying stripes and lines:

  1. I have swapped around ram, starting with SIM 1. I tried several iteration of this with no success, including using the original ram which came with the machine and ram from a donor board.
  2. I inspected the ROM chips and found very light corrosion which I cleaned with a fiberglass pen and deoxit. I also applied deoxit to the ROM IC socket. I also tried ROM from a donor board with the same effect.
  3. I have re-soldered several dubious joints on the analog board which had zero effect. I want to emphasize that the analog board is immaculate, including 4.95v / 11.95v output.

 

Observed changes:

  1. No ram produces no image (shocking, I know)
  2. 1 stick in SIM 1 produces several rows of ~5 pixel high bars every ~10 pixels (similar to the below but zero waviness)
  3. 2+ sticks of RAM produces the image above.

 

This is the screen with the original RAM:

img.thumb.png.a7ccab6129ecf92b51763c4c63105c73.png

 

This is a photo of the logic board I took before posting this evening:

IMG_1711.thumb.jpg.7ec892746cae01ce8efc999a17efe4bf.jpg

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Maybe try the other ram again and clip R8 if the other ram is 1mb simms.

 

Quote

Memory Upgrade

Memory upgrading procedure is quite simple if you know what you're doing. First, you have to get 30-pin SIMM modules with sufficient timings. Mac was designed to work with 120-150ns SIMMs, some 80ns should do the job, while 60-70 may cause errors. To test memory you can run Level 2 Testing from bootable MacTest 7.0 service diskette (if you get one). Mac was designed to support 8-bit SIMMs, but 9-bit (8+parity) should work. If you have 8 or 9-chip modules and 3-chip modules, these 8/9-chip ones work in more cases.

Your Mac Plus has 4 SIMM sockets. Here you can install memory in the following configurations:
 - 512K - two 256K SIMMs in sockets closer to CPU, lower 2 sockets empty.
 - 1MB - 4 256K SIMMs.
 - 2MB - 2 1MB SIMMs in sockets closer to CPU, lower 2 sockets empty.
 - 2.5MB - 2 1MB SIMMs in sockets closer to CPU, 2 256K SIMMs populating other two sockets.
 - 4MB - 4 1MB SIMMs.

But if you think that putting SIMMs to sockets is enough, you're in mistake. If you, for example, upgrade 1MB Plus to 4MB with changing SIMMs only, it'll see a half of it - 2MB. To tell Mac how much memory it has, you have to dig in the mainboard.

Look near reset/interrupt buttond and the serial port sockets, there are 2 resistors (or 1 resistor and 1 place for it, or 2 empty places) in the white square labeled "RAM SIZE". One resistor has label "256K BIT" while the other has "ONE ROW".

memexps.jpg

Generally, install "ONE ROW" resistor when only 2 SIMMs (doesn't matter which size) are installed.
Install "256K BIT" resistor if all of SIMMs are 256K (1MB config).

It means that to install 4MB, there must be no resistors. While upgrading, cut only one resistor's pin, then move it not to connect - it helps with downgrading.

Resistors are 150 Ohms.

 

http://oldcomputer.info/apple/Plus/index.htm

 

Edited by 360alaska
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  • 68kMLA Supporter
9 minutes ago, 360alaska said:

Maybe try the other ram again and clip R8 if the other ram is 1mb simms.

The individual sticks have 8 "TMS 4256FML" chips. If the "256" is of any significance, wouldn't that put these sticks at 2mb each?

edit: tried it — no change

Edited by Stephen
tried it but no change!
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FML is right! Only kidding...

 

According to the datasheet, these chips are 256k x 1 bit, the way you calculate a total for a simm is to add all the bits and then divide by 8 to get bytes. so 256k x 1 bit x 8 chips / 8 = 256kb per simm. (I realize multiplying by 8 then dividing by 8 is redundant but there are sims with less chips. ) So that resistor being there is correct for these simms. My point was if you swap in some other simms you may need to cut it. you can also add a resistor to the one row holes and test them two at a time. I have feeling you have a bad stick and your replacements my not work with this machine either. Especially if they are 2 or 3 chip sticks.

Edited by 360alaska
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24 minutes ago, 360alaska said:

FML is right! Only kidding...

The same thought crossed my mind while I searched for more information about them. Sounds like the best recommendation right now is to buy replacement ram?

 

Thanks for including the additional information (reference guide, calculation and example). I tried a bunch of iterations but the only change I got was snow-like static going back and forth across the screen and the same glitchiness from the original (suspected) bad ram.

 

edit:

Adding photos — I also have this ram laying around, but maybe it's OP for this Macintosh?

ram.thumb.jpg.eb88147b5ea3a115175d7946083eb8bf.jpg

 

And this is what the "glitchiness" looks like which I described earlier.

glitchy.thumb.jpg.f0f0d993dbdce5c3f36f81acebff4ac0.jpg

Edited by Stephen
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  • 68kMLA Supporter

The good news — the Macintosh Plus logic board (and the spare I purchased prior to this cursed plus) are good!

The bad news — both of the motherboards have faulty ROM chips.

 

I am fortunate in that I have a working Macintosh Plus and the audacity to remove my working ROMs and test them in potentially faulty logic boards. I see replacement ROMs for $30 on eBay which seems sort of high for ~$3.50 in chips and the time to burn an image... and I have an EEPROM programmer on the way from Amazon.

 

Can anyone recommend a good tutorial for ripping and burning ROMs or a cheaper alternative to the options I see on eBay? Thank you for your help @360alaska!

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I've not used a PICKIT to flash EEPROMs.

 

But using a TL866 family of programmers isn't hard assuming its a supported chip. The basics would be to convert the raw file into a format used by the flashing software. After that load the chip and write then verify.

 

If your reading the ROMs from working chips the file conversion isn't needed. Just read then write.

 

I do notice that the PICKIT2 is listed as supported by flashrom but not the PICKIT3. I don't know what software you were hoping to use.

https://flashrom.org/Supported_hardware#External_flashers.2Fprogrammers

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31 minutes ago, karrots said:

I've not used a PICKIT to flash EEPROMs.

 

But using a TL866 family of programmers isn't hard assuming its a supported chip. The basics would be to convert the raw file into a format used by the flashing software. After that load the chip and write then verify.

 

If your reading the ROMs from working chips the file conversion isn't needed. Just read then write.

 

I do notice that the PICKIT2 is listed as supported by flashrom but not the PICKIT3. I don't know what software you were hoping to use.

https://flashrom.org/Supported_hardware#External_flashers.2Fprogrammers

That's a great starting point — thank you! Once the pickit3 arrives I'll try out flashrom.

 

If anyone else has experience or suggestion they would like to share it would be warmly welcome!

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5 hours ago, dochilli said:

You can use 27C512 Eproms.

This may help:

https://oldcrap.org/2019/03/22/macintosh-plus/

 

Here you can download ROMs and see a decription of a repair:

https://www.nightfallcrew.com/25/04/2014/macintosh-plus-sad-face-repair/

 

More ROMS:

https://www.macintoshrepository.org/7038-all-macintosh-roms-68k-ppc-

 

 

Super useful resources, thank you!

 

8 hours ago, 360alaska said:

Have a look here before you commit to anything: https://www.bigmessowires.com/mac-rom-inator/

 

they are not sold anymore by Steve but you can make them.

I had been looking at that but I don't really want to hotrod my Macintosh Plus'. Appreciate the suggestion though

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6 hours ago, dochilli said:

I use a TL866II plus and it worked for me. Perhaps you can try to read your old ROMs and compare them to the downloaded ROMs. If there are differences then the ROMs are defect.

 

 

The TL866II is what I use and it's pretty dang nice. It also has a TTL logic IC tester built in, which is incredibly useful in troubleshooting 74-series logic chips. It's about $50-60 on Amazon and worth every penny.

 

As dochilli said, you'll want to dump your Plus' ROMs and compare them (the checksum, at least) to a known good copy of the ROMs; if the checksums are different, the ROMs need replacement.

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As a point of clarification, I have already confirmed the ROMs are fubar. That being said, I'm tempted to solder the fubar ROM directly to some header and re-testing; it could be corrosion damage on the legs... franken-rom :evil:

 

42 minutes ago, warmech said:

 

The TL866II is what I use and it's pretty dang nice. It also has a TTL logic IC tester built in, which is incredibly useful in troubleshooting 74-series logic chips. It's about $50-60 on Amazon and worth every penny.

 

As dochilli said, you'll want to dump your Plus' ROMs and compare them (the checksum, at least) to a known good copy of the ROMs; if the checksums are different, the ROMs need replacement.

Thank you!

Edited by Stephen
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Taking everyone’s advice, I cancelled the PICKIT3 (I realized my mistake) and purchased a TL866II Plus. 
 

If I can get this working then the community can expect a new source of programmed ROM chips for much less than the $30 on eBay - it’s predatory and not in the spirit nor best interest of the community. 

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  • 2 weeks later...
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I had a little learning curve figuring out the best chip as a modern ROM replacement. I got close and some friendly Redditors helped me figure out the best options.

 

I have a few chips on order from China and expect them in a few weeks. Once I receive them I’ll be documenting the process and publishing a YouTube video as well as an Instructable guide (if all goes smoothly).

 

In fact, I’ve ordered enough chips that I will have many more than I need immediately. If anyone is interested I would be willing to provide a free set of ROMs to 3 people - just pay shipping. Of course, this is all contingent on me completing the process.

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Very glad to see this being looked into.  Just as an aside here, I’m going to defended the $29 ROM eBay guy ... wanting to get a Plus working in a hurry a while back, I bought a pair from him, there was a minor issue with the order (not the checksum I’d expected) and he quickly and courteously sent a corrected order with no questions asked and without asking for a return.  I don’t begrudge him charging a premium for the service, it’s not like these things are moving in volume.  That said, would always welcome an alternative and wish you success with your project!

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Did you go with EEPROMs or EPROM?

 

Also good instructions at the bottom of this thread.

 

In general the process is not difficult assuming you can get parts. Also don't assume EPROMs are shipped blank. Verify before writing.

 

I've only burned only for my Classic.

 

https://68kmla.org/forums/topic/61249-help-fixing-up-some-macintosh-plusses/?do=findComment&comment=656550

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  • 1 month later...
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Something incredible happened today — it wasn't that my chips finally arrived (I guess I can scratch out my "ONE Apus" theory)... and it wasn't that the chips were actually good... the incredible thing is that everything worked on the first try. I'll be publishing the tutorial in a few days!

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Okay, here's the tutorial — feel free to leave feedback or ask questions in the comments on Instructables or here.

 

 

I'm giving away 3 sets of ROMs (just pay shipping!)

Please post in here if you're interested and I'll reach out to qualifying parties to get your mailing information 

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